The use of small-angle X-ray scattering to study archaeological and experimentally altered bone

1. Bone diagenesis and biomolecular preservationArchaeological biomolecular resources are crucial to severalforms of analysis, including radiocarbon dating, stable isotopestudies of diet, and genetic analysis using ancient DNA orprotein samples [6,11,52]. Biomolecules such as DNA andcollagen are unstable over archaeological time, however, andthe information they contain is easily lost to posterity. Thecontinued exploitation of these assets as windows onto pastlifeways, environments, and evolutionary histories requiresan understanding of diagenetic processes leading to their pres-ervation and decay over time. A measure of degree of preser-vation of archaeological samples used in biomolecular studieshas been introduced as an important criterion to validate re-sults [11]. Additionally, screening methods have been devel-oped to predict which samples are likely to yield resultsprior to the destructive analysis involved in extracting biomo-lecular material [57].Bone is a composite material, consisting of a mineral phase(primarily carbonated hydroxyapatite) embedded in and over-growing an organic fraction. The primary protein componentof bone is type I collagen, composed of three helical polypep-tide chains wound together into a triple helical structure. Thehelices are arranged in bundles (fibrils) with a staggered spac-ing, and grouped together to form fibers of a tightly wovenrope-like structure. The mineral forms within and around thefibers, but the exact processes and mechanisms of mineraliza-tion remain unknown. It is the composite nature of bone andthe associated mineral structure that lends it more robusticitythan observed in soft tissue following burial, but degradationstill occurs. Additionally, there are several non-collagenousproteins, of which osteocalcin is the most relevant to biomo-lecular archaeology. Osteocalcin is a small acidic proteinthat contains an unusual amino acid, g-carboxy-glutamicacid (Gla); this can bind tightly to bone mineral, which stabil-izes the protein in the bone and contributes to its longevity inarchaeological samples [9].Bone mineral consists primarily of a carbonated form of hy-droxyapatite, Ca