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Heat-Induced Gelation Of Proteins
[Thesis]. Manchester, UK: The University of Manchester; 2012.
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Abstract
In this study the heat-induced gelation of two (readily available) proteins, which contain disulphide bonds, has been investigated over a range of protein concentrations in the presence and absence of the presence of the reductant, dithiothreitol at neutral pH. The proteins selected in this study were: β-Lactoglobulin and bovine serum albumin. These proteins have different number of disulphide bonds and possess different protein secondary structures. The influences of the reductant and protein concentration on their heat-induced gelation were explored to see whether the proteins were able to form protein hydrogels and that the mechanical properties of the resulting protein hydrogels were controllable. The tilting test tube method revealed that both proteins formed macroscopic hydrogels, at protein concentrations above the critical gelation concentration and that the critical gelation concentration was constantly lower in the presence of the reductant. Micro-DSC revealed that both proteins had completely denatured upon heating and that the denaturation temperature and enthalpy were significantly lower in the presence of the reductant. IR spectroscopy revealed that both proteins undergo major secondary structure transitions that resulted in the formation of fibers that are rich in β-sheet structure upon heating and that the protein lost some secondary structure before any heating and gained more β-sheet structure in the presence of the reductant. Both proteins had partially denatured before any heating in the presence of the reductant and that β-LG underwent aggregation that was accompanied by the loss of native β-sheet structure and the formation of intermolecular β-sheet structure, while that BSA underwent aggregation that was accompanied by the loss of native α-helix structure and the formation of intermolecular β-sheet structure. Cryo-TEM revealed that both proteins formed fibers (10 nm in diameter) that exist as single entities at low protein concentrations and become entangled into macroscopic networks, at protein concentrations above the critical gelation concentration and that more fibers and denser macroscopic networks were formed in the presence of the reductant. Oscillatory rheology revealed that both proteins formed macroscopic networks exhibit viscoelastic behaviour and that their elastic modulus had increased in the presence of the reductant and with increasing protein concentration.