Differential scanning calorimetric studies on bovine serum albumin: I. Effects of pH and ionic strength.

Using defatted and SH-blocked bovine serum albumin (BSA), the measurement of differential scanning calorimetry (d.s.c.) was performed in the range pH 3-11 and ionic strength 0.001-1 M. The shape of the d.s.c. curve was classified into four regimes: (i) the curve with no peak, (ii) that with a peak, (iii) that with a peak having a shoulder, and (iv) that with two peaks. The presence of two peaks was interpreted by the concept of 'heat-induced transition'. The BSA molecule is composed of two domains, thermodynamically independent owing to the formation of a crevice in BSA in a particular range of pH and ionic strength; this gives two peaks in the d.s.c. curve. The enthalpy (delta H) from the d.s.c. curve was plotted against pH and against the NaCl concentration. The value of delta H increased with the increase in the ionic strength in the pH range 5.6-9.0. The temperature of thermal denaturation (the temperature of the peak maximum, Td) was raised with the increase in the ionic strength in the pH range 4.5-9.0, but was lowered in the pH range 3.5-4.0. BSA was stabilized in the neutral-alkaline pH range by the presence of NaCl, but was destabilized in the acidic pH range.