pH Stability Study of Lectin from Black Turtle Bean (Phaseolus vulgaris) as Influenced by Guanidinium–HCl and Thermal Treatment

The pH stability of lectin from black turtle bean was characterized by a variety of biophysical techniques over the pH range from 2.0 to 10.0. The zero-order and second-derivative UV spectra at different pH values indicated that the tertiary structure of the protein does not change significantly with changes in pH. The maxima emission fluorescence of the lectin upon excitation at 280 nm and 295 nm were found to be blue shifted 1.2 nm at pH 2.0, and with a red-shift of 1.0 nm at pH 10.0. The chemical denaturation results obtained by monitoring the intrinsic fluorescence of the lectin indicated that unfolding of the protein induced by guanidine hydrochloride (GdnHCl) could be described using a three-state model. Complete unfolding of the protein was observed at pH 7.2 in the presence of 6.5 M GdnHCl after 2 weeks, which suggested that the lectin was stable at various pHs. Irreversible thermal denaturation of lectin was also investigated at various pHs by differential scanning calorimetry (DSC). The first-order two-state kinetic model was applied to explain the scan-rate dependent DSC transitions. Both the higher activation energy (Ea) computed from the irreversible thermal denaturation and the three-state chemical denaturation process suggested that the lectin structure remained significantly unchanged from pH 2.0 to 10.0, which indicated that the structure of the lectin was stable over a wide pH range.