Heat capacity and thermodynamic characteristics of denaturation and glass transition of hydrated and anhydrous proteins.
Calorimetric measurements of absolute heat capacity have been performed for hydrated (11)S-globulin (0 < C(H(2)O) < 25%) and for lysozyme in a concentrated solution, both in the native and denatured states. The denaturation process is observed in hydrated and completely anhydrous proteins; it is accompanied by the appearance of heat capacity increment (Delta(N)(D)C(p)), as is the case for protein solutions. It has been shown that, depending on the temperature and water content, the hydrated denatured proteins can be in a highly elastic or glassy states. Glass transition is also observed in hydrated native proteins. It is found that the denaturation increment Delta(N)(D)C(p) in native protein, like the increment DeltaC(p) in denatured protein in glass transition at low water contents, is due to additional degrees of freedom of thermal motion in the protein globule. In contrast to the conventional notion, comparison of absolute C(p) values for hydrated denatured proteins with the C(p) values for denatured proteins in solution has indicated a dominant contribution of the globule thermal motion to the denaturation increment of protein heat capacity in solutions. The concentration dependence of denaturing heat absorption (temperature at its maximum, T(D), and thermal effect, DeltaQ(D)) and that of glass transition temperature, T(g), for (11)S-globulin have been studied in a wide range of water contents. General polymeric and specific protein features of these dependencies are discussed.
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Research Institute of Physics, St.-Petersburg State University, Petrodvorets, 198904 St.-Petersburg, Russia.