Molecular and microstructural studies of thermal denaturation and gelation of beta-lactoglobulins A and B

The thermal properties of beta-lactoglobulins (beta-lg) A and B at pH 3, 5, 7, and 8.6 were studied by differential scanning calorimetry. Fourier transform infrared spectroscopy was used to monitor changes in the secondary structure of the proteins when heated from 25 to 95 degrees C. The microstructure of beta-lg A and B gels made from 10% (w/v) protein solutions by heating at 90 degrees C for 30 min was studied by scanning and transmission electron microscopy. beta-Lg B had greater thermal stability and required more energy to denature than beta-lg A; denaturation of beta-lg B was also more cooperative. Infrared spectroscopy showed that beta-lg B had a higher proportion of beta-sheet than the A variant at pH 3 and 5. At pH 7 and 8.6 the secondary structures of the two variants were similar. At all four pH values, aggregation bands (1682 and similar to 1622 cm(-1)) were observed when the proteins were heated. Electron microscopy showed that the gel matrix of beta-lg B at both acid and alkaline pH was made up of larger aggregate structures than beta-lg A. The aggregates formed by both variants were large (1-2 mu m) and globular at acid pH but much smaller (nanometer range) and amorphous at alkaline pH. This information provides a useful model for studying the relationship between protein structure and function.