Water-ice equilibrium in hydrated proteins, and structural relaxation and H-bond equilibrium in a high protein food, beef

Configurational degrees of freedom available to the structure of most biopo lymers in their native state further increase when the extent of their hydr ation, particularly of proteins and nucleic acids, is raised, These allow w ater and ice to coexist in the proteins below 273 K, and increase the distr ibution of energy barriers for their structural relaxation. Hence, their vi trification occurs over an extremely broad, almost indiscernible, temperatu re range, as the configurational contribution to the thermodynamic function s is lost gradually on their cooling. When annealed, a hydrated protein los es part of its enthalpy and entropy spontaneously as certain of its H-bonde d subunits and H2O molecules relax during the annealing period. The time an d temperature dependence of the thermodynamics and kinetics of this process of structural relaxation is studied by calorimetry of a native protein, be ef, and mathematically formulated. A detailed analysis shows that at each t emperature during the cooling of a hydrated beef sample, the native state o f protein, the H-bond equilibrium becomes frozen-in, and that the frozen-in equilibrium quotient increases on isothermal annealing, as the number of H -bonds increases isothermally. The configuration contribution and the H-bon ding contribution to the enthalpy and entropy become inseparable in biopoly mers, as they should be for simpler materials, Description of both phenomen a in general terms underscores the premise that simple model systems are no t necessarily required for understanding the native protein's behaviour.