Cyclodextrins and their uses in Textiles: Review

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Cyclodextrins and their uses in Textiles: Review

  1. 1. Process Biochemistry xxx (2003) xxx–xxx Review Cyclodextrins and their uses: a review E.M. Martin Del Valle Department of Chemical Engineering, University of Salamanca, Plaza de los Caidos S/N, 37007 Salamanca, Spain Received 17 February 2003; received in revised form 20 May 2003; accepted 2 July 2003Abstract Cyclodextrins are a family of cyclic oligosaccharides composed of ␣-(1,4) linked glucopyranose subunits. Cyclodextrins are useful molecularchelating agents. They possess a cage-like supramolecular structure, which is the same as the structures formed from cryptands, calixarenes,cyclophanes, spherands and crown ethers. These compounds having supramolecular structures carry out chemical reactions that involveintramolecular interactions where covalent bonds are not formed between interacting molecules, ions or radicals. The majority of all thesereactions are of ‘host–guest’ type. Compared to all the supramolecular hosts mentioned above, cyclodextrins are most important. Becauseof their inclusion complex forming capability, the properties of the materials with which they complex can be modified significantly. As aresult of molecular complexation phenomena CDs are widely used in many industrial products, technologies and analytical methods. Thenegligible cytotoxic effects of CDs are an important attribute in applications such as rug carrier, food and flavours, cosmetics, packing, textiles,separation processes, environment protection, fermentation and catalysis.© 2003 Elsevier Ltd. All rights reserved.Keywords: Cyclodextrins; Applications; Inclusion complex; Equilibrium; Complexation techniques1. History crystalline products, dextrins A and B, which were described with regard to their lack of reducing power. The bacterial Cyclodextrins are cyclic oligosaccharides consisting of six strain capable of producing these products from starch was␣-cyclodextrin, seven ␤-cyclodextrin, eight ␥-cyclodextrin unfortunately not maintained.or more glucopyranose units linked by ␣-(1,4) bonds In 1904, Schardinger [2] isolated a new organism ca-(Fig. 1). They are also known as cycloamyloses, cyclomal- pable of producing acetone and ethyl alcohol from sugartoses and Schardinger dextrins [1,2]. They are produced as and starch-containing plant material. In 1911, he describeda result of intramolecular transglycosylation reaction from that this strain, called Bacillus macerans, also producesdegradation of starch by cyclodextrin glucanotransferase large amounts of crystalline dextrins (25–30%) from starch.(CGTase) enzyme [3]. Schardinger [2] named his crystalline products ‘crystallised They were first discovered in 1891 [1], when in addition dextrin ␣’ and ‘crystallised dextrin ␤’. It took until 1935to reducing dextrins a small amount of crystalline mate- before ␥ dextrin was isolated. Several fractionation schemesrial was obtained from starch digest of Bacilus amylobacter for the production of cyclodextrins [4–6] were also de-“. . . there is formed in very small amounts (about 3 g/kg of veloped. At that time the structures of these compoundsstarch) a carbohydrate which forms a beautiful radiate crys- were still uncertain, but in 1942 the structures of ␣ andtals after a few weeks in the alcohol from which the dextrins ␤-cyclodextrin were determined by X-ray crystallographywere precipitated. . . . having the composition represented by [7]. In 1948, the X-ray structure of ␥-cyclodextrin followeda multiple of the formula (C6 H10 O3 )·3H2 O. . . ” According and it was recognised that CDs can form inclusion com-to other authors, Villiers [1] probably used impure cultures plexes. In 1961, evidence for the natural existence of ␦-, ␨-,and the cyclodextrins were produced by a Bacillus macer- ␰- and even ␩-cyclodextrin (9–12 residues) was providedans contamination. Villiers [1] named his crystalline product [8]. The main interest in cyclodextrins lies in their ability to‘cellulosine’. In 1903, Schardinger was able to isolate two form inclusion complexes with several compounds [9–13]. From the X-ray structures it appears that in cyclodextrins the secondary hydroxyl groups (C2 and C3 ) are locatedE-mail address: emvalle@usal.es (E.M.M. Del Valle). on the wider edge of the ring and the primary hydroxyl0032-9592/$ – see front matter © 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S0032-9592(03)00258-9
  2. 2. 2 E.M.M. Del Valle / Process Biochemistry xxx (2003) xxx–xxx Fig. 1. Chemical structure of ␤-cyclodextrin.groups (C6 ) on the other edge, and that the apolar C3 and allel double helix, indicate that ␣-cyclodextrin is the formC5 hydrogens and ether-like oxygens are at the inside of in which the steric strain due to cyclization is least whilethe torus-like molecules. This result in a molecule with a ␥-cyclodextrin is most strained [3].hydrophilic outside, which can dissolve in water, and an ap- Apart from these naturally occurring cyclodextrins,olar cavity, which provides a hydrophobic matrix, described many cyclodextrin derivatives have been synthesised. Theseas a ‘micro heterogeneous environment’ [14]. derivatives usually are produced by aminations, esterifica- As a result of this cavity, cyclodextrins are able to form in- tions or etherifications of primary and secondary hydroxylclusion complexes with a wide variety of hydrophobic guest groups of the cyclodextrins. Depending on the substituent,molecules. One or two guest molecules can be entrapped by the solubility of the cyclodextrin derivatives is usuallyone, two or three cyclodextrins. different from that of their parent cyclodextrins. Virtually all derivatives have a changed hydrophobic cavity volume and also these modifications can improve solubility, stabil-2. Properties ity against light or oxygen and help control the chemical activity of guest molecules [1]. Cyclodextrins are of three types: ␣-cyclodextrin, Cyclodextrins are frequently used as building blocks. Up␤-cyclodextrin and ␥-cyclodextrin, referred to as first gener- to 20 substituents have been linked to ␤-cyclodextrin in a re-ation or parent cyclodextrins. ␣-, ␤- and ␥-cyclodextrins are gioselective manner. The synthesis of uniform cyclodextrincomposed of six, seven and eight ␣-(1,4)-linked glycosyl derivatives requires regioselective reagents, optimisation ofunits, respectively [15]. ␤-Cyclodextrin is the most acces- reaction conditions and a good separation of products. Thesible, the lowest-priced and generally the most useful. The most frequently studied reaction is an electrophilic attack atmain properties of those cyclodextrins are given in Table 1. the OH-groups, the formation of ethers and esters by alkyl Studies of cyclodextrins in solution are supported by halides, epoxides, acyl derivatives, isocyanates, and by inor-a large number of crystal structure studies. Cyclodextrins ganic acid derivatives as sulphonic acid chloride cleavage ofcrystallise in two main types of crystal packing, channel C–OH bonds has also been studied frequently, involving astructures and cage structures, depending on the type of nucleophilic attack by compounds such as azide ions, halidecyclodextrin and guest compound. ions, thiols, thiourea, and amines; this requires activation of These crystal structures show that cyclodextrins in com- the oxygen atom by an electron-withdrawing group [3].plexes adopt the expected ‘round’ structure with all gluco- Because of their ability to link covalently or noncovalentlypyranose units in the 4 C1 chair conformation. Furthermore, specifically to other cyclodextrins, cyclodextrins can be usedstudies with linear maltohexaoses, which form an antipar- as building blocks for the construction of supramolecularTable 1Cyclodextrins propertiesProperty ␣-Cyclodextrin ␤-Cyclodextrin ␥-CyclodextrinNumber of glucopyranose units 6 7 8Molecular weight (g/mol) 972 1135 1297Solubility in water at 25 ◦ C (%, w/v) 14.5 1.85 23.2Outer diameter (Å) 14.6 15.4 17.5Cavity diameter (Å) 4.7–5.3 6.0–6.5 7.5–8.3Height of torus (Å) 7.9 7.9 7.9Cavity volume (Å3 ) 174 262 427
