A chain of peptides: a new mass spectrometric tool for modelling protein diagenesis

Understanding the mechanisms of protein breakdown has long been a major focus of research in the AAR scientific community. The models of protein diagenesis proposed have generally been based on the extensive use of high temperature experiments to track the kinetics of the reaction by chiral amino acid analysis. However, this knowledge has not yet been able to produce a model which is fully able to explain the patterns of breakdown at low (burial) temperatures. By performing high temperature experiments on a range of biominerals (e.g. corals and marine gastropods) and comparing the racemisation patterns with those obtained in fossil samples of known age, some of our studies have highlighted a range of discrepancies in the datasets which we attribute to the interplay of a network of diagenesis reactions which are not yet fully understood. In particular, an accurate knowledge of the temperature sensitivity of the two main observable diagenetic reactions (hydrolysis and racemisation) is still elusive. This is mainly due to lack of data on the primary structure of the proteins which are targeted for dating purposes, e.g. in mollusc shells or avian eggshell. The amino acid sequence of struthiocalcin (SCA), the main protein of ostrich eggshell, has recently been determined (Mann & Siedler, 2004). This offers a major opportunity for degradation studies, as the primary chemical environment of each amino acid is known. We are coupling soft-ionization mass spectrometry (MALDI TOF-TOF) and liquid chromatography (RP-HPLC) to investigate the degradation pathways of four peptides from the sequence of SCA-1. The extents of racemisation and hydrolysis are coupled to the mass shifts of the peptides observed over time and at different temperature. Informed by this data, we aim to build a model of struthiocalcin breakdown and test its ability to mimic the natural breakdown observed in archaeological struthiocalcin of known age.