Reversibility of heat-induced conformational changes and surface exposed hydrophobic clusters of beta-lactoglobulin: their role in heat-induced sol-gel state transition.

The effects of heat-treatment on the conformational changes of beta-lactoglobulin were monitored by differential scanning calorimetry (DSC), binding properties to 1-anilino-8-naphthalenesulphonic acid (ANS) and to 5,5'-dithio-bis (2-nitrobenzoic acid) (DTNB). The thermal transition of beta-lactoglobulin was 100% reversible on re-heating and its binding properties to the ANS fluorescent-dye and to the DTNB probe did not significantly change when the first heating was made to a temperature T < Tmax, that of the DSC maximal peak deviation of unheated solutions. When the solutions were heated to higher temperatures, the degree of reversibility of the thermal transition decreased, while the beta-lactoglobulin surface hydrophobicity increased. Furthermore, the time (tg) needed for the sol-gel state transitions was highly temperature-dependent for the solutions showing no significant reactivity with the DTNB probe, a high percentage of residual tertiary structure but a low surface hydrophobicity. For beta-lactoglobulin showing < 50% residual tertiary structure but high surface hydrophobicity, tg values were hardly temperature-dependent. The results are discussed in terms of the role of hydrophobic interactions in the aggregation process of denatured beta-lg molecules.