N-Methylamino acids in peptide synthesis. VI. A method for determining the enantiomeric purity of N-methylamino acids and their derivatives by ion-exchange chromatography as their C-terminal lysyl dipeptides

Abstract

A method capable of detecting one part in one thousand of the other isomer is described for determining the enantiomeric purity of N-methylamino acids and their cleavable derivatives. The method consists in converting the N-methylamino acid to its N-benzyloxycarbonyl derivative, and/or coupling the derivative with benzyl N^ε-εbenzyloxycarbonyl-L-lysinate using N,N′-dicyclohexylcarbodiimide, followed by removal of protecting groups by catalytic hydrogenation or other cleavage methods not affecting the chirality of the product. The resulting diastereomeric lysyl peptides are analyzed by ion-exchange chromatography on a 15 cm column of Aminex A-5 resin using an amino-acid analyzer. The method is applicable to samples contaminated by the corresponding unmethylated amino acid or derivative, and in effect, provides a new method for determining the enantiomeric purity of amino acids and their derivatives as well.Examples are given where, in some cases, optical purity verification or configurational assignment for N-methylamino acids can be achieved by inspection of the nmr spectra of related lysyl dipeptide derivatives.