Transition temperatures of heat-induced structural changes in ovalbumin solutions at acid and neutral pH

Heat-induced changes in secondary and tertiary structures of ovalbumin solutions at pH 3 and 7 were studied by far UV-circular dichroism, differential scanning calorimetry and intrinsic viscosity measurements Despite the difference in concentration of the solutions used by the two first techniques (0.015 and 5% respectively), it seemed that the transition temperatures T-[theta] and T-i, corresponding to the inflexion of the ellipticity values at 222 nm (alpha-helical structures) and the first increase in heat flow signal respectively, were in fair agreement. Compared with the observations at pH 7, at pH3 the absolute value of ellipticity at 213 nm (from beta-structures) increased while the ellipticity at 222 nm decreased at this pH, heat-treatment enhanced the development of apparent beta-sheet structures at the expense of helical structures. At room temperature, the intrinsic viscosity of ovalbumin at pH 3 was similar to 30% higher than at pH 7. Increasing values were found when protein solutions were pre-heated over a critical temperature which was lower (pH 3) or slightly higher (pH 7) than T-[theta] and T-i. At pH 3 this temperature corresponded to the increase in beta-structures whereas it corresponded to the decrease in alpha-structures at pH 7. All of the results reported in this study could be explained by a close relationship between the heat-induced development of beta-structure and aggregation mechanisms at pH 3-unlike pH 7, where the disappearance of the helical structure coincided with an increase of unordered structures.