pH-dependence of the alpha-lactalbumin structure: a fluorescence study.

The fluorescence parameters of demetallized alpha-lactalbumin in the range from pH 8 to 2 show an extreme around pH 5-4 (a minimum in quantum yield and wavelength and a maximum in polarization). This extreme is not due to a competition between Ca2+ and protons but rather to a stabilization of the conformation of the protein near the isoelectric pH by the ionic interactions between local positive and negative charges on the protein. The calcium-free protein has similar fluorescence characteristics at pH 2 and 8 but the thermal transition curve is different. The influence of 0.1 M NaCl is also considered.