  3. 3. E.M.M. Del Valle / Process Biochemistry xxx (2003) xxx–xxx 3complexes. Their ability to form inclusion complexes with metabolite incorporation and 7–14% as metabolites in fae-organic host molecules offers possibilities to build supra ces and urine, mainly excreted unchanged by the renal routemolecular threads. In this way molecular architectures such after i.v. injections with t1/2 = 25 min in rats, LD50 oral, ratas catenanes, rotaxanes, polyrotaxanes, and tubes, can be >10,000 mg/kg, LD50 i.v., rat: between 500 and 750 mg/kg.constructed. Such building blocks, which cannot be preparedby other methods can be employed, for example, for the sep- 2.1.2. β-Cyclodextrinaration of complex mixtures of molecules and enantiomers The main properties are: less irritating than ␣-cyclodextrin[3]. after i.m. injection; binds cholesterol; very small amounts Each year cyclodextrins are the subject of almost 1000 (1–2%) absorbed in the upper intestinal tract after oral ad-research articles and scientific abstracts, large numbers of ministration; no metabolism in the upper intestinal tract;which deal with drugs and drug-related products. In ad- metabolised by bacteria in caecum and colon; currently thedition, numerous inventions have been described which most common cyclodextrin in pharmaceutical formulationsinclude cyclodextrins (over 1000 patents or patent applica- and, thus, probably the best studied cyclodextrin in humans.tions in the past 5 years). From a regulatory standpoint, a Application of high doses may be harmful and is not recom-monograph for ␤-cyclodextrin is already available in both mended; bacterial degradation and fermentation in the colonthe US Pharmacopoeia/National Formulary (USP 23/NF may lead to gas production and diarrhoea, LD50 oral, rat18, 1995) the European Pharmacopoeia (3rd ed., 1997). A >5000 mg/kg, LD50 i.v., rat: between 450 and 790 mg/kg.monograph for 2-hydroxypropyl-b-cyclodextrin is in thepreparation for US Pharmacopoeia/National Formulary, 2.1.3. γ-Cyclodextrinand various monographs for cyclodextrins are included in The main properties are: insignificant irritation after i.m.compendial sources, e.g. the Handbook of Pharmaceutical injection; rapidly and completely degraded to glucose in theExcipients [16]. Thus, more than one century after their dis- upper intestinal tract by intestinal enzymes (even at highcovery cyclodextrins are finally, but rapidly, being accepted daily dosages, e.g. 10–20 g/kg); almost no (0.1%) absorptionas ‘new’ pharmaceutical excipients. (of intact ␥-cyclodextrin) after oral administration; practi- cally no metabolism after i.v. administration; probably the2.1. Toxicological considerations least toxic cyclodextrin, at least of the three natural cy- clodextrins. Actively promoted as food additive by its main The safety profiles of the three most common natural manufactures; complexing abilities, in general, less thancyclodextrins and some of their derivatives have recently those of ␤-cyclodextrin and the water soluble ␤-cyclodextrinbeen reviewed [17,18]. In general, the natural cyclodextrins derivatives; its complexes frequently have limited solubil-and their hydrophilic derivatives are only able to permeate ity in aqueous solutions and tend to aggregate in aqueouslipophilic biological membranes, such as the eye cornea, solutions, which makes the solution unclear (opalescence)with considerable difficulty. Even the somewhat lipophilic [20], LD50 oral, rat 8000 mg/kg, LD50 i.v., rat: aboutrandomly methylated ␤-cyclodextrin does not readily per- 4000 mg/kg.meate lipophilic membranes, although it interacts morereadily with membranes than the hydrophilic cyclodextrin 2.2. Inclusion complex formationderivatives [19]. All toxicity studies have demonstrated thatorally administered cyclodextrins are practically non-toxic, The most notable feature of cyclodextrins is their ability todue to lack of absorption from the gastrointestinal tract [17]. form solid inclusion complexes (host–guest complexes) withFurthermore, a number of safety evaluations have shown a very wide range of solid, liquid and gaseous compounds bythat ␥-cyclodextrin, 2-hydroxypropyl-b-cyclodextrin, sul- a molecular complexation [1]. In these complexes (Fig. 2),phobutylether ␤-cyclodextrin, sulphated ␤-cyclodextrin and a guest molecule is held within the cavity of the cyclodex-maltosyl ␤-cyclodextrin appear to be safe even when ad- trin host molecule. Complex formation is a dimensional fitministered parenterally. However, toxicological studies have between host cavity and guest molecule [21]. The lipophilicalso shown that the parent ␣- and ␤-cyclodextrin and the cavity of cyclodextrin molecules provides a microenviron-methylated ␤-cyclodextrins are not suitable for parenteral ment into which appropriately sized non-polar moieties canadministration. enter to form inclusion complexes [22]. No covalent bonds are broken or formed during formation of the inclusion com-2.1.1. α-Cyclodextrin plex [23]. The main driving force of complex formation is The main properties are: relatively irritating after i.m. the release of enthalpy-rich water molecules from the cavity.injection; binds some lipids; some eye irritation; between Water molecules are displaced by more hydrophobic guest2 and 3% absorption after oral administration to rats; no molecules present in the solution to attain an apolar–apolarmetabolism in the upper intestinal tract; cleavage only by association and decrease of cyclodextrin ring strain resultingthe intestinal flora of caecum and colon. Excretion after oral in a more stable lower energy state [3].administration to rats were: 60% as CO2 (no CO2 exhala- The binding of guest molecules within the host cy-tion after oral administration to germ-free rats), 26–33% as clodextrin is not fixed or permanent but rather is a dynamic
  4. 4. 4 E.M.M. Del Valle / Process Biochemistry xxx (2003) xxx–xxx Fig. 2. Cyclodextrins structure and inclusion complex formation.equilibrium. Binding strength depends on how well the of cyclodextrins are expanded. CDs are modified through‘host–guest’ complex fits together and on specific local in- substituting various functional compounds on the primaryteractions between surface atoms. Complexes can be formed and/or secondary face of the molecule. Modified CDs areeither in solution or in the crystalline state and water is typ- useful as enzyme mimics because the substituted functionalically the solvent of choice. Inclusion complexation can be groups act in molecular recognition. The same property isaccomplished in a co-solvent system and in the presence of used for targeted drug delivery and analytical chemistry asany non-aqueous solvent. Cyclodextrin architecture confers modified CDs show increased enantioselectivity over nativeupon these molecules a wide range of chemical proper- CDs [1].ties markedly different from those exhibited by non-cyclic The ability of a cyclodextrin to form an inclusion com-carbohydrates in the same molecular weight range. plex with a guest molecule is a function of two key factors. Inclusion in cyclodextrins exerts a profound effect on the The first is steric and depends on the relative size of thephysicochemical properties of guest