The Kinetics of the Thermal Denaturation of Collagen in Unrestrained Rat Tail Tendon Determined by Differential Scanning Calorimetry

Abstract

This paper shows that the position and shape of the denaturation endothem of collagen fibrils are governed by the kinetics of an irreversible rate process. This was proved by measuring the rate of denaturation in rat tail tendons held isothermally at different temperatures, thereby determining rate constant characteristics such as the activation enthalpy and entropy and predicting endotherm position and shape therefrom. Comparison with actual scanning results showed good correspon dence. Isothermal measurements of the rate of collagen denaturation, measured continuously using a calorimetric method, were used to determine rate constants for collagen denaturation in tendons immersed in water and 0.5 M acetic acid. The temperature dependence of the rate constants were fitted to the three rate process models, previously examined theoretically: the Dand zformulation, the Arrhenius equation and the absolute rate theory. For example, in water the activation enthalpy was 0.518 (±0.016) MJ mol −1and the activation entropy 1.485 (±0.049) kJ mol −1K −1, while in acetic acid the corresponding figures were 1.306 (±0.099) MJ mol −1and 4.142 (±0.323) kJ mol −1K −1. These characteristics are discussed in terms of the thermal activation of a region of the molecule, the co-operative unit. The ratio of the activation enthalpy to the calorimetry enthalpy of denaturation indicated a co-operative unit that was 66 (±5) residues long when fibrils were swollen in acetic and the collagen molecules acted essentially independently. 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 stabilized by other molecules in the fibre. It is int;eresting 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. Indeed such a domain exists near the C terminus of the triple helix from Gly877 to Pro941, i.e. 65 residues. In acetic acid, activation is similar to that of collagen molecules in solution. All the inter α-chain 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 interactions. In tendons bathed in water, the collagen molecules are more tightly packed together in fibrils and the number of possible conformations of the activated state is limited by inter-molecular interactions. This reduces the size of the co-operative unit to about 26 amino residues.