Extraction of soluble collagen and its feasibility in the palaeodietary research

In current palaeodietary research, gelatinization is the main method to extract insoluble collagen (ISC) from ancient bones. However, the degradation products of ISC, i.e., soluble collagen (SC), is often neglected and abandoned. In this work, we try to separate the extracts of ancient bones using gel chromatography and compare the contents of carbon and nitrogen, atomic C/N ratio, and stable carbon and nitrogen isotopic values of the extracts from three peaks to determine which peak can be attributed to SC. At last, the potential application of SC in palaeodietary research is discussed based on the comparison of stable isotopic values between ISC and SC. Among the three peaks, the second with the retention time between 17.5 min and 27.5 min had the most broad peak shape, indicating that the molecular weights of proteins collected were most variable. Besides, the contents of carbon and nitrogen and atomic C/N ratio of extracts in this peak were closest to the corresponding ISC. Based on the above, we conclude that the extract in second peak is SC. More important, the δ ¹³C and δ ¹⁵N values of ISC and SC are very similar. For ISC and SC with atomic C/N ratios within the normal range (2.9–3.6), the mean difference of δ ¹³C value was only (0.3±0.2)‰ (n=2) while δ ¹⁵N value was (0.6±0.1)‰ (n=2). Although the atomic C/N ratios of some SC are slightly beyond the normal range, the mean differences of δ ¹³C and δ ¹⁵N values were still only (0.4±0.1)‰ and (0.3±0)‰ (n=2) respectively. These isotopic differences are quite below the isotope fractionation in one trophic level (δ ¹³C values of 1‰–1.5‰ and δ ¹⁵N values of 3‰–5‰), suggesting that SC had great application potentials in palaeodietary research.