A study on gelation of soybean globulin solutions

Abstract

A study has been undertaken on the linear viscoelastic properties (stress relaxation) of thermotropic gels of soybean globulins (SBG) over a wide range of concentration and temperature. Experimental data on isothermal stress relaxation for SBG gels of various concentrations were generalized in the form of a concentration-invariant relaxation curve using the reduction by modulus, but without the reduction by time. The concentration dependence of the modulus reduction parameter was shown to be similar to that of the equilibrium gel modulus with an accuracy of up to a constant factor. The temperature-invariant relaxation curve of 17.5% gel was obtained in the traditional way in the range of reduced time equal to about seven decimal orders. Its form is characteristic of ordinary cross-linked high elastic polymers at the end of the transition zone and the beginning of plateau zone. The activation energy for relaxation of SBG gels amounts to 136 kJ/mol irrespective of their concentration. Based on assaying of sol-fraction content and its composition, it has been found that 7 and 11 S SBG fractions play a predominant part in gelation. SBG gel are soluble in 8 M urea, but insoluble in 0.01 M mercaptoethanol. Under the action of urea the gels and native SBG are decomposed into practically identical molecular fragments (subunits). These facts attest that the hydrophobic interaction of subunits play a leading role in SBG gelation. Gelation is not accompanied by changes in the integral intensity of a comparatively well resolved leucine band in PMR (proton magnetic resonance) spectrum of SBG at 100 MHz. This fact serves as a basis for assumption that the packing densities of polypeptide chains in the structural elements of gels and native molecules of SBG are comparable. The relaxation properties of gels at the end of the transition zone and the beginning of the plateau zone are determined predominantly by the “internal viscosity” of their elastic structural elements. It should be noted in this connection that the role of the “local viscosity” as an effective criterium for hydrodynamic interaction of these elements is apparently small due to their limited flexibility. The elastic elements of gels are long-term fluctuations of protein concentration. In the event of gel with 17.5% concentration one elastic element consists of approximately 20 subunits. In their thermo-rheological behaviour SBG gels can be classed with the systems having entropic elasticity.