Changes of glycinin conformation due to pH, heat and salt determined by differential scanning calorimetry and circular dichroism

Conformational changes of glycinin caused by either pH, heat or salt were investigated using differential scanning calorimetry (DSC) and circular dichroism (CD). Glycinin of more than 97% purity was used for the experiments. Without the presence of salt (NaCl), glycinin (protein concentration of 16%) was most stable at pH 4.5; the temperature for midpoint of transition (Tm) and enthalpy change ((delta)H) were 93.5 °C and 18.1 J g^-1 protein, respectively. Both parameters decreased gradually as the pH of the protein solution was increased to 9.5. The endothermic peak of glycinin disappeared when the pH was (greater than)11.5 or (less than)<3.0. DSC results showed that with increasing of salt concentration, glycinin was substantially stabilized even in acidic (pH 3.0) and alkaline (pH 11.5) conditions; the degree of stabilization was more significantly affected at the acidic pH. The CD profiles of soy glycinin were, however, not significantly altered even in acidic and alkaline conditions. Thermal denaturation was suppressed by NaCl even in 90 °C, pH 8.0, when the salt concentration was 0.45 m. The stabilizing effects of NaCl coincided with an increase in the percentage of (alpha)-helix in glycinin. Therefore, soy glycinin appeared to have a well-ordered structure under the pH conditions studied, especially in the presence of NaCl. The (lambda) min signal at 222 nm remained unchanged up to 80 °C, but the negative intensity increased substantially when the temperature was (greater than)80 °C.