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

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In this review we have critically evaluated the application of the diagenetic racemization of amino acids to geochronology and geothermometry. Although there has been enthusiastic support given to this new method, it is our opinion that recent developments suggest a more cautious approach. We have discussed the pitfalls and inhereent complications, while outlining the advances which have been accomplished. We conclude that this is an innovative approach which will add valuable information to the scientific literature. However, since our fundamental understanding of diagnetic racemization is still limited, many of the age and paleotemperature estimates which have been assigned to fossil specimens may be unreliable.

7321 I ' ,,4 I I \ /' 0 -9 I -1 3 7 II 15 PH Figure 4. Experimental and calculated racemization rate constants for phenylalanine at various pH values and 142 O C : (A) experimental observed rate constants by Bada and Show2 (A) calculated observed rate constants using the six absolute rate constants from this study; and (B) calculated observed rate constants using the four absolute rate constants from Bada and Shou.Z Bada and Shou's2 data points are shown by the A' s in all three plots. Curve B is the computer-calculated curve using only the four rate constants k,, k2, k4, and k6 calculated by Bada and Shou.'Curve A is the one obtained by using all six rate constants, which follows the experimental points precisely.The pK, values used in the calculations of both curves A and B were those obtained in this study using computer analysis. However, there was very little effect on the plots when using pK, values reported by Bada and Shou2 or obtained by applying Robinson and stoke^'^ equations. Experimental SectionAbsolute Rate Constants and pK,'s. The values of different pH's and observed rate constants were entered into the computer. The concentrations of H+, OH-, and each ionic species of the amino acids (using the Henderson-Hasselbach e q~a t i o n )~ were then calculated for each pH. After this, a minimization routine (ZXSSQ from the IMSL Library) using least-squares analysis was called to calculate the best six absolute rate constants. The px"s of the amino acids were determined by manually varying the pK values until the differences between the calculated and experimental values of observed rate constants were minimal.Observed Rate Constants. The calculated observed rate constants were obtained by entering previously determined pK, values and absolute rate constants in the computer program and then calculating the resultant observed constants at various values of pH. The pH profile data (A points in all plots) are those reported by Bada and Shou.2 Curve B was obtained by using Bada and Shou's equation (2) with only their four rate constants. Curve A was obtained by using eq 1 with all six rate constants from this study. W e thank NASA for research Grant NSG-7038 and the Research Office a t Utah State University for computer time. Registry No. Alanine, 56-41-7; valine, 72-18-4; leucine, 61-90-5; phenylalanine, 63-91-2. Acknowledgment.( 5 ) Eisenberg, D.; Crothers, D. Physical Chemistry with Applications to the Lqe Sciences; Benjamin/Cumming: Abstract: Dipeptides, their methyl esters, diketopiperazines (DKP), and N-substituted derivatives were racemized at high

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