Ionic strength and pH effects on optical thermographs for bovine serum albumin (BSA)

An optical method using a custom-built system was investigated to study heating effects on bovine serum albumin (BSA) as affected by pH and ionic strength (IS). Changes in the optical rotation (OR) and transmitted light (TL) of BSA in 0.01 M phosphate buffer at pH 6.1, 7, and 7.9 with IS maintained at 0.04, 0.08, and 0.16 were monitored while heating the solution at ∼6 °C/min from room temperature to ∼85 °C. There was no significant effect of IS on BSA's denaturation temperature (T_d) values. T _d was affected by pH with lower values observed at pH 7.9 than at pH 7 but not statistically different from values at pH 6.1. Values at pH 6.1 and 7 were not significantly different either. These observations were consistent with literature values determined by differential scanning calorimetry. Changes in the TL signal, reflecting the formation of an opaque gel, were not detected at pH 7.9, identified only at the two highest IS at pH 7, and observed always at pH 6.1, i.e., closest to the pH value for the isoelectric point (pI) of BSA reported to be around pH 4.7 to 5.2. Promoting of gel formation by IS was attributed to the screening of intra- and intermolecular electrostatic forces.