The ratio of entropy to enthalpy for thermal transitions in biological cells, tissues and materials, and its implications for biology

The process of irreversible thermal denaturation of macromolecules involves cooperative bond breakage. Many bonds must break at the same time to allow denaturation. Hence, molecules are stabilized against thermal damage. However, these multiple bonds enforce a structural order on the macromolecule. A review of the literature on the entropy AS (J/(mole K)) and enthalpy ΔH (J/mole) of various endpoints of irreversible thermal denaturation (eg., whitening, contraction, loss of birefringence, necrosis, onset of heat shock proteins) indicate that the ratio ΔS/ΔH is constant at a value of 31.47×10^-4 K^-1, or 1/T_crit where T _crit ≈ 44.6°C. The free energy of denaturation is ΔG = ΔH - TΔS. At temperatures below T_crit, more cooperative bonds yield more stability because AH dominates over TΔS, but at temperatures above T_crit more bonds yield less stable structure because TΔS dominates over ΔH. Only one free parameter describes the kinetics of irreversible denaturation of macromolecules involving simultaneous breakage of multiple cooperative bonds, the ΔH of the transition.