Thermal stabilization of collagen fibers by calcification

Mineralized collagenous tissue is known to be more stable than soft collagenous tissue both mechanically and thermally. We find that the denaturation temperature of collagen in bone scanned in differential scanning calorimetry at 5 degrees C/min is 155 degrees C, 90 degrees C higher than that in skin. Furthermore, when the bone is partially demineralized with citrate, a discrete intermediate denaturation temperature appears at 113 degrees C, indicating that the mineral is retained at preferential binding sites in the collagen until it is completely leached out. It is shown by electron microscopy that these sites are located in the overlap regions of the collagen fibrils. Collagen in bovine hide can be synthetically mineralized in vitro by impregnation with calcium acid phosphate solution, followed by raising the pH, causing the phosphate to precipitate. Some of the collagen in this synthetically calcified tissue has an elevated denaturation temperature, 104 degrees C. This temperature compares favorably with collagen that is tanned with chromium oxide-sulfate complexes. Calcium phosphate in synthetically mineralized hide, hydroxyapatite in bone, and chromium oxide-sulfate complexes in leather might share the same mechanism of thermal stabilization.