VITRIFICATION AND STRUCTURAL RELAXATION OF A WATER-SWOLLEN PROTEIN, WHEAT GLUTEN AND THE THERMODYNAMICS OF ITS WATER-PROTEIN[--]ICE EQUILIBRIUM

To understand the chemical physics of molecular motions in a structura lly simple, vegetable native protein containing absorbed water, in whi ch both intermolecular and intramolecular interactions occur, the natu re of the glass-transition and structural relaxation of vitrified 52.8 wt.% water swollen wheat gluten have been studied by differential sca nning calorimetry (DSC), as has the water <-> ice phase transformation . Experiments performed during both cooling and heating and with sampl es of different thermal histories show a broad endothermic feature beg inning at ca. 160 K which is barely interrupted by a partial-crystalli zation exotherm at ca. 250 K. The endothermic features resembled those observed for several simpler hydrated proteins (Biophys. J., 1994, 66 , 249; J. Phys. Chem., 1994, 98, 13780), a hydrated crosslinked polyme r (J. Phys. Chem., 1990, 94 2689) and a dry interpenetrating network p olymer blend (J. Polym. Sci. B, 1992, 32, 683). Their broadness is a r esult of the closely spaced multiplicity of small but sharp mini-endot herms and has its origin in the onset of different configurational sub strates that become available to the protein's structure as the temper ature is increased. The remarkable similarity of these features amongs t a broad class of materials is a reflection of the predominant role o f the intermolecular energy barriers in determining the structural rel axation kinetics. This has been described in terms of a time-dependent potential-energy surface, which represent a hierarchy of molecular an d segmental motions. Ice and freeze-concentrated solution coexist at a thermodynamic equilibrium at T < 273 K. Their respective amounts have been measured between 260 and 273 K and formalism based on equilibriu m thermodynamics has been developed, and the DSC scans for cooling sim ulated. This formalism agrees with the data. The temperature variation of the equilibrium constant for the protein-water <-> protein-ice coe xistence does not agree with that given by the Gibbs-Helmholtz equatio n, which is a reflection of strong interactions between the water mole cules and H-bonding protein segments. The interpretation in terms of t he role of protein dynamics in the crystallization of water, and the f ormalism developed, are general and useful for studies on other comple x biomaterials.