Thermodynamic and kinetic features of vitrification and phase transformations of proteins and other constituents of dry and hydrated soybean, a high protein cereal
To understand the features of molecular motions in mixtures of structurally complex native proteins in which both intermolecular and intramolecular interactions occur, the nature of the glass transition and structural relaxation of vitrified soybean constituents have been studied by differential scanning calorimetry (DSC), as has the phase transformation in its dry and hydrated states. Experiments done during both cooling and heating and with samples of different thermal histories show a broad endothermic feature beginning at about 160 K which is interrupted by a partial crystallization exotherm at about 230 K. The endothermic features resembled those observed for several simpler hydrated proteins (Sartor, Mayer and Johari, 1994a; Green, Fan and Angell, 1994), a hydrated cross-linked polymer (Hofer, Mayer and Johari, 1990) and a dry interpenetrating network polymer blend (Sartor, Mayer and Johari, 1992b). Their broadness is a result of the closely spaced multiplicity of small but sharp miniendotherms and has its origin in the onset of different configurational substates that become available to the protein’s structure as the temperature is increased. The remarkable similarity of these features amongst a broad class of materials is a reflection of the predominant role of the intermolecular energy barriers in determining the structural relaxation kinetics. On heating the vitrified constituents of the soybean two exotherms appear in the 180–220 K range, which correspond to the crystallization of its constituents, and two corresponding endotherms of their melting, both below 273 K. Ice and freeze-concentrated solution coexist at a thermodynamic equilibrium at T < 273 K, for which a formalism based on equilibrium thermodynamics has been developed, and the DSC scans for cooling simulated. The interpretation in terms of the role of protein dynamics in the crystallization of water and the formalism developed are general and useful for studies of other complex biomaterials.
KeywordsEntropy Crystallization Hydrated Convection Enthalpy
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