Thermal properties of Phaseolus angularis (red bean) globulin

The thermal properties of Phaseolus angularis (red bean) globulin were studied by differential scanning calorimetry under various medium conditions. Red bean globulin (RGB) was fractionated by ion-exchange chromatography into a major fraction, with SDS–PAGE pattern corresponding to the 7S vicilin, and two minor fractions, probably representing residual vicilin and the 11S legumin, respectively. The thermogram of RBG showed a major endothermic peak at 86.4°C and a minor transition at 92.2°C. Vicilin exhibited two endothermic peaks (87.7 and 94.1°C), while legumin showed one transition at 89.5°C. The progressive increase in denaturation temperature (Td) with increase in salt concentration, suggests a more compact conformation for RBG with higher thermal stability. Decreases in enthalpy and Td were observed under the influence of highly acidic and alkaline pHs, chaotropic salts, and protein perturbants such as sodium dodecyl sulfate, urea and ethylene glycol, indicating partial denaturation and decrease in thermal stability. Dithiothreitol and N-ethylmaleimide have little effect on the thermal properties of RBG since the major protein component, vicilin, is devoid of disulfide bonds.