Chiral

CHIRAL SEPARATIONS

Separations of optical isomers requires interaction withA chiral selector. Such a chiral selector can be presentIn the stationary phase or in the mobile phase. Thepreferred appraoch is one in which the chiral selectorIs bound to a solid stationary phase in a solid-liquidSystem or distributed into a liquid in a liquid-liquidSystem.The approaches to chiral separation are:1. Ligand exchange – mobile phase additive; amino acids2. Inclusion complex formation with cyclodextrins.3. Interaction with a chiral selector based on 3 specificInteractions on of which is strong and the other two areWeak – 3 point contact model based on drug-receptorInteraction.4. Molecular imprinting – lock and key approach

LIGAND EXCHANGE

Introduced by V. Davankov in the late 1960’s. TypicallyPerformed 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 forThe complexes CuD and CuL between the mobile phaseAnd C18 stationary phase. The difference in the Capacity factors is bound to be small. The chiralRecognition occurs through differences in KD

and KL . This method does not yield pure analytes afterSeparation. It is a useful analytical tool.

LIGAND EXCHANGE (CONTD)

Chiral Center

L-proline is typically used as the exchanging ligand

INCLUSION COMPLEX FORMATION

CYCLODEXTRIN COLUMNS

CYCLODEXTRIN STATIONARY PHASES

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. - interaction.2. Ion pair formation3. Hydrogen bonding4. Charge transfer interaction5. Long range forces (van der Waal’s forces)6. Steric interactions

PIRKLE COLUMNS

MACROCYLIC ANTIBIOTICS

Teicoplanin

Other antibiotics used: vancomycin

CHAPTER 29SUPERCITICAL FLUID CHROMATOGRAPHY

Problems assigned: All problems at the endof the chapter (29-1 to 29-10).

SUPERCITICAL FLUID CHROMATOGRAPHY

Critical Temp.

Triple Point

SUPERCITICAL FLUIDTriple point is the temperature and pressure at which the three phases solid, liquid, and gas coexist.Supercritical point corresponds to the temperatureAnd pressure at which the distinction between liquid andGas 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 itsCritical pressure.

At temperatures and pressures above the critical pointThe substance is called a supercritical fluid.

PROPERTIES OF SUPECRITICAL FLUIDS

Properties of supercitical fluids are inermediateBetween its properties in the gaseous and liquid states.

1. Supercritical fluids have much lower surface tensionThan their liquid form. This allows them spread acrossSurfaces very easily.2. They maintain their ability to dissolve a wide rangeOf substances as in their liquid state.3. Analytes dissolved in supercirtical fluids are easilyRecovered by lowering the pressure at relatively low temp.

PROPERTIES OF SUPECRITICAL FLUIDS (CONTD)

The supercritical pressure of CO2 at 32 C is 1070 psi andAt 49 C is 3500 psi. These pressures are well withinThe pressures under which HPLC can be operated.Supercritical CO2 can dissolve a variety of compoundsSuch as n-alkanes (5 –30 C atoms), aromatics includingPolycyclic compounds, and di-n-alkylphthalates withAlkyl groups containing 4-16 C atoms.

SUPERCRITICAL FLUID CHROMATOGRAPHY

SFC is a hybrid of gas and liquid chromatography. It isUseful for the separation of compounds that are not Easily handled in gas or liquid chromatography.ItIs applicable for the following general cases:1. Nonvolatile and thermally labile compounds that makesGas chromatography unsuitable.2. Poor solubility in solvents employed in liquidChromatography.3. Compounds do not have functional groups that makesSpectroscopic and electrochemical detection possible.

SUPERCRITICAL FLUID CHROMATOGRAPHY (CONTD.)

The column must be in a thermostated oven to achieveThe critical temperature. A restrictor or back pressureDevice is used to maintain the pressure at or above theCritical pressure.Analytes are detected by flame ionization.


Column: Open tubular and packed columns are employed.Open tubular columns are fused silica columns with Internal coatings of bonded and cross-linked siloxanePolymers of various types. Packed columns contain various packing materialssuch 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 m.Packed columns contain particles of size 3 – 10 m.Column inner diameter can be 0.5 – 4.6 mm.Particles can be normal or reverse phase or polymersSuch as polystyrene as in HPLC.



Pressure changes in SFC have a pronounced effect onRetention and hence capacity factor. This is consequenceOf change in density with pressure. Density increasesWith pressure. Increase in density lowers retnetionAs the analytes dissolve better in the mobile phaseAnd distribute less into the stationary phase.

The van Deemter plot for SFC indicates that for a givenLinear mobile phase velocity the HETP in SFC is aboutA factor of 3 smaller than in HPLC.


When a molecule dissolves in a supercritical fluid, theProcess resembles volatilization at a much lowerTemperature than would be required for this compoundIn GC.At a given temperature the vapor pressure for a largeMolecule in a supercritical fluid may be 1010 timesGreater than its vapor pressure in the absence of theFluid. As a result high molecular weight compoundsSuch as biomolecules and thermally unstable compoundsCan be separated at low temperatures usingSupercritical fluid chromatography.


The analyte and the supercritical fluid must interact forIt to dissolve in the supercritical fluid. The fundamentalThermodynamic parameter that helps to determineIf a given molecule will dissolve in a supercritical fluidIs its fugacity.The limitation of supercritical chromatography lies in theAbility of the supercritical fluid to solubilize analyteMolecules. Modifiers such as methanol and other solventsCan 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 isTermed is CO2philicity.

EFFECT OF MODIFIERS ON SOLUBILITY IN SUPERCRITICAL CO2

Diuron – phenylDerivative of ureaTCDD – 2,3,7,8-Tetrachlorodibenzo-p-dioxinLAS – linear AlkylbezenesulfonateDetergent.

Education

Chiral