Racemization of Amino Acids in Dipeptides Shows COOH > NH 2 for Non-Sterically Hindered Residues

Abstract

The relative rates of racemization for amino acid residues at the NH₂ and COOH ends of 37 different dipeptides were determined. In nine dipeptides containing alanine, leucine, phenylalanine, aspartic acid, and methionine, the amino acid residue racemized faster at the COOH-terminal position than at the NH₂-terminal position (COOH > NH₂). The sterically hindered amino acids isoleucine and valine showed NH₂ > COOH. Six proline dipeptides showed NH₂ > COOH. Intramolecular effects have been invoked to explain these surprising results.