1.2 Classification of Complexation
1.3 Applications, Methods of Analysis
1.4 Protein Binding
1.5 Complexation and the drug actions
1.6 Crystalline Structures of Complexes and Thermodynamic Treatment of Stability Constants.
1. PHYSICAL PHARMACEUTICS-I
COMPLEXATION AND PROTEIN BINDING.
1.2 CLASSIFICATION OF COMPLEXATION
1.3 APPLICATIONS, METHODS OF ANALYSIS
1.4 PROTEIN BINDING
1.5 COMPLEXATION AND DRUG ACTIONS,
1.6 CRYSTALLINE STRUCTURES OF COMPLEXES AND THERMODYNAMIC
TREATMENT OF STABILITY CONSTANTS.
2. 1.1 INTRODUCTION
COMPLEXATION – Complexation is defined as the association
of two of more species capable of independent existence.
• Complexes result from a DONOR-ACCETOR MECHANISM or a
LEWIS ACID- BASE REACTION between 2 or more different chemical
constituents, forming co-ordination compounds.
• In this the DONOR Compound is a NON-METALLIC ATOM/ION which can
donate an electron pair and an ACCEPTOR is a METTALIC ION/NEUTRAL
ATOM which is capable of accepting a pair of electrons.
3. 1.2 CLASSIFICATION OF COMPLEXES
(I) Metal Ion Complexes (II) Organic Molecular Complexes (III) No-Bond
- Inorganic type - Quinhydrone type - Clathrate
- Chelates - Picric Acid Type - Channel Lattice
- Olefin Type - Caffeine Type & Other Drug Complexes - Layer Type
- Aromatic Type - Polymer Type - Monomolecular
Pi-bond type - Macromolecular
4. (I) METAL ION COMPLEXES
a. Inorganic Type: In this type of complex the central atom(ACCEPTOR)in the
complex is a metal/metal ion which accepts electron from the donor.
Donor compound is also known as LIGAND coordinated with the acceptor molecule
Example of inorganic type complexes is Hexamine Cobalt III Chloride [Co(NH3)6]Cl is
formed due to reaction between ammonia and cobalt chloride.
The coordination no. of Cobalt ion = 6 (i.e. no. of ammonia groups coordinated with
5. b. Chelates: A substance containing 2 or more donor groups may combine with a metal
ion to form a complex is known as a chelate.
Ionic/Primary covalent type
Ligands may have more than one group capable of bonding with the metal ion. A molecule
with 2 DONOR Groups is k/a BIDENTATE
A molecule with 3 DONOR Groups is k/a TRIDENTATE
Example of chelate/chelating agent is Ethylenediamine Tetra acetic Acid (EDTA). It has 6 points
for attachment for the metal ion (2N &4 O2) and is therefore called HEXADENTATE. Both
metal & ligand molecules or complexes may exhibit isomerism, due to this only cis-coordinated
ligands are readily replaced by reaction with chelating agents.
6. Vitamin B12 and Haemoproteins, the trans coordination positions of the metal are
available & they are not able to react with chelating agents.
Another example of chelates involved in life processes of plants and animals are
Chlorophyll and Haemoglobin are 2 important naturally occurring compounds.
Albumin is the main carrier of various metal ions & small molecules in blood serum.
c. Olefin Complexes : Aqueous solution of certain metal ion such as Platinum, Iron,
Palladium, Mercury and Silver can absorb olefins such as ethylene to yield water
soluble complexes. Example: Silver-olefin complex.
7. d. Aromatic Complexes:
Pi-bond complexes: Aromatic bases such as Benzene, toluene, xylene form pi-bond
complexes with metal ion such as silver by Lewis acid-base reactions.
Stability of complex increases with increasing strength of aromatic hydrocarbon.
-Iodine forms pi-bond complex with benzene to give RED-COLOURED SOLUTION,
whereas no such complexation takes place when iodine reacts with chloroform and
CCL4 (Carbon Tetrachloride) to give VIOLET-COLOURED SOLUTION.
-Toluene forms a pi-bond complex with HCl.
Pi-bond complexes between toluene and HCl
8. Sigma-bond complexes:
These complexes involve the formation of a sigma bond between an
ion & a carbon of the aromatic ring.
-These complexes are very reactive and difficult to isolate.
-Toluene forms a sigma-bond complex with a catalyst couple
Sigma-bond complexes of toluene with HCl.AlCl3
9. Sandwich compounds: These are STABLE COMPLEXES involving a delocalized
covalent bond between the d-orbital of a transition metal and a molecular
orbital of the aromatic ring.
An example of sandwich complexes is Ferrocene or Bisdicyclopentadienyl iron II
Here 1 Pi-electron of each ring is used to bind with the metal atom.
Ferrocene exhibits an aromatic character, such compounds are known as
sandwich compounds, because of the layered structure of ring-metal complexes.
10. (II) ORGANIC MOLECULAR COMPLEXES.
