Collagen proteins exchange oxygen with demineralisation and gelatinisation reagents and also with atmospheric moisture

Rationale: The oxygen (O) isotope composition of collagen proteins is a potential indicator of adult residential location, useful for provenancing in ecology, archaeology and forensics. In acidic solution, proteins can exchange O from carboxylic acid moieties with reagent O. This study investigated whether this exchange occurs during demineralisation and gelatinisation preparation of bone/ivory collagen. Methods: EDTA and HCl demineralisation or gelatinisation reagents were made up in waters with different delta O-18 values, and were used to extract collagen from four skeletal tissue samples. Aliquots of extracted collagen were exposed to two different atmospheric waters, at 120 degrees C and ambient temperature, and subsequently dried in a vacuum oven at 40 degrees C or by freeze drying. Sample delta O-18 values were measured by HT-EA pyrolysis/IRMS using a zero-blank autosampler. Results: Collagen samples exchanged O with both reagent waters and atmospheric water, which altered sample delta O-18 values. Exchange with reagent waters occurred in all extraction methods, but was greater at lower pH. Damage to the collagen samples during extraction increased O exchange. The nature of exchange of O with atmospheric water depended on the temperature of exposure: kinetic fractionation of O was identified at 120 degrees C but not at ambient temperature. Exchange was difficult to quantify due to the high variability of delta O-18 values between experimental replicates. Conclusions: Studies of delta O-18 values in collagen proteins should avoid extraction methods using acidic solutions.