Chemical, enzymatic and spectroscopic characterization of "collagen" and other organic fractions from prehistoric bones
Three organic fractions from 9 modern bones were prepared: collagen, the HC1 insoluble fraction, and the HC1 soluble fraction. The fraction that has the same solubility characteristics as collagen (referred to herein as "collagen") and the HC1 soluble and insoluble fractions from 44 prehistoric bones were also prepared. Of these 44,17 had "collagen" with δ 13C and δ 15N values and C/N ratios within the ranges displayed by collagen from modern animals that ate the same types of food, whereas "collagen" in the remaining samples displayed clear evidence of diagenetic isotopic and/ or elemental alteration based on comparisons with modern collagen. The bones were characterized in terms of the amino acid compositions and infrared spectra of the three organic fractions, and the amino acid compositions of the low molecular weight products of the reaction between collagenase and "collagen." For all three analytical procedures, prehistoric samples whose "collagen" had not suffered diagenetic isotopic and/or elemental alteration produced results similar to those obtained from modern samples, whereas altered prehistoric samples did not. Application of the chemical, enzymatic and spectroscopic methods presented here should permit unequivocable identification of prehistoric samples that have suffered postmortem alteration of their "collagen" isotopic compositions, including those that have undergone more subtle isotopic shifts than can be identified by changes in the "collagen" C/N ratio, the only criterion applied to date for identifying altered samples. Some of the techniques we used to identify altered samples can be applied to fractions more readily prepared than "collagen", thereby facilitating screening of large numbers of samples in order to eliminate those unsuitable for isotopic paleodietary analysis. The same criteria we have established to identify "collagen" samples that have not suffered postmortem stable isotopic alteration could be applied to characterize samples prepared for radiocarbon analysis, leading to more accurate dating of bone.