molecules as they are cyclodextrin to the size of the guest molecule or certaintemporarily locked or caged within the host cavity giving key functional groups within the guest. If the guest is therise to beneficial modifications of guest molecules, which wrong size, it will not fit properly into the cyclodextrinare not achievable otherwise [24]. These properties are: cavity. The second critical factor is the thermodynamic in-solubility enhancement of highly insoluble guests, stabil- teractions between the different components of the systemisation of labile guests against the degradative effects of (cyclodextrin, guest, solvent). For a complex to form, thereoxidation, visible or UV light and heat, control of volatility must be a favourable net energetic driving force that pullsand sublimation, physical isolation of incompatible com- the guest into the cyclodextrin.pounds, chromatographic separations, taste modification While the height of the cyclodextrin cavity is the sameby masking off flavours, unpleasant odours and controlled for all three types, the number of glucose units determinesrelease of drugs and flavours. Therefore, cyclodextrins are the internal diameter of the cavity and its volume. Based onused in food [25], pharmaceuticals [26], cosmetics [27], these dimensions, ␣-cyclodextrin can typically complex lowenvironment protection [28], bioconversion [29], packing molecular weight molecules or compounds with aliphaticand the textile industry [30]. side chains, ␤-cyclodextrin will complex aromatics and The potential guest list for molecular encapsulation in cy- heterocycles and ␥-cyclodextrin can accommodate largerclodextrins is quite varied and includes such compounds as molecules such as macrocycles and steroids.straight or branched chain aliphatics, aldehydes, ketones, al- In general, therefore, there are four energeticallycohols, organic acids, fatty acids, aromatics, gases, and polar favourable interactions that help shift the equilibrium tocompounds such as halogens, oxyacids and amines [24]. Due form the inclusion complex:to the availability of multiple reactive hydroxyl groups, thefunctionality of cyclodextrins is greatly increased by chem- • The displacement of polar water molecules from theical modification. Through modification, the applications apolar cyclodextrin cavity.
  5. 5. E.M.M. Del Valle / Process Biochemistry xxx (2003) xxx–xxx 5• The increased number of hydrogen bonds formed as the librium constants, guest properties are usually most easily displaced water returns to the larger pool. assessed.• A reduction of the repulsive interactions between the hydrophobic guest and the aqueous environment. D + CD DCD• An increase in the hydrophobic interactions as the guest [DCD] inserts itself into the apolar cyclodextrin cavity. Kc = (1) [D][CD] While this initial equilibrium to form the complex is very Connors has evaluated the population characteristics of cy-rapid (often within minutes), the final equilibrium can take clodextrin complex stabilities in aqueous solution [40,41].much longer to reach. Once inside the cyclodextrin cavity, The stability constant (Kc ) is better expressed as Km:n tothe guest molecule makes conformational adjustments to indicate the stoichiometric ration of the complex. It can betake maximum advantage of the weak van der Waals forces written [32,42]:that exist. Km:n Complexes can be formed by a variety of techniques that mL + nS Lm Sn (a−mx)(b−nx) (x)depend on the properties of the active material, the equi-librium kinetics, the other formulation ingredients and pro- So that,cesses and the final dosage form desired. However, each [x]of these processes depends on a small amount of water to Km:n = (2)help drive the thermodynamics. Among the methods used [a − mx]m [b − nx]nare simple dry mixing, mixing in solutions and suspensions In addition, dissociation constant can also be defined:followed by a suitable separation, the preparation of pastes [a − mx]m [b − nx]n 1 1and several thermo-mechanical techniques. Kd = = or (3) [x] Kc Km:n Dissociation of the inclusion complex is a relatively rapidprocess usually driven by a large increase in the number of One of the most useful and widely applied analyticalwater molecules in the surrounding environment. The result- approaches in this context is the Phase–solubility methoding concentration gradient shifts the equilibrium in Fig. 2 described by Higuchi and Connors [42]. Phase–solubilityto the left. In highly dilute and dynamic systems like the analysis involves an examination of the effect of a solubi-body, the guest has difficulty finding another cyclodextrin to lizer, i.e. cyclodextrin or ligand on the drug being solubi-reform the complex and is left free in solution. lized, i.e. the substrate. Experimentally, the drug of interest is added to several vials such that it is always in excess.2.2.1. Equilibrium The presence of solid drug in these systems in necessary to The central cavity of the cyclodextrin molecule is lined maximise the thermodynamic activity of the dissolved sub-with skeletal carbons and ethereal oxygens of the glucose strate. To the drug or substrate (S) a constant volume ofresidues. It is, therefore, lipophilic. The polarity of the cavity water containing successively larger concentrations of thehas been estimated to be similar to that of aqueous ethanolic cyclodextrin or ligand (L) is added. The vials are mixed atsolution [31]. It provides a lipophilic microenvironment into constant temperature until equilibrium is established (whichwhich suitably sized drug molecules may enter and include. frequently takes about 1 week). The solid drug is then re- One drug molecule forms a complex with one cyclodex- moved and the solution assayed for the total concentrationtrin molecule. of S. A Phase–solubility diagram is constructed by plotting Measurements of stability or equilibrium constants (Kc ) the total molar concentration of S on the y-axis and the totalor the dissociation constants (Kd ) of the drug–cyclodextrin molar concentration of L added on the x-axis (Fig. 3).complexes are important since this is an index of changes in Phase–solubility diagrams prepared in this way fall intophysicochemical properties of a compound upon inclusion. two main categories, A- and B-types. A-type curves are in-Most methods for determining the K-values are based on dicative for the formation of soluble inclusion complexestitrating changes in the physicochemical properties of the while B-type behaviour are suggestive of the formation ofguest molecule, i.e. the drug molecule, with the cyclodextrin inclusion complexes of poor solubility. ABS -type responseand then analysing the concentration dependencies. Addi- denotes complexes of limited solubility and a BI -curve aretive properties that can be titrated in this way to provide indicative of insoluble complexes. The A-curves are sub-information on the K-values include [32] aqueous solubility divided into AL (linear increases of drug solubility as a[33–35], chemical reactivity [36,37], molar absorptivity and function of cyclodextrin concentration), AP (positively de-other optical properties (e.g. optical rotation dispersion), viating isotherm) and AN (negatively deviating isotherms)phase solubility measurements [38], nuclear magnetic res- subtypes.onance chemical shifts, pH-metric methods, calorimetric While ␤-cyclodextrin often gives rise to B-type curves duetitration, freezing point depression [39], and liquid chro- to the poor water solubility of the ligand itself, the chemi-matography chromatographic retention times. While it is cally modified CDs including HP␤CD and SBE␤CD usuallypossible to use both guest or host changes to generate equi- produce soluble complexes (i.e. A-type systems). AL -type
