A correlation between the distribution of biological apatite and amino acid sequence of type I collagen

We have determined the localization of apatite within type I collagen fibrils of calcifying turkey leg tendons by both bright field and selected-area dark field (SADF) electron microscopy and have compared this to computer-modeled, chick type I collagen amino acid sequence data. Apatite crystals occur in both the gap and overlap zones at early stages of mineralization in an asymmetric pattern that corresponds to the polarity, N- to C- orientation, of the collagen molecule. Based on comparisons with computer-generated models of known amino acid sequence of collagen, it was determined for early stages of mineral deposition that apatite is restricted by areas of high hydrophobicity. The gap zone is less hydrophobic than the overlap zone on average but each of these zones had areas of high hydrophobicity that correlated with sites of low localization of mineral. Possible interactions between hydrophobic regions and the process of mineral deposition are discussed.