Effects of heating at neutral and acid pH on the structure of β-lactoglobulin A revealed by differential scanning calorimetry and circular dichroism spectroscopy

The structural change of β-lactoglobulin A (βLG A) on heating was measured at pH 3.0 and 7.5 with UV absorption difference spectra, differential scanning calorimetry (DSC), and circular dichroism (CD). At pH 3.0, βLG A showed a reversible structural change by heating at 80 °C, while an irreversible change was observed and molecular aggregates of βLG were formed by heating at 95 °C. DSC analysis of βLG A gave endothermic peaks at 75 °C and 90 °C at pH 7.5, and 90 °C at pH 3.0. At pH 7.5, βLG A modified with N-ethylmaleimide (NEM-βLG A) gave two endothermic peaks: at 72 °C and 90 °C. CD spectra of βLG A heated at various temperatures and pHs were measured and the spectra at pH 3.0 and 7.5 were not changed by heating to 95 °C and 80 °C, respectively. Unheated NEM-βLG A gave a spectrum similar to that of heated βLG A, suggesting that the secondary structure was changed by NEM treatment.