Amino Acids
Collagen molecules are composed of linear, unbranching sequences of approx 20 naturally occurring amino acids. The structure of the molecule is stabilised by hydrogen bonds; the most common being between the amino group (-NH2) of one residue and the carboxyl group (-COOH) of a second residue, resulting in both acidic and basic properties. Uncharged side-chains also interact with one another, but by excluding water from their mutual interfaces (i.e. hydrophobic reaction) (Woodhead-Galloway 1980:10, 23-24, 38). All amino acids, except glycine, exhibit optical activity, existing in the natural state as laevo-rotary compounds, a property apparently restricted to amino acids of a biological origin (Wyckoff 1972:53), a property which is exploited in amino acid racemisation dating (e.g. Masters 1987).
In general the composition of mammalian collagens shows little variability (see Hare 1980:209). Of special significance to recent AMS works is the amino acid hydroxyproline (i.e. Stafford et al. 1987, 1988, 1991). Hydroxyproline is found rarely in other proteins but comprises about 10% of all amino acids in collagen. However, from a practical point of view the use of hydroxyproline for 14C analysis is limited as it does not occur in large quantities in fossil bones, has been detected in natural waters (Long et al. 1989), is excreted in the urine, and can be found in some plants (Waterlow et al. 1978:510; Woodhead-Galloway 1980:12; Taylor 1982:468).
Apatite
Seventy percent of bone is made up of the inorganic mineral hydroxyapatite, which includes calcium phosphate, calcium carbonate, calcium fluoride, calcium hydroxide and citrate. This inorganic component ([Ca3(P)4)2]3.Ca(OH)2) is predominantly crystalline, though may be present in amorphous forms (Hedges and van Klinken 1992:284). The crystals are platelets or rods, about 8 to 15A thick, 20 to 40A wide and 200 to 400A long. The substitution mechanisms that occur in the hydroxyapatite of bone include intercrystalline exchange and a recrystallisation due to dissolution and reformation of crystals, with the addition of new ions into the crystal structure replacing Ca2+ or being adsorbed on the crystal surfaces (Smith et al. 1983:446).