0016-7037/79/0901-1443$02.00/O zyxwv Grochimicu cf Cmmochimicu Aclu Vol. 43. pp. 1443 to 1448 0 Pe rg a mo n Press Ltd. 1979. Printed in Great Britain Effect of glucose on aspartic acid racemization JOHN E. ZUMBERCE* Laboratory of Organic Geochemistry, Department of Geosciences, The University of Arizona, Tucson, AZ 85721, U.S.A. (Received 12 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED December 1978; accepted in reoisedform 25 April 1979) Abstract-The racemization of aspartic acid in aqueous solution with and without phosphate buffer was found to increase in the presence of o-glucose. This increase in racemization appears to involve the formation of an imine intermediate (Schiff base). Subsequent proton tautomerization leading to the loss of asymmetry during the early stages of melanoidin formation could account for the observed increase in racemization. The racemization reaction did not follow reversible first order kinetics when elucose was added to buffered asnartic acid solution at 80°C; in unbuffered solution reversible first order kinetics were followed up to 624 hr. INTRODUCTION THE RACEMIZATION rates of various amino acids in natural systems have been used in developing a dating method and paleotemperature indicator. The time- temperature dependent interconversion of the L- to D-HMItiOmerS after enzymatic activity has ceased is the basis for these studies. In some cases, rates of racemization have been reported to follow reversible first order kinetics (e.g. SCHROEDER and BADA, 1976; ENGEL et al., 1977). A number of recent articles have reviewed the rather extensive research in this field which has been performed during the past few years (KVENVOLDEN,1975; DUNGWORTH, 1976; SCHROEDER and BADA, 1976; WILLIAMS and SMITH, 1977). Much of this research has focused on the abundances and stereochemistry of amino acids in ancient shell, bone, sediment and wood. Amino acid racemization in fossil shells has been employed by a number of investiga- tors to determine or clarify stratigraphic correlations. For example, MITTERER (1974, 1975, 1976) has measured the extent of isoleucine epimerization in fossil Mercenaria shells from Pleistocene deposits in Florida and the Carolinas to facilitate stratigraphic studies. Additional racemization investigations using fossil molluscs from Pacific coast marine terrace deposits have been conducted by WEHMILLER et al. (1977), MASTERS and BADA (1977), and WEHMILLER (1977). In addition to fossil shells, Bada and co- workers have used amino acid racemization kinetic data to date fossil bones from a number of localities (e.g. BADA 1972a; BADA and SCHROEDER, 1975; BADA and HELFMAN, 1975). DUNGWORTH et al. (1976) have also studied the kinetics of amino acid racemization in fossil bones; they reported a range of activation energies and Arrhenius frequency factors for different amino acids from mammoth collagen. Arrhenius par- ameters were also calculated for four amino acids iso- lated from sequoia heartwood by ENGEL et al. (1977). These latter values, along with a reversible first order * Present address: Cities Service Co., Energy Resources Group, Exploration and Production Research, Box 50408, Tulsa, OK 74150, U.S.A. rate constant determined from the extent of racemiza- tion of aspartic acid in dendrochronologically dated sequoia heartwood, allowed for the calculation of an average temperature experienced by bound aspartic acid in sequoia heartwood during the past z 2200 yr. This temperature (6°C) was found to be in close agreement with modern and extrapolated past tem- peratures near the sample location. Other physical and chemical factors besides tem- perature affect racemization such as Hz0 (HARE, 1973), pH (BADA, 1972b), ionic strength and buffer concentration (SMITH et al., 1978), metal ions (SCH- ROEDER and BADA, 1976), sequential position in pep- tides (KRIAUSAKUL and MITTERER, 1978), species vari- ation (MILLER and HARE, 1975; KING and NEVILLE, 1977; ENGEL et al., 1978), bacterial contaminations (POLLOCK et al., 1977; ENGEL et al., 1977) clay inter- actions (AKIYAMA,1978), etc. It is apparent from these studies that a variety of factors influence amino acid reactions both in laboratory experiments as well as natural environments. The evaluation of the various possible amino acid reactions in shell, bone, sediment and wood and the factors controlling these reactions will no doubt contribute eventually to more compre- hensive geochronological and paleoclimatic interpre- tations. A reaction which is important during the early dia- genesis of organic matter is the interaction between amino acids and carbohydrates to form melanoidins (e.g. GOTTSCHALK, 1966; HEM;ES, 1976). This conden- sation between amines and sugars is also known as the Maillard reaction. The effect of o-glucose and subsequent melanoidin polymer formation on the apparent racemization rate of aspartic acid is another factor which must be considered where significant quantities of carbohydrates are present. As is shown in this report, o-glucose affects the racemization of aspartic acid by increasing the apparent rate of inter- conversion of the L-and n-enantiomers. EXPERIMENTAL . ,D-Glucose and L-aspartic acid (asp) were dissolved in triple distilled water at a molar ratio of 15/l (glucose/asp). 1443