  6. 6. 6 E.M.M. Del Valle / Process Biochemistry xxx (2003) xxx–xxx and uncomplexed) and [L]t is the total concentration of L. For Phase–solubility systems that are first order with respect to the cyclodextrin (n = 1), the following equation may be derived: mKSm [L]t [S]t = 0 + S0 (10) 1 + KSm 0 A plot of [S]t versus [L]t for the formation of Sm L should give a straight line with the y-intercept representing S0 and the slope being: mKSm slope = 0 (11) 1 + KSm0 Therefore, if m is known, K can be calculated. If m = 1 Fig. 3. Phase–solubility relationships. (i.e. a 1:1 drug:cyclodextrin complex forms), the following equation can be applied:diagrams are first order with respect to the cyclodextrin (L) slopeand may be first or higher order with respect to the drug K1:1 = (12) S0 (1 − slope)(S), i.e. SL, S2 L, S3 L, . . . , Sm L. If the slope of an AL -typesystem is greater than one, higher order complexes are indi- 2.2.2. Temperaturecated. A slope of less than one does not necessarily exclude The thermodynamic parameters, i.e. the standard freehigher order complexation but 1:1 complexation is usually energy change ( G), the standard enthalpy change ( H)assumed in the absence of other information. AP -type sys- and the standard entropy change ( S), can be obtainedtems suggest the formation of higher order complexes with from the temperature dependence of the stability constantrespect to the ligand at higher ligand concentrations, i.e. SL2 , of the cyclodextrin complex [43]. The free energy of re-SL3 , . . . , SLn . The stoichiometry of AP -type systems can be action is derived from the equilibrium constant using theevaluated by curve fitting. AN -type systems are problematic relationship:and difficult to interpret. The negative deviation from linearity may be associated G = −RT ln K1:1 (13)with ligand-induced changes in the dielectric constant of the The enthalpies of reactions can likewise be determinedsolvent or self-association of the ligands at high cyclodextrin from K1:1 obtained at various temperatures using the van’tconcentrations. Hoff equation. If two sets of data are available (i.e. two These Phase–solubility systems not only allows a quali- Kc values determined at two different temperatures in K)tative assessment of the complexes formed but may also be then:used to derive equilibrium constants. The equilibrium con-stant (K) for the formation of [Sm Ln ] can be represented by: K2 − H T2 − T1 log = (14) K1 2.303R T1 T2 [Sm Ln ]K= , (4) [S]m [L]n On the other hand, if a range of values are available, the H values can be obtained from a plot of ln K versus 1/Twhere, using the following relationship:[S] = S0 (5) H 1 log K = − + constant, (15)[S]t = S0 + m[Sm Ln ] (6) 2.303R T where the slope will provide the enthalpy data.[L]t = [L] + n[Sm Ln ] (7) The entropy for the complexation reaction can the be cal-Therefore, the values of [Sm Ln ], [S] and [L] can be culated using the expression:obtained: G= H −T S (16)[S] = S0 (5) Complex formation is usually associated with a relative large [S]t − S0 negative H and a S, which can either be negative, but[Sm Ln ] = (8) m also depends on the properties of the guest molecules.[L] = [L]t − n[Sm Ln ], (9) The association of binding constants with substrate po- larizability suggest that van der Waal’s forces are importantwhere S0 is the equilibrium solubility of S (i.e. in the absence in complex formation. Hydrophobic interactions are associ-of solubilizer), [S]t is the total concentration of S (complexed ated with a slightly positive H and a large positive S and,
  7. 7. E.M.M. Del Valle / Process Biochemistry xxx (2003) xxx–xxx 7therefore, classical hydrophobic interactions are entropy 2.3. Preparation methoddriven suggesting that they are not involved with cyclodex-trin complexation since, as indicated, these are enthalpically Cyclodextrin inclusion is a stoichiometric moleculardriven processes. Furthermore, for a series of guests there phenomenon in which usually only one guest moleculetends to be a linear relationship between enthalpy and en- interacts with the cavity of the cyclodextrin molecule totropy, with increasing enthalpy related with less negative become entrapped. A variety of non-covalent forces, suchentropy values. This effect, termed compensation, is often as van der Waals forces, hydrophobic interactions and othercorrelated with water acting as a driving force in complex forces, are responsible for the formation of a stable complex.formation. However, Connors has pointed out that, in gen- Generally, one guest molecule is included in one cy-eral, the most nonpolar portions of guest molecules are clodextrin molecule, although in the case of some lowenclosed in the cyclodextrin cavity and, thus, hydrophobic molecular weight molecules, more than one guest moleculeinteractions must be important in many cyclodextrin com- may fit into the cavity, and in the case of some high molecu-plexes [40]. The main driving force for complex formation lar weight molecules, more than one cyclodextrin moleculeis considered by many investigators to be the release of may bind to the guest. In principle, only a portion of theenthalpy-rich water from the cyclodextrin cavity [44]. The molecule must fit into the cavity to form a complex. Aswater molecules located inside the cavity cannot satisfy a result, one-to-one molar ratios are not always achieved,their hydrogen bonding potentials and therefore are of especially with high or low molecular weight guests.higher enthalpy. The energy of the system is lowered whenthese enthalpy-rich water molecules are replaced by suit- 2.3.1. Solution dynamicsable guest molecules which are less polar than water. Other In the crystalline form, only the surface molecules ofmechanisms that are thought to be involved with complex the cyclodextrin crystal are available for complexation. Information have been identified in the case of ␣-cyclodextrin. solution, more cyclodextrin molecules become available. In this instance, release of ring strain is thought to be Heating increases the solubility of the cyclodextrin as wellinvolved with the driving force for compound-cyclodextrin as that of the guest, and this increases the probability ofinteraction. Hydrated ␣-cyclodextrin is associated with complex formation. Complexation occurs more rapidlyan internal hydrogen bond to an included water molecule when the guest compound is either in soluble form or inwhich perturbs the cyclic structure of the macrocycle. Elim- dispersed fine particles.ination of the included water and the associated hydrogenbond is related with a significant release of steric strain 2.3.2. Temperature effectsdecreasing the system enthalpy. In addition, ‘non-classical Temperature has more than one effect upon cyclodextrinhydrophobic effects’ have been invoked to explain com- complexes. Heating can increase the solubility of the com-plexation [40]. These non-classical hydrophobic effects are plex but, at the same time also destabilises the complex.a composite force in which the classic hydrophobic effects These effects often need to be balanced.