Separation of enantiomeric beta-methyl amino acids and of beta-methyl amino acid containing peptides.

Erythro-D,L- and threo-D,L-beta-methylphenylalanine, -beta-methyltyrosine and -beta-methyl-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid were synthesized. High-performance liquid chromatographic methods were developed for the separation and identification of the enantiomers of the beta-methyl amino acids, with the application of 1-fluoro-2,4-dinitrophenyl-5-L-alanine amide and 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyl isothiocyanate as derivatizing reagents. These amino acids were incorporated into the mu-agonist/delta-antagonist opioid peptides H-beta-MeTyr-Tic-Phe-Phe-NH2, H-Tyr-Tic-beta-MePhe-Phe-NH2 and H-Tyr-Tic-Phe-beta-MePhe-NH2, and the delta-antagonist H-Tyr-beta-MeTic-Phe-Phe-OH, by solid-phase peptide synthesis. Each peptide has four stereoisomers. The peptide stereoisomers were separated on different columns and in different eluent systems and the elution order of the peptide epimers was determined.