An investigation of the hydrothermal stability and mineralisation of collagen and their relationship

Mineralised collagen displays an improved hydrothermal stability compared to collagen that is unmineralised. The possibility of using in-vitro partial mineralisation of collagen as a method of increasing the hydrothermal stability was investigated. Remineralisation experiments using demineralised turkey leg tendon and chemically modified bovine hide collagen showed that although it was possible to grow hydroxyapatite mineral crystallites on the collagen substrate they were only present at the substrate-solution interface and as such did not give rise to an increase in hydrothermal stability. The morphology of the mineral crystallites produced in-vitro were compared with those in the naturally mineralised tendon using Scanning Electron Microscopy (SEM), Small Angle X-ray scattering (SAXS), X-ray Diffraction (XRD) and Fourier-Transform Infrared Spectroscopy (FT-IR). Differential scanning calorimetry (DSC) studies on demineralised tendon identified a previously unknown high temperature endothermic transition to be present in the thermal scan of both mineralised and unmineralised collagen during denaturation. The position of this transition was found to be affected by hydration, presence of mineral, pH, and crosslinking similar to that of the first transition. Experiments using reagents known to selectively break various non-covalent interactions within collagen indicated that the transition was due to the breaking of covalent bonds via an endothermic chemical reaction, with the most likely candidate being the hydrolysis of peptide bonds within the polypeptide backbone. Optical microscopy of collagen after heating indicated that the fibrillar structure of the collagen was destroyed during the second transition, forming an amorphous gel. Finally, the effect of the mineral phase on the hydrothermal stability of naturally mineralised collagen was discussed in context to its location within the collagen structure. It was postulated that the presence of mineral dehydrates the collagen structure, as well as decreasing the available space within the hole region.