Thermally induced changes in tissue birefringence

We have previously demonstrated that the thermal denaturation of collagen results in a repeatable loss of linear birefringence (LB), which can be measured with birefringence compensafion techniques or transmitted or remitted intensity measurements. Consequently, we have used LB to measure the kinetic changes in collagenous tissues. Analysis of the birefringence data results in the calculation of the kinetic parameters entropy, ΔS, enthalpy, ΔH and the rate constant, k. The birefringence data show that multiple kinetic processes are occurring during collagen denaturation. The most likely physical interpretaüon of the observed data is that structural changes are occurring on the molecular (iropocollagen) as well as the collagen fibril structural levels. Each of these siructures contributes to the total birefringence. When a parallel kinetic model, which is composed of two first order exponential decay models, is applied to the birefringence data, excellent agreement between the data and model is seen. Moreover, the data also show that the relative contributions of the two constituent reactions to the total birefringence measured are in excellent agreement with reported values of intrinsic and form birefringence contributions in collagen. The knowledge that multiple reactions exist during collagen denaturation impacts the understanding of how denaturation occurs and what information can be utilized for the conirol of the heating process.