The thermal denaturation of collagen in solution and its structural implications

The thermal denaturation in solution of five purified vertebrate collagens has been examined by viscometry and optical polarimetry. The intrinsic viscosity and specific optical rotation values of undenatured collagen in solution were found to be independent of pH under the conditions used. The temperature dependences of the denaturation processes have been examined for the five collagens as a function of pH and the activation energies and entropy changes involved have been calculated from the changes of intrinsic viscosity with temperature. Considerable and similar variations in the denaturation temperatures TD with change of pH have been observed for all the collagens; for three collagens this variation is examined in detail. The activation energy involved in the denaturation of a given collagen is found to be independent of pH and the entropy change on denaturation is shown to be a function of the distribution of electric charges along the collagen molecule. The TD values for the collagens at a given pH are separately correlated with the contents of hydroxyproline, proline, and total pyrrolidine residues. It is considered that the correlation between TD and total pyrrolidine content is the most fundamental and that the variation in thermal stability from collagen to collagen may be explained entirely on stereochemical grounds. The results afford no direct evidence to allow a decision between the two structures collagen I and collagen II proposed to explain the collagen high-angle X-ray diagram but put both structures on an equal basis. Since collagen II is more stereochemically satisfactory than collagen I this structure would appear to be the more likely.