MOLECULAR AND MICROSTRUCTURAL STUDIES OF THERMAL-DENATURATION AND GELATION OF BETA-LACTOGLOBULIN-A AND BETA-LACTOGLOBULIN-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. Fo urier transform infrared spectroscopy was used to monitor changes in t he secondary structure of the proteins when heated from 25 to 95 degre es C. The microstructure of beta-lg A and B gels made from 10% (w/v) p rotein solutions by heating at 90 degrees C for 30 min was studied by scanning and transmission electron microscopy. beta-Lg B had greater t hermal stability and required more energy to denature than beta-lg A; denaturation of beta-lg B was also more cooperative. Infrared spectros copy 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 b ands (1682 and similar to 1622 cm(-1)) were observed when the proteins were heated. Electron microscopy showed that the gel matrix of beta-l g B at both acid and alkaline pH was made up of larger aggregate struc tures than beta-lg A. The aggregates formed by both variants were larg e (1-2 mu m) and globular at acid pH but much smaller (nanometer range ) and amorphous at alkaline pH. This information provides a useful mod el for studying the relationship between protein structure and functio n.