title: Thermodynamics of structural fluctuations in lysozyme as revealed by hydrogen exchange kinetics.
abstract: A new method is described that makes use of the empirical enthalpy--entropy compensation behavior of a related series of processes for deriving the activation enthalpy and entropy probability density functions from the corresponding rate constant density function. The method has been applied to data obtained from a study of the temperature dependence of hydrogen-tritium exchange in lysozyme. Analysis of the temperature dependence of tj, the time required to reach a particular number of hydrogens remaining unexchanged, provides estimates of delta G#, delta H#, and delta S# for the exchange process. The results are consistent with the notion of two mechanisms of exchange characterized by different activation energies. Increases in delta H# are compensated by corresponding increases in delta S#. The compensation plot, however, reveals two distinct apparent compensation temperatures, which reflect the operation of two qualitatively different mechanisms of exchange. The faster hydrogens exchange with delta H# values between 8 and 18 kcal X mol-1 and are characterized by a high compensation temperature of 470 K. The slower hydrogens exchange with delta H# values that reach 40 kcal X mol-1 and display a compensation temperature of congruent to 360 K. The latter is associated with a thermal unfolding mechanism of exchange.
mesh_terms: D006859:Hydrogen / Q000378:metabolism*; D007700:Kinetics; D008433:Mathematics; D009113:Muramidase / Q000378:metabolism*; D013816:Thermodynamics
chemical_list: D006859:Hydrogen; D009113:Muramidase