Journal of Biomedical OpticsRatio of entropy to enthalpy in thermal transitions in biological tissues
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Thermal transitions in biological tissues that have been reported in the literature are summarized in terms of the apparent molar entropy (ΔS) and molar enthalpy (ΔH) involved in the transition. A plot of ΔS versus ΔH for all the data yields a straight line, consistent with the definition of free energy, ΔG=ΔH+TΔS. Various bonds may be involved in cooperative bond breakage during thermal transitions; however, for the sake of description, the equivalent number of cooperative hydrogen bonds can be cited. Most of the tissue data behave as if 10 to 20 hydrogen bonds are cooperatively broken during coagulation, with one transition, the expression of heat shock protein, involving 90 cooperative hydrogen bonds. The data are consistent with ΔS=a+bΔH, where a=–327.5 J/(mol K) and b=31.47×10–4 K–1. If each additional hydrogen bond adds 19×103 J/mol to ΔH, then each additional bond adds 59.8 J/(mol·K) to ΔS. Hence, the dynamics of irreversible thermal transitions can be described in terms of one free parameter, the apparent number of cooperative hydrogen bonds broken during the transition.