(characterised by large positive DS) and van der Waal’s As heat stability of the complex varies from guest toeffects (characterised by negative H and negative S) are guest, most complexes start to decompose at 50–60 ◦ C, whileoperating in the same system. Using adamantanecarboxy- some complexes are stable at higher temperatures, espe-lates as probes, ␣-, ␤- and ␥-cyclodextrins were examined. cially if the guest is strongly bound or the complex is highlyIn the case of ␣-cyclodextrin, experimental data indicated insoluble.small changes in H and S consistent with little interac-tion between the bulky probe and the small cavity. In the 2.3.3. Use of solventscase of ␤-cyclodextrin, a deep and snug-fitting complex was Water is the most commonly used solvent in which com-formed leading to a large negative H and a near-zero S. plexation reactions are performed. The more soluble theFinally, complexation with ␥-cyclodextrin demonstrated cyclodextrin in the solvent, the more molecules becomenear-zero H values and large positive S values consis- available for complexation. The guest must be able to dis-tent with a classical hydrophobic interaction. Evidently, the place the solvent from the cyclodextrin cavity if the solventcavity size of ␥-cyclodextrin was too large to provide for a forms a complex with the cyclodextrin. Water, for exam-significant contribution by van der Waal’s-type interactions. ple is very easily displaced. The solvent must be easilyThese various explanations show that there is no simple removed if solvent-free complexes are desired. In the caseconstruct to describe the driving force for complexation. of multi-component guests, one of the components may actAlthough release of enthalpy-rich water molecules from the as a solvent and be included as a guest.cyclodextrin cavity is probably an important driving force Not all guests are readily solubilised in water, makingfor the drug-cyclodextrin complex formation other forces complexation either very slow or impossible. In such cases,may be important. These forces include van der Waals the use of an organic solvent to dissolve the guest is desir-interactions, hydrogen bonding, hydrophobic interactions, able. The solvent should not complex well with cyclodextrinrelease of ring strain in the cyclodextrin molecule and and be easily removed by evaporation. Ethanol and diethylchanges in solvent-surface tensions [45]. ether are good examples of such solvents.
  8. 8. 8 E.M.M. Del Valle / Process Biochemistry xxx (2003) xxx–xxx2.3.4. Effects of water vatives to reduce the cyclodextrin complexation of various As the amount of water is increased, the solubility of both steroids [48]. On the other hand, additives such as ethanolcyclodextrin and guest are increased so that complexation can promote complex formation in the solid or semisolidoccurs more readily. However, as the amount of water is state [49]. Un-ionised drugs usually form a more stable cy-further increased, the cyclodextrin and the guest may be so clodextrin complex than their ionic counterparts and, thus,dilute that they do not get in contact as easily as they do in a complexation efficiency of basic drugs can be enhancedmore concentrated solution. Therefore, it is desirable to keep by addition of ammonia to the aqueous complexationthe amount of water sufficiently low to ensure complexation media. For example, solubilisation of pancratistatin withoccurs at a sufficiently fast rate. hydroxypropyl-cyclodextrins was optimised upon addition Some high molecular weight compounds such as oils have of ammonium hydroxide [50].a tendency to associate with themselves rather than interact-ing with cyclodextrin. In such cases, using more water allied 2.4.2. Slurry complexationwith good mixing will allow better dispersion and separa- It is not necessary to dissolve the cyclodextrin completelytion of the oil molecules or isolation of the oil molecules to form a complex. Cyclodextrin can be added to water asfrom each other. When the oil molecules come into contact high as 50–60% solids and stirred. The aqueous phase willwith the cyclodextrin, they form a more stable complex than be saturated with cyclodextrin in solution. Guest moleculesthey would if less water were present. will complex with the cyclodextrin in solution and, as the cyclodextrin complex saturates the water phase, the complex2.3.5. Volatile guests will crystallise or precipitate out of the aqueous phase. The Volatile guests can be lost during complexation, especially cyclodextrin crystals will dissolve and continue to saturateif heat is used. With highly volatile guests, this can be pre- the aqueous phase to form the complex and precipitate orvented by using a sealed reactor or by refluxing the volatile crystallise out of the aqueous phase, and the complex canguests back to the mixing vessel. be collected in the same manner as with the co-precipitation method.2.4. Complexation techniques The amount of time required to complete the complexa- tion is variable, and depends on the guest. Assays must be Several techniques are used to form cyclodextrin com- done to determine the amount of time required. Generally,plexes [32,45]. slurry complexation is performed at ambient temperatures. With many guests, some heat may be applied to increase2.4.1. Co-precipitation the rate of complexation, but care must be applied since too This method is the most widely used method in the lab- much heat can destabilise the complex and the complexationoratory. Cyclodextrin is dissolved in water and the guest reaction may not be able to take place completely. The mainis added while stirring the cyclodextrin solution. The con- advantage of this method is the reduction of the amount ofcentration of ␤-cyclodextrin can be as high as about 20% water needed and the size of the reactor.if the guest can tolerate higher temperatures. If a suffi-ciently high concentration is chosen, the solubility of the 2.4.3. Paste complexationcyclodextrin–guest complex will be exceeded as the com- This is a variation of the slurry method. Only a smallplexation reaction proceeds or as cooling is applied. In amount of water is added to form a paste, which is mixedmany cases, the solution of cyclodextrin and guest must be with the cyclodextrin using a mortar and pestle, or on acooled while stirring before a precipitate is formed. large scale using a kneader. The amount of time required is The precipitate can be collected by decanting, centrifu- dependent on the guest.gation or filtration. The precipitate may be washed with a The resulting complex can be dried directly or washedsmall amount of water or other water-miscible solvent such with a small amount of water and collected by filtration oras ethyl alcohol, methanol or acetone. Solvent washing may centrifugation. Pastes will sometimes dry forming a hardbe detrimental with some complexes, so this should be tested mass instead of a fine powder. This is dependent on the guestbefore scaling up. and the amount of water used in the paste. Generally, the The main disadvantage of this method lies in the hard mass can be dried thoroughly and milled to obtain ascale-up. Because of the limited solubility of the cyclodex- powdered form of the complex.trin, large volumes of water have to be used. Tank capacity,time and energy for heating and cooling may become im- 2.4.4. Damp mixing and heatingportant cost factors. Treatment and disposal of the mother This method uses little or no added water. The amountliquor obtained after collecting the complex may also be a of water can range from the amount of water of hydrationconcern. This can be diminished in many cases by recycling in the cyclodextrin and added guest to up to 20–25% waterthe mother liquor [46,47]. on a dry basis. This amount of water is typically found in a In addition, non-ionic surfactants have been shown to filter cake from the co-precipitation or slurry methods. Thereduce cyclodextrin complexation of diazepam and preser- guest and cyclodextrin are thoroughly mixed and placed in
