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    Chiral Chiral Presentation Transcript

    • CHIRAL SEPARATIONS Separations of optical isomers requires interaction with A chiral selector. Such a chiral selector can be present In the stationary phase or in the mobile phase. The preferred appraoch is one in which the chiral selector Is bound to a solid stationary phase in a solid-liquid System or distributed into a liquid in a liquid-liquid System. The approaches to chiral separation are: 1. Ligand exchange – mobile phase additive; amino acids 2. Inclusion complex formation with cyclodextrins. 3. Interaction with a chiral selector based on 3 specific Interactions on of which is strong and the other two are Weak – 3 point contact model based on drug-receptor Interaction. 4. Molecular imprinting – lock and key approach
    • LIGAND EXCHANGE Introduced by V. Davankov in the late 1960’s. Typically Performed on a C18 column. The principle is: CuL + D (or R)  CuD + L; (KD) CuL + L (or S)  CuL + L; (KL) The separation depends on the differences between KD and KL and the differences in the capacity factors for The complexes CuD and CuL between the mobile phase And C18 stationary phase. The difference in the Capacity factors is bound to be small. The chiral Recognition occurs through differences in KD and KL . This method does not yield pure analytes after
    • LIGAND EXCHANGE (CONTD) Chiral Center L-proline is typically used as the exchanging ligand
    • THREE POINT CONTACT MODEL A B C C B A A' B' C' A' C' B' Enantiomer Enantiomer Chiral Recognition Molecule
    • MODES OF INTERACTION 1. π-π interaction. 2. Ion pair formation 3. Hydrogen bonding 4. Charge transfer interaction 5. Long range forces (van der Waal’s forces) 6. Steric interactions
    • MACROCYLIC ANTIBIOTICS Teicoplanin Other antibiotics used: vancomycin
    • CHAPTER 29 SUPERCITICAL FLUID CHROMATOGRAPHY Problems assigned: All problems at the end of the chapter (29-1 to 29-10).
    • SUPERCITICAL FLUID Triple point is the temperature and pressure at which the three phases solid, liquid, and gas coexist. Supercritical point corresponds to the temperature And pressure at which the distinction between liquid and Gas does not exist. Super critical temperature is the temperature above which a distinct liquid phase does not exist. Vapor Pressure of a substance at its critical temperature is its Critical pressure. At temperatures and pressures above the critical point The substance is called a supercritical fluid.
    • PROPERTIES OF SUPECRITICAL FLUIDS Properties of supercitical fluids are inermediate Between its properties in the gaseous and liquid states. Supercritical fluids have much lower surface tension han their liquid form. This allows them spread across urfaces very easily. They maintain their ability to dissolve a wide range f substances as in their liquid state. Analytes dissolved in supercirtical fluids are easily ecovered by lowering the pressure at relatively low temp.
    • PROPERTIES OF SUPECRITICAL FLUIDS (CONTD) The supercritical pressure of CO2 at 32 C is 1070 psi and At 49 C is 3500 psi. These pressures are well within The pressures under which HPLC can be operated. Supercritical CO2 can dissolve a variety of compounds Such as n-alkanes (5 –30 C atoms), aromatics including Polycyclic compounds, and di-n-alkylphthalates with
    • SUPERCRITICAL FLUID CHROMATOGRAPHY SFC is a hybrid of gas and liquid chromatography. It is Useful for the separation of compounds that are not Easily handled in gas or liquid chromatography.It Is applicable for the following general cases: 1. Nonvolatile and thermally labile compounds that makes Gas chromatography unsuitable. 2. Poor solubility in solvents employed in liquid Chromatography. 3. Compounds do not have functional groups that makes Spectroscopic and electrochemical detection possible.
    • SUPERCRITICAL FLUID CHROMATOGRAPHY (CONTD.) The column must be in a thermostated oven to achieve The critical temperature. A restrictor or back pressure Device is used to maintain the pressure at or above the Critical pressure. Analytes are detected by flame ionization.
    • SUPERCRITICAL FLUID CHROMATOGRAPHY (CONTD.) Column: Open tubular and packed columns are employed. Open tubular columns are fused silica columns with Internal coatings of bonded and cross-linked siloxane Polymers of various types. Packed columns contain various packing materials such as silica and polymeric beads. Column length = 10 – 20 m; inner diameter = 0.05 – 0.1 mm; Film thickness for open tubular columns = 0.05 – 0.1 µm. Packed columns contain particles of size 3 – 10 µm. Column inner diameter can be 0.5 – 4.6 mm. Particles can be normal or reverse phase or polymers Such as polystyrene as in HPLC.
    • SUPERCRITICAL FLUID CHROMATOGRAPHY (CONTD.) Pressure changes in SFC have a pronounced effect on Retention and hence capacity factor. This is consequence Of change in density with pressure. Density increases With pressure. Increase in density lowers retnetion As the analytes dissolve better in the mobile phase And distribute less into the stationary phase. The van Deemter plot for SFC indicates that for a given Linear mobile phase velocity the HETP in SFC is about A factor of 3 smaller than in HPLC.
    • SUPERCRITICAL FLUID CHROMATOGRAPHY (CONTD.) When a molecule dissolves in a supercritical fluid, the Process resembles volatilization at a much lower Temperature than would be required for this compound In GC. At a given temperature the vapor pressure for a large Molecule in a supercritical fluid may be 1010 times Greater than its vapor pressure in the absence of the Fluid. As a result high molecular weight compounds Such as biomolecules and thermally unstable compounds Can be separated at low temperatures using Supercritical fluid chromatography.
    • SUPERCRITICAL FLUID CHROMATOGRAPHY (CONTD.) The analyte and the supercritical fluid must interact for It to dissolve in the supercritical fluid. The fundamental Thermodynamic parameter that helps to determine If a given molecule will dissolve in a supercritical fluid Is its fugacity. The limitation of supercritical chromatography lies in the Ability of the supercritical fluid to solubilize analyte Molecules. Modifiers such as methanol and other solvents Can be used to enhance the solubility of analytes in Supercritical fluids. Most commonly employed supercritical fluid is CO2. The affinity of a compound for supercritical CO2 is Termed is CO2philicity.
    • EFFECT OF MODIFIERS ON SOLUBILITY IN SUPERCRITICAL CO2 Diuron – phenyl Derivative of urea TCDD – 2,3,7,8- Tetrachlorodibenzo -p-dioxin LAS – linear Alkylbezenesulfonat Detergent.