RACEMIZATION IN A BOX; DOES A CLOSED SYSTEM IMPROVE KINETICS?

Glenn Goodfriend had a talent for making amino acid racemization (AAR) work and was instrumental in setting up our laboratory. Using his favourite land snails Glenn was able to demonstrate time dependent racemization kinetics and its application to a wide range of historical, archaeological and geological questions. However mollusc shells do not appear to be as consistently reliable as ratite (e.g. ostrich) eggshells, arguably because the latter operate as a closed system. According to models of racemization, AAR values in ‘leaky systems’ are sensitive to flux. In a closed system because all the reactants and products are trapped increasing racemization corresponds to predictable decreases in the concentrations of bound and free amino acids. We report results from an investigation of closed (bleached) vs. open (unbleached) system AAR in snails from the UK Quaternary. The closed system behaviour of bleached samples means that both free amino acids and total yields (following hydrolysis) can be determined and these co-vary in a predictable manner (R2 0.93, n=3 species, 30 samples) something which has previously been observed from the equivalent range of DL values for eggshells (R2 0.92 n=30; Miller, GH., et al. 2000; Perspectives in Amino Acid and Protein Geochemistry; Ed. by G.A. Goodfriend, et al., 161-181). The predictable behaviour allows us to develop the concept of the Degradation model kinetic (DMK). This is an aIle/Ile ratio equivalent value, calculated for the D/L values of multiple amino acids (currently Asx, Glx, Val, Phe, Ala), assuming idealised degradation of the protein in a closed system. The DMK increases the precision of AAR estimation (as multiple amino acids are included in the estimate), whilst deviations from the model identify compromised (and hence unreliable) samples. Bleach can turn molluscs into eggshells – at least for the purpose of AAR.