Stability and mobility of the collagen structure.

The thermodynamic analysis of microcalorimetric and hydrogen-exchange data on the stability and mobility of collagen structures from different species with different physiological temperatures has shown that not only the thermostability but the enthalpy and entropy of disruption of the native collagen structure are increasing functions of the total prolyl and hydroxyprolyl content. At the same time the number of stable hydrogen bonds maintaining the native structure is constant and consists of one stable and a less stable (0.7) bond per triplet for all the collagens studied, in agreement with Ramachandran's model (Ramachandran &; Kartha, 1955). Thus the observed difference between the enthalpies of disruption of collagens with a different imino acid content cannot be explained by the difference in the amount of stable hydrogen bonds involved in maintaining the native collagen structure. With an increase in the imino acid content of collagen, the Gibbs energy of micro-unfolding, which determines the mobility of a structure, also increases. It seems probable that the rigidity and order of the core of a native macromolecule influence the order of the surrounding water and that they are the main source of the observed large values for the enthalpy and entropy of collagen unfolding. The observed link between the stability and mobility properties of collagens explains the correlation that exists between the thermostabilities of collagens and the physiological temperatures of different species if it is assumed that some definite level of mobility of collagen structure is required for its efficient functioning in living systems. The experimental data presented here support the validity of this assumption.