  9. 9. E.M.M. Del Valle / Process Biochemistry xxx (2003) xxx–xxx 9a sealed container. The sealed container and its contents are labile guests and soluble complexes such as hydroxypropy-heated to about 100 ◦ C and then the contents are removed lated cyclodextrin complexes.and dried. The amount of water added, the degree of mixingand the heating time have to be optimised for each guest. 2.6. Release2.4.5. Extrusion Once a complex is formed and dried, it is very stable, Extrusion is a variation of the heating and mixing method exhibiting long shelf life at ambient temperatures under dryand is a continuous system. Cyclodextrin, guest and water conditions. Displacement of the complexed guest by anothercan be premixed or mixed as added to the extruder. Degree guest requires heating. In many cases, water can replace theof mixing, amount of heating and time can be controlled guest.in the barrel of the extruder. Depending upon the amount When a complex is placed in water, two steps are in-of water, the extruded complex may dry as it cools or the volved in the release of the complexed guest. First, thecomplex may be placed in an oven to dry. complex is dissolved. The second step is the release of the Extrusion has the advantages of being a continuous complexed guest when displaced by water molecules. Anprocess and using very little water. Because of the heat equilibrium will be established between free and complexedgenerated, some heat-labile guests decompose using this cyclodextrin, the guest and the dissolved and undissolvedmethod. complex. In the case of complexes containing multiple guest com-2.4.6. Dry mixing ponents or cyclodextrin types, guest molecules are not neces- Some guests can be complexed by simply adding guest sarily released in the same proportion as in the original guestto the cyclodextrin and mixing them together. This works mixture. Each guest complex may have different solubilitybest with oils or liquid guests. The amount of mixing time and rate of release from the complex. If release rates arerequired is variable and depends on the guest. Generally, different for each component, it is possible to obtain an in-this method is performed at ambient temperature and is a tended release pattern by alteration of the guest formulation.variation on the paste method. The main advantage is that no water need be added, un- 2.7. Applications of cyclodextrinsless a washing step is used. Its disadvantages are the riskof caking on scale-up, resulting in mixing not being suffi- Since each guest molecule is individually surrounded by aciently thorough leading to incomplete complexation, and, cyclodextrin (derivative) the molecule is micro-encapsulatedwith many guests, the length of time required. from a microscopical point of view. This can lead to advan- tageous changes in the chemical and physical properties of2.5. Drying of complexes the guest molecules. • Stabilisation of light- or oxygen-sensitive substances. The complexes can be dried in an oven, fluid bed dryer • Modification of the chemical reactivity of guest molecules.or other dryer. Care has to be taken that the complex is not • Fixation of very volatile substances.destroyed during the drying process. • Improvement of solubility of substances. • Modification of liquid substances to powders.2.5.1. Highly volatile guests • Protection against degradation of substances by micro- For guests with boiling temperatures below 100 ◦ C, a organisms.lower temperature must be used during drying. Less guest • Masking of ill smell and taste.will be lost during drying when reducing the drying tem- • Masking pigments or the colour of substances.perature a few degrees below the boiling temperature of the • Catalytic activity of cyclodextrins with guest molecules.guest. These characteristics of cyclodextrins or their derivatives2.5.2. Spray drying make them suitable for applications in analytical chemistry, Complexes can also be spray-dried. Precipitation must be agriculture, the pharmaceutical field, in food and toilet arti-controlled in order to avoid the particles becoming too large cles [51].and blocking the atomiser or spray nozzle. With volatileguests, some optimisation of drying conditions is required 2.8. Cosmetics, personal care and toiletryin order to reduce the losses. Spray drying is not a viablemeans for drying highly volatile and heat-labile guests. Cosmetic preparation is another area which demands cy- clodexytrin use, mainly in volatily suppression of perfumes,2.5.3. Low temperature drying room fresheners and detergents by controlled release of A desiccator or freeze dryer may be used to dry com- fragrances from inclusion compounds.plexes. The low temperature minimises the loss of extremely The major benefits of cyclodextrins in this sector arevolatile guests. Freeze-drying is especially useful for heat stabilisation, odour control and process improvement upon
  10. 10. 10 E.M.M. Del Valle / Process Biochemistry xxx (2003) xxx–xxxconversion of a liquid ingredient to a solid form. Applica- were reported to have a texture-improving effect on pastrytions include toothpaste, skin creams, liquid and solid fabric and on meat products. Other applications arise from theirsofteners, paper towels, tissues and underarm shields. ability to reduce bitterness, ill smell and taste and to sta- The interaction of the guest with CDs produces a higher bilise flavours when subjected to long-term storage. Emul-energy barrier to overcome to volatilise, thus producing sions like mayonnaise, margarine or butter creams can belong-lasting fragrances [52]. Fragrance is enclosed with the stabilised with ␣-cyclodextrin. Using ␤-cyclodextrin may beCD and the resulting inclusion compound is complexed with removed cholesterol from milk; to produce dairy productscalcium phosphate to stabilise the fragrance in manufactur- low in cholesterol [3,30].ing bathing preparations [53]. Holland et al. [27] prepared Cyclodextrins act as molecular encapsulants, protectingcosmetic compositions containing CDs to create long-lasting the flavour throughout many rigorous food-processing meth-fragrances. CD-based compositions are also used in various ods of freezing, thawing and microwaving. ␤-CD as a molec-cosmetic products to reduce body odours [54]. The major ular encapsulant allows the flavour quality and quantity to bebenefits of CDs in this sector are stabilisation, odour control, preserved to a greater extent and longer period compared toprocess improvement upon conversion of a liquid ingredient other encapsulants and provides longevity to the food itemto a solid form, flavour protection and flavour delivery in [21]. In Japan, cyclodextrins have been approved as ‘modi-lipsticks, water solubility and enhanced thermal stability of fied starch’ for food applications for more than two decades,oils [7]. Some of the other applications include use in tooth- serving to mask odours in fresh food and to stabilise fishpaste, skin creams, liquid and solid fabric softeners, paper oils. One or two European countries, for example Hungary,towels, tissues and underarm shields [3]. have approved ␥-cyclodextrin for use in certain applications The use of CD-complexed fragrances in skin preparations because of its low toxicity.such as talcum powder stabilises the fragrance against the The complexation of CDs with sweetening agents suchloss by evaporation and oxidation over a long period. The as aspartame stabilises and improves the taste. It also elimi-antimicrobial efficacy of the product is also improved [30]. nates the bitter aftertaste of other sweeteners such as stevio- Dry CD powders of size less than 12 mm are used for side, glycyrrhizin and rubusoside. CD itself is a promisingodour control in diapers, menstrual products, paper towels, new sweetener. Enhancement of flavour by CDs has beenetc. and are also used in hair care preparations