No abstract

No abstract

Four series of diastereomeric peptide pairs (X-Lys, Lys-Y, Gly-X-Lys, and Gly-Lys-Y; where X and Y = Ala, Leu, Val, Ile, Phe, and Pro) have been synthesized by conventional procedures using benzyl-based protecting groups. Chromatographic separation of each pair, except Gly-Lys-Pro, has been achieved by elution from a 15-cm Aminex A-5 resin column using pH 5.5 buffer for tripeptides, pH 6.5 buffer for dipeptides, and pH 7.5 buffer for phenylalanyl peptides. The isomers have been identified by chromatography of the L-L isomers. Examples of the use of the model peptides for optical purity determinations and as tests for racemization are presented. The series Gly-X-Lys allows comparison of the tendencies to racemize of residues X during couplings. The separations of other peptides where Y = MeVal and X = N- and O-methylhydroxyamino acids are also described.

A laboratory experiment involving the coupling of N‐benzoylamino acids with methyl Nε‐benzyloxycarbonyl‐L‐lysinate using various methods is described. The extents of racemization are determined by analysis for the protected diastereomeric dipeptide ester products by quantitation of the separated ester methyl proton peaks of their nuclear magnetic resonance (60 MHz) spectra. Synthesis of the starting materials and additional exercises are included as options.

The series of diastereomeric peptide derivatives N‐benzoyl‐d,l‐X‐N‐benzyl‐oxycarbonyl‐L‐lysine methyl ester where X = alanyl, valyl, leucyl, phenylalanyl and isoleucyl are submitted as model systems for studying racemization in peptide synthesis. The diastereomers can be analyzed by quantitation of the separated ester methyl proton peaks of their nuclear magnetic resonance spectra. Data on the tendency to racemize of the different residues are presented. In polar solvents, valyl and isoleucyl residues racemize more readily than the other residues.