Mechanism of the Racemization of Amino Acids. Kinetics of Racemization of Arylglycines

Article abstract of DOI:10.1021/jo00153a001

In a study of the rates of racemization of substituted arylglycines at pH 10, the Hammett ? value was found to be surprisingly low, 1.15, suggesting a concerted reaction or charge stabilization in a manner other than by the substituent.The rate of methine hydrogen exchange was, however, the same as the rate of racemization, which argues against a concerted reaction mechanism.A pH profile study demonstrated that the most reactive species was the zwitterion <⁺NH3CH(C6H5)CO2^-> in basic media.The racemization reaction showed general-base catalysis when the buffer concentration was changed at constant ionic strength.Within the aryl series, the entropy of activation was more significant than the enthalpy of activation.The ΔS(excit.) ranged from -24.5 to +29.0 eu, while ΔH(excit.) values ranged from 19.9 to 20.4 kcal.Racemization of arylglycines followed reversible first-order kinetics similar to that found for aliphatic amino acids in solution.The extent of racemization was studied as a function of pH.The details of the mechanism of this reaction are presented in light of these data.