THE KINETICS OF THE THERMAL-DENATURATION OF COLLAGEN IN UNRESTRAINED RAT TAIL TENDON DETERMINED BY DIFFERENTIAL SCANNING CALORIMETRY

This paper shows that the position and shape of the denaturation endot hem of collagen fibrils are governed by the kinetics of an irreversibl e rate process. This was proved by measuring the rate of denaturation in rat tail tendons held isothermally at different temperatures, there by determining rate constant characteristics such as the activation en thalpy and entropy and predicting endotherm position and shape therefr om. Comparison with actual scanning results showed good correspondence . Isothermal measurements of the rate of collagen denaturation, measur ed continuously using a calorimetric method, were used to determine ra te constants for collagen denaturation in tendons immersed in water an d 0.5 M acetic acid. The temperature dependence of the rate constants were fitted to the three rate process models, previously examined theo retically: the D and z formulation, the Arrhenius equation and the abs olute rate theory For example, in water the activation enthalpy was 0. 518 (+/-0.016) MJ mol(-1) and the activation entropy 1.485 (+/-0.049) kJ mol(-1) K-1, while in acetic acid the corresponding figures were 1. 306 (+/-0.099) MJ mol(-1) and 4.142 (+/-0.323) kJ mol(-1) K-1. These c haracteristics are discussed in terms of the thermal activation of a r egion of the molecule, the co-operative unit. The ratio of the activat ion enthalpy to the calorimetry enthalpy of denaturation indicated a c o-operative unit that was 66 (+/-5) residues long when fibrils were sw ollen in acetic and the collagen molecules acted essentially independe ntly On the other hand the intact fibrils in water gave a co-operative unit of 26 (+/-1) residues long. The reason for the reduction in size of the co-operative unit is that it is surrounded, and therefore stab ilized by other molecules in the fibre. It is interesting to note that the suggested co-operative unit lies almost entirely within the ''gap '' zone of the collagen fibril in its quarter-staggered arrangement of molecules. We believe that the co-operative unit would be represented by a domain that is free of stabilising hydroxyproline residues. Inde ed such a domain exists near the C terminus of the triple helix from G ly877 to Pro941, i.e. 65 residues. In acetic acid, activation is simil ar to that of collagen molecules in solution. All the inter alpha-chai n hydrogen bonds in the co-operative unit are broken and the separate chains in this short region are free to flail around under the action of thermal collisions relatively unimpeded by intermolecular interacti ons. In tendons bathed in water, the collagen molecules are more tight ly packed together in fibrils and the number of possible conformations of the activated state is limited by inter-molecular interactions. Th is reduces the size of the co-operative unit to about 26 amino residue s.