First-Order Rate Constans for the Racemization of Each Component in a Mixture of Isomeric Dipeptides and their Diketopiperazines

Article abstract of DOI:10.1021/jo00387a027

L-Alanylglycine (L-Ala-Gly), glycyl-L-alanine (Gly-L-Ala), and c-L-Ala-Gly were racemized at 120 deg C in aqueous phosphate-buffered solutions at pH 8.0, a pH value near maximum racemization.The kinetics were followed by regression analysis.The racemization of Ala-Gly and Gly-Ala closelly followed reversible first-order kinetics.The initial rate of racemisation of DKP was fast but soon slowed, likely because of hydrolysis to the dipeptides.The resulting rate was similar to that of the dipeptides.The observed racemization rate constans of the dipeptides and DKP were shown to be independent of the concentration of the peptides and the concetration of buffer.Component isolation studies using preparative TLC and chiral-phase GC analysis, coupled with computer analysis, showed an equilibrium existing between Ala-Gly, Gly-Ala, and DKP and the individual rates of racemization.At equilibrium, the mole fractions are as follows: Ala-Gly, 0.57; DKP, 0.22; Gly-Ala, 0.21.The rate constant for racemization of DKP was only 2 times that of Gly-Ala and 7 times the rate of Ala-Gly.Ala-Gly racemized 20 times and Gly-Ala 66 times faster than free alanine.The results support the influence of neighboring groups in the racemization of dipeptides.Factors that contribute to the rapid racemization (epimerization) are discussed.