for the reduc- also claimed for alcoholic beverages such as whisky and beertion of volatility of odorous mercaptans. The hydoxypropyl [60]. The bitterness of citrus fruit juices is a major problem␤-cyclodextrin surfactant, either alone or in combination in the industry caused by the presence of limonoids (mainlywith other ingredients, provides improved antimicrobial limonin) and flavanoids (mainly naringin). Cross-linked cy-activity [55]. clodextrin polymers are useful to remove these bitter com- Dishwashing and laundry detergent compositions with ponents by inclusion complexes.CDs can mask odours in washed items [56,57]. The most prevalent use of CD in process aids is the re- CDs used in silica-based toothpastes increase the avail- moval of cholesterol from animal products such as eggs,ability of triclosan (an antimicrobial) by cyclodextrin dairy products. CD-treated material shows 80% removalcomplexation and resulting in an almost threefold enhance- of cholesterol. Free fatty acids can also be removed fromment of triclosan availability [58]. CDs are used in the fats using CDs, thus improving the frying property of fatpreparation of sunscreen lotions in 1:1 proportion (sun- (e.g. reduced smoke formation, less foaming, less brown-screen/hydroxypropyl ␤-CD) as the CD’s cavity limits the ing and deposition of oil residues on surfaces) [30]. Fruitsinteraction between the UV filter and the skin, reducing the and vegetable juices are also treated with CD to removeside effects of the formulation. Similarly, by incorporating phenolic compounds, which cause enzymatic browning. InCD in self-tanning emulsions or creams, the performance juices, polyphenol-oxidase converts the colourless polyphe-and shelf life are improved. An added bonus is that the nols to colour compounds and addition of CDs removestan looks more natural than the yellow and reddish tinge polyphenoloxidase from juices by complexation. Sojo et al.produced by traditional dihydroxyacetone products [59]. [61] studied the effect of cyclodextrins on the oxidation of o-diphenol by banana polyphenol oxidase and found2.9. Foods and flavours that cyclodextrins act as activator as well as inhibitor. By combining 1–4% CD with chopped ginger root, Sung [62] Cyclodextrins are used in food formulations for flavour established that it can be stored in a vacuum at cold tem-protection or flavour delivery. They form inclusion com- perature for 4 weeks or longer without browning or rotting.plexes with a variety of molecules including fats, flavours Flavonoids and terpenoids are good for human health be-and colours. Most natural and artificial flavours are volatile cause of their antioxidative and antimicrobial properties butoils or liquids and complexation with cyclodextrins provides they cannot be utilised as foodstuffs owing to their verya promising alternative to the conventional encapsulation low aqueous solubility and bitter taste. Sumiyoshi [63]technologies used for flavour protection. Cyclodextrins are discussed the improvement of the properties of these plantalso used as process aids, for example, to remove cholesterol components (flavanoids and terpenoids) with cyclodextrinfrom products such as milk, butter and eggs. Cyclodextrins complexation. CDs are used in the preparation of foodstuffs
  11. 11. E.M.M. Del Valle / Process Biochemistry xxx (2003) xxx–xxx 11in different ways. For example, highly branched CDs are Inclusion complexes can also facilitate the handling ofused in flour-based items like noodles, pie doughs, pizza volatile products. This can lead to a different way of drugsheets and rice cakes to impart elasticity and flexibility to administering, e.g. in the form of tablets. Cyclodextrins aredough [25]. They are also used in the preparation of an- used to improve the stability of substances to increase theirtimicrobial food preservatives containing trans-2-hexanalin resistance to hydrolysis, oxidation, heat, light and metalin apple juice preparation [64] and in the processing of salts. The inclusion of irritating products in cyclodextrinsmedicinal mushrooms for the preparation of crude drugs can also protect the gastric mucosa for the oral route, andand health foods. CDs are used in the preparation of con- reduce skin damage for the dermal route.trolled release powdered flavours and confectionery items Furthermore, cyclodextrins can be applied to reduce theand are also used in chewing gum to retain flavour for effects of bitter or irritant tasting and bad smelling drugslonger duration, a property highly valued by customers [6]. [3,30,73,74]. Administered cyclodextrins are quite resistant to starch2.10. Pharmaceuticals degrading enzymes, although they can be degraded at very low rates by ␣-amylases. ␣-Cyclodextrin is the slowest, A drug substance has to have a certain level of water sol- and ␥-cyclodextrin is the fastest degradable compound,ubility to be readily delivered to the cellular membrane, but due to their differences in size and flexibility. Degrada-it needs to be hydrophobic enough to cross the membrane. tion is not performed by saliva or pancreas amylases, but One of the unique properties of cyclodextrins is their abil- by ␣-amylases from microorganisms from the colon flora.ity to enhance drug delivery through biological membranes. Adsorption studies revealed that only 2–4% of cyclodex-The cyclodextrin molecules are relatively large (molecular trins were adsorbed in the small intestines, and that theweight ranging from almost 1000 to over 1500), with a hy- remainder is degraded and taken up as glucose. This candrated outer surface, and under normal conditions, cyclodex- explain the low toxicity found upon oral administration oftrin molecules will only permeate biological membranes cyclodextrins [14].with considerable difficulty [65,66]. It is generally recog-nised that cyclodextrins act as true carriers by keeping the 2.11. Agricultural and chemical industrieshydrophobic drug molecules in solution and delivering themto the surface of the biological membrane, e.g. skin, mucosa Cyclodextrins form complexes with a wide variety ofor the eye cornea, where they partition into the membrane. agricultural chemicals including herbicides, insecticides,The relatively lipophilic membrane has a low affinity for fungicides, repellents, pheromones and growth regulators.the hydrophilic cyclodextrin molecules and therefore, they Cyclodextrins can be applied to delay germination of seed.remain in the aqueous membrane exterior, e.g. the aqueous In grain treated with ␤-cyclodextrins some of the amylasesvehicle system (such as o/w cream or hydrogel), salvia or that degrade the starch supplies of the seeds are inhibited.the tear fluid. Conventional penetration enhancers, such Initially the plant grows more slowly, but later on this isas alcohols and fatty acids, disrupt the lipid layers of the largely compensated by an improved plant growth yielding abiological barrier. Cyclodextrins, on the other hand, act as 20–45% larger harvest [3]. Recent developments involve thepenetration enhancers by increasing drug availability at the expression of cyclodextrin glucanotransferases (CGTases) insurface of the biological barrier. For example, cyclodextrins plants [3,30].have been used successfully in aqueous dermal formulations In the chemical industry, cyclodextrins are widely used[67], an aqueous mouthwash solution [68], nasal drug de- to separate isomers and enantiomers, to catalyse reactions,livery systems [69] and several eye drop solutions [70–72]. to aid in various processes and to remove or detoxify waste The majority of pharmaceutical active agents do not have materials.sufficient solubility in water and traditional