In a study of the rates of racemization of substituted arylglycines [ArCH(NH2)COOH] 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)C02'"] 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 AS* ranged from -24.5 to +29.0 eu, while AH* 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.Amino acid racemization studies are of interest to diverse specialists. Analytical chemists find it a challenge to measure the rate of racemization using small quantities while synthetic peptide chemists take precautions to avoid the process. On the other hand, geochemists relate racemization to the age of fossils, while evolutionists study racemization in their search for the origin of life. This report concerns the physical organic chemistry of the mechanism of racemization.The research attempted to answer five questions: (1) Does the racemization of arylglycines follow reversible, psuedo-first-order kinetics? (2) What are the sign and magnitude of p in a Hammett plot of the racemization of C-arylglycines, and how do they relate to the reactive intermediate? (3) Do racemization and tritium exchange take place at the same rate? (4) Is racemization primarily entropy or enthalpy controlled? (5) What is the most reactive species in the raceipization and exchange of aryl glycines? The results include new insight into the mechanism of amino acid racemization. Phenylglycine and its

Die Racemisierung zahlreicher Aminosäuren (I) und (II) wird untersucht, z.Tl. unter Verwendung tritiierter Verbindungen.