These are formed by union of 2 organic molecules held together, either by
electrostatic force or hydrogen bonding.
a. Quinhydrone Complex: Formed by mixing of alcoholic solutions of
equimolar quantities of benzoquinone & hydroquinone, when green crystals
of crystals of quinhydrone complex settle done.
11. b. Picric acid type complexes: Picric acid (2,4,6 Trinitrophenol) forms
complexes with many polynuclear aromatic compounds.
Stability increases with increased No. of EWG (Electron Withdrawing Groups) on
the nitro group and ring complexity.
Stability increases with increases presence of EDG (Electron Donating Groups) on
the second compound.
Example of Picric Acid Complex is Butesin Picrate, a local anaesthetic.
12. C. Hydrogen Bonded Complexes: A large no. of compounds containing the
-OH and -NH- linkage exhibit hydrogen bonding. Hydrogen bonding is an example
of Dipole-Dipole Interaction. In this, one molecules positive hydron (H+) are
attracted to the negative oxygen atom of a second molecule. Complex formation in
such compounds occurs only if INTERMOLECULAR HYDROGEN BONDS are formed.
Caffeine and other drug Complexes – The most extensively studied example of
hydrogen bond complexes is that of caffeine compounds. Caffeine forms complexes
with a no. of drugs such as benzocaine, tetracaine or procaine and it enhances the
stability & appearance of pharmaceutical preparations of these drugs.
13. d. Polymer Type Complexes:
Polymeric materials such as PEG (Polyethylene glycols), Polystyrene,
Polyvinylpyrrolidone and sodium carbomethyl cellulose which are
usually present in suspensions, emulsions, suppositories and some
solid dosage forms, can form complexes with large no. of drugs.
Such interactions can result in precipitation, flocculation,
solubilisation, alteration in bioavailability, undesirable physical,
chemical and pharmacological effects.
14. (III) INCLUSION COMPOUNDS (NO BOND
These complexes are formed due to the ability of one of the constituents of the
complex to get entrapped in the open lattice or cage-like crystals structure of the
There are no adhesive force acting between their constituent molecules. Therefore
they are known as NO-BOND.
a. Clathrates- A molecules of a ‘guest’ compounds gets entrapped within the cage
like structure formed by the association/union of several molecules of a ‘host’
compound. The size of guest molecule is very important for complex formation.
If the size is too small, it will escape from the cage-lie structure of the host and
if the size of guest molecule is too big, it will not be accommodated inside the
15. For Example: Hydroquinone crystallizes in cage like structure (hydrogen bonded)
leaving holes of diameter 4.2 Armstrong. This permits the entrapment of
molecules such as methyl alcohol, HCl and CO2 but SMALLER molecules such as
H2 & LARGER molecules such as ethanol cannot be entrapped.
b. Channel Lattice Complexes:
In this the host component crystallizes to form a channel-like structure into
which the guest molecule can fit. Channel lattice complexes offers a
number of applications, in separation of petroleum products.
Examples: Starch iodine solution is a channel lattice type complex
consisting of iodine molecules entrapped within the spirals of starch
16. C. Layer-type Complexes: In this complex the guest molecule is diffused
between the layers of carbon atom, hexagonally oriented to form alternate
layer of guest & host molecules. Example: Clay, Graphite.
D. Monomolecular type compounds: These compounds involve the
entrapment of one guest molecule into the cage like structure formed from a
single host molecule.
Example: Cyclodextrin molecule represents a monomolecular host structure into
which a no. of guest molecules can get entrapped.
E. Macromolecular Inclusion compounds: These are commonly k/a Synthetic
Zeolites, dextrin, silica gels and related substances. The atoms in these are
arranged n 3D to provide cages and channels and the guest molecule are
17. 1.3 APPLICATIONS & METHODS OF ANALYSIS
Applications of Complexation:
a. Physical state- Complexation process improves processing characteristics by converting
liquid into solid complex, 𝛽-cyclodextrin complexes with nitro-glycerine.
b. Volatility- Complexation process reduces drug volatility for following benefits: Stabilize
system & Overcome unpleasant odour (iodine complexes with PVP Polyvinyl Pyrrolidone)
c. Solid state stability- Complexation process enhances the solid state stability of drug. β-
cyclodextrin complexes with Vitamin A & D
d. Chemical stability- Complex formation inhibit chemical reactivity (mostly inhibit). The
hydrolysis of Benzocaine is decreased by complexing with caffeine.
e. Solubility- Enhances solubility of drug. Caffeine enhances solubility of PABA (Para Amino
Benzoic Acid) by complex formation.
f. Dissolution- Enhances dissolution of drug. 𝛽 -cyclodextrin increases dissolution of
phenobarbitone by inclusion complex.
