24 OPTICAL RESOLUTION OF D, L-AMINO ACID DERIVATIVES IN PEPTIDE SYNTHESIS-DESIGN OF LIQUID-SOLID CHROMATOGRAPHIC SYSTEM CONTAINING A CHIRAL STATIONARY PHASE
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The optical resolution of amino acid derivatives by the normal phase HPLC was performed using chemically bonded chiral silica and binary solvent systems. It is our aim to inquire the driving force based on hydrogen bonding association between the stationary surface and solutes concerning with the resolution of the enantiomers. Five grafted silicas were obtained by bonding of N-acyl-L-valines Me_2CHCH(COOH)NHCOR (R=H, Me, Et, n-Pr, n-Bu) onto 3-aminopropylsilanized (APS) silica. The most effective separation was obtained by the column which contains N-formyl-L-valylaminopropylsilanized (FVA) silica. Derivatization of amino acids was carried out by the acylation of amino group and esterification of carboxyl group. Effects of bulkiness of the alkyl moieties on each functional group on the chromatographic separation were examined carefully using several binary solvents consisted of n-hexane as the nonpolar component and 2-propanol, diethyl ether, dichloromethane, or chloroform as the stronger component. Highest values of the separation factor a were obtained with N-acetyl-O-t-butyl derivatives of seventeen pairs of enantiomers of natural and synthetic amino acid family and aprotic solvent systems. The peak tailing was minimized with protic solvent system such as n-hexane and 2-propanol mixture affording base-line separations. Enantiomers and diastereomers of di- and tripeptides containing benzyloxycarbonyl substituent as protecting group were also resolved by the FVA-column so that the method will be applied to the peptide synthesis with high optical purities of target molecules.