formulation sys- Cyclodextrins are widely used in the separation oftems for insoluble drugs involve a combination of organic enantiomers by high performance liquid chromatographysolvents, surfactants, and extreme pH conditions, which (HPLC) or gas chromatography (GC). The stationary phasesoften cause irritation or other adverse reactions. Cyclodex- of these columns contain immobilised cyclodextrins ortrins are not irritant and offer distinct advantages such as derived supra molecular architectures. Other analytical ap-the stabilisation of active compounds, reduction in volatility plications can be found in spectroscopic analysis. In nuclearof drug molecules, and masking of malodours and bitter magnetic resonance (NMR) studies they can act as chiraltastes. shift agents and in Circular Dichroism as selective (chiral) There are numerous applications for cyclodextrins in agents altering spectra. In electrochemical chemistry theythe pharmaceuticals field. For example, the addition of ␣- can be used to mask contaminating compounds, allowingor ␤-cyclodextrin increases the water solubility of several more accurate determinations [3].poorly water-soluble substances. In some cases this results One novel use of CDs in catalytic reactions is their abilityin improved bioavailability, increasing the pharmacolog- to serve as enzyme mimics. These are formed by modifyingical effect allowing a reduction in the dose of the drug naturally occurring CDs through substituting various func-administered. tional compounds on the primary or secondary face of the
  12. 12. 12 E.M.M. Del Valle / Process Biochemistry xxx (2003) xxx–xxxmolecule or by attaching reactive groups. These modified determining bioavailability of pollutants using CD and itsCDs are useful as enzyme mimics because of the molecular derivatives. CD complexation also resulted in the increaserecognition phenomenon [3] attributed to the substituted of water solubility of three benzimidazole-type fungicidesgroups on the CD. This ability results from binding of (thiabendazole, carbendazim and fuberidazole) makingsubstrates in the hydrophobic cavity with the subsequent re- them more available to soil. In addition to its ability toaction initiated by catalytic groups linked to the CD. Rates increase the solubility of the hydrocarbon for biodegrada-of reaction are enhanced by almost 1000-fold by such mod- tion and bioremediation, CDs also decrease the toxicityified CDs versus free solution due to the chelating effect resulting in an increase in microbial and plant growth.of the CD catalysts. The enantiomeric specificity of CDs in ␤-Cyclodextrins accelerated the degradation of all types ofsuch applications also promises to be a significant attribute hydrocarbons influencing the growth kinetics, producing[1]. The first chymotrypsin mimic was produced by [75] higher biomass yield and better utilisation of hydrocarbonmodifying ␤-CD, which enhanced the rates of hydrolysis of as a carbon and energy source. The low cost, biocompatibleactivated esters and formation of amine bonds by 3.4-folds and effective degradation makes ␤-cyclodextrins a useful[76] modified ␤-CD for the purpose of catalysis and used tool for bioremediation process [83].it for the selective hydroxy-ethylation and hydroxymethy-lation of phenol. They observed that chemical modification 2.12. Adhesives, coatings and other polymersgreatly promoted the catalytic activity, and the resultingCD derivative served as a transamine mimic, catalysing the Cyclodextrins increase the tackiness and adhesion ofconversion of phenylpyruvic acid to phenylalanine. Atwood some hot melts and adhesives. They also make additives[77] explained the use of modified ␣-cyclodextrin in the and blowing agents compatible with hot melt systems.reduction of Mn(III) porphyrin. The interaction between polymer molecules in associative Due to their steric effects, CDs also play a significant thickening emulsion-type coatings such as paints tends torole in biocatalytic processes by increasing the enantios- increase viscosity, and CDS can be used to counteract thiselectivity. After the formation of inclusion complex with undesirable effect.the prochiral guest molecule, the preferential attack by thereagent takes place only from one of the enantioselectivefaces, resulting in higher enantioselectivity. It was reported 3. Conclusionsby Kamal et al. [78] that the hydrolysis of racemic arylpro-pionic esters by bovine serum albumin, a carrier protein, Many types of encapsulation are available which coat aresulted in low enantioselectivity (50–81% ee), while addi- fine particle of an active core with an outer shell. Encapsu-tion of ␤-CD to this reaction not only enhanced the enan- lation also can occur on a molecular level. This can be ac-tioselectivity (80–99% ee) but also accelerated the rate of complished using a category of carbohydrates called CDs,hydrolysis. Rao et al. [79] demonstrated that chiral recogni- encapsulates made with these molecules may possibly holdtion during cycloaddition reaction of nitriloxides or amines the key for many future encapsulated formulation solutions.to the C≡C triple bond using baker’s yeast as a chiral cat- The ability of cyclodextrins to form inclusion complexesalyst was improved by the addition of CDs, increasing the with many guest molecules by taking up a whole molecule,enantioselectivity of yeast by up to 70%. or some part of it, into the cavity place cyclodextrins is a Cyclodextrins can play a major role in environmental unique class of encapsulation technique. This type of molec-science in terms of solubilisation of organic contaminants, ular encapsulation will affect many of the physicochemicalenrichment and removal of organic pollutants and heavy properties of the guest molecules. The ability of cyclodex-metals from soil, water and atmosphere [80]. CDs are also trins to form complexes with a wide variety of organic com-applied in water treatment to increase the stabilising action, pounds helps to alter the apparent solubility of the molecule,encapsulation and adsorption of contaminants [81]. Using to increase the stability of compound in the presence of light,cyclodextrins, highly toxic substances can be removed from heat and oxidising conditions and to decrease volatility ofindustrial effluent by inclusion complex formation. In the compound. These properties have resulted in the growingmother liquor of the insecticide trichlorfon, the uncrystallis- importance of the applications of cyclodextrins in food, phar-able trichlorfon can be converted into a ␤-CD complex and maceutical, agriculture and chromatographic techniques.in a single treatment 90% of the toxic material is removed The versatility of cyclodextrins and modified cyclodex-[3,30]. Wastewaters containing environmentally unaccept- trins is demonstrated in their range of applications fromable aromatic compounds such as phenol, p-chlorophenol cosmetics and food to drugs. Recent biotechnological ad-and benzene after treating with ␤-CD have considerably vancements have resulted in dramatic improvements in thereduced levels of these aromatic hydrocarbons from their efficient manufacture of cyclodextins lowering the cost ofinitial levels. Cyclodextrins are used to scrub gaseous ef- these materials making highly purified cyclodextyrins andfluent from organic chemical industries [3,30]. Solubility cyclodextrins derivates available.enhancement phenomenon of CDs is used for testing of In conclusion, due to the unique architecture and thesoil remediation. Reid et al. [82] discussed the soil test for chelating properties, cyclodextrins are becoming an impor-
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