18. g. Partition Co-efficient: Complexation process enhances the Partition Co-efficient of
certain drugs. Permanganate ion with benzene.
h. Absorption & Bioavailability: Complexation process reduces the absorption of
tetracycline by complexing cations like Ca2+, Mg2+ and Al3+. Enhances the
absorption of indomethacin and barbiturates by complexing with 𝛽-cyclodextrin.
i.Reduced toxicity: 𝛽-cyclodextrin reduces ulcerogenic effects of indomethacin. 𝛽-
cyclodextrin reduces local tissue toxicity of chlorpromazine.
j. Antidote for metal poisoning: BAL (British Anti Lewisite) reduces toxicity of heavy
metals by complexing with gold, mercury and antimony.
k. Drug acting through metal poisoning: 8-Hydroxy quinoline complexes with Fe
exhibit greater antimalarial activity.
l. Anti-tubercular activity- PAS (Para Amino Salicylic Acid) complexes with Cupric ion
exhibit greater antitubercular activity.
19. ASSAY OF DRUGS/METHODS OF ANALYSIS
1. Dielectric constant, Referactive index, Spectrophotometric extinction coefficient-
When there is complexation between the species, the value property is ADDITIVE.
On complexation these properties CHANGE but additive rule do not hold good.
The change in characteristics proves that the complexation has taken place.
2. pH Titration Method- This method is applicable for that complex that produces the
change in pH will determine that complexation has been taken place.
3. Distribution method- the distribution behaviour of a solute between two immiscible
liquid is expressed by distribution coefficient or partition coefficient. When a solute
complexes with an added substance, the solute distribution pattern changes
depending on the nature of complex.
20. 4. Solubility method- When the mixture form complexes solubility may
5. Spectroscopy method- The UV Spectroscopy is used as extensively in
determining rate constant, equilibrium constant, acid-bbase dissociation constant
etc for chemical reaction.
6. Miscellaneous method- Several other methods are available for the analysis
of complexes like NMR and IR spectroscopy, Polarography, Circular dicromism,
kinetics, X-Ray diffraction and electron diffraction.
21. 1.4 PROTEIN BINDING
The binding of drug to protein in the body is called as protein binding.
The phenomenon of complex formation of drug with protein is called as protein
binding of drug. The interacting molecules are generally the macromolecules
such as proteins (Albumin, Globulins, ∝1 acid glycoproteins also called as AGP
OR lipoproteins) are present in blood, DNA, Adipose. These molecules have
been known to bind with the large number of drug molecules.
As a protein bound drug is neither metabolized nor excreted hence it is
pharmacologically INACTIVE due to its pharmacokinetic and pharmacodynamics
22. The protein binding alters the biological properties of drug molecules as free
drugs concentration is reduced the bound drug inherits the diffusional
characteristics of the protein molecules.
KINETICS OF PROTEIN BINDING
An equation relating reaction velocity to Drug Concentration (Mol/L) for a
system where a Drug D binds reversibility to any Protein (P) of to form a
This system can be represented schematically as follows:
Protein + Drug ==== Protein-Drug Complex
P + D ==== PD
23. Applying the law of mass action, the equilibrium or association constant (K) is:
K= [PD]/[P] [D]
25. The graph is plotted between 1/R versus 1/[D], called Klotz reciprocal plot,
gives a straight line whose slope is 1/VK and intercept is V.
26. 1.5 COMPLEXATION AND DRUG ACTION
Protein binding inactivates the drugs because sufficient concentration of drug
cannot be build up in the receptor sight for action. Ex. Naphthoquinone.
Only free drug participate in the drug action. Complexation can alter the
pharmacological action of drug by interfering the interaction with the receptor
the action of drug to remove the toxic effect of metal ion from the human
bodies is through the complexation reaction
It has been seen that in some instance complexation can also lead to poor
solubility or decreased absorption of drug in the body, which decreases the bio
availability of drug in blood. Thus, the drug action gets altered.
27. Drug complex with hydrophilic drug also enhance the drug elimination, thus
helps in drug action termination and reduction in drug toxic action.
• Tetracycline and Calcium – Poor absorbed complex
• Polar drug and complexing agent - Well absorbed lipid soluble complex
• Carboxy methyl cellulose and amphetamine - Poor absorbed complex
• PVP and Iodine – Better Absorption
28. 1.6 THERMODYNAMIC TREATMENT OF STABILITY
The relationship between the standard free energy change of complexation
and the overall stability constant K is related as;
∆𝐺 = −2.303𝑅𝑇 𝐿𝑜𝑔𝐾
The standard enthalpy change maybe obtained from the slope of a plot of
Log K versus 1/T, thus the equation will be ;
Log K=-(∆H/2.303R) × (1/T) + Constant
When the value of K at two temperatures are known , the following equation
can be written as;
Log (𝐾1/𝐾2) = -(∆H/2.303R) × (𝑇2-𝑇1/ 𝑇1𝑇2)
29. The Standard Entropy change maybe obtained from the expression
∆𝐺 = ∆H−T ∆S
• As the stability constant for molecular complexation increases, ∆H and ∆S
becomes more negative
• As binding between the donor and receptor becomes stronger , ∆H becomes
• Since the specificity of interacting sites become negative, ∆S also becomes
But extent of change in ∆H is large enough to overcome the un favourable
entropy change resulting in negative ∆G value and hence complexation