The pH dependent binding of α-lactalbumin to the phospholipid vesicles was studied sedimenting the vesicles after the chemical equilibrium was reached. Unilamellarity of these vesicles was checked by electron microscopy.
Binding reactions carried out in 10 mM buffers at several pH values with and without added NaCl. The amount of binding of α-lactalbumin to the vesicles in the 10 mM buffers showed maximum at pH 4 through the each pH range : (pH7-2). However, at 100 mM ionic strength(90 mM NaCl and 10 mM buffer), the binding affinity increased with decreasing pH value and the rate of decrease was steepest between pH value of 3 and 4. The binding was found to be reversible at pH 4 but it was irreversible at pH 2. It appears that, therefore, at pH ranges below 4, hydrophobic interactions are involved for the binding reaction. The pH dependent structural changes of secondary and tertiary structure of α-lactalbumin were also studied by monitoring the circular dichroism spectra. The difference ultraviolet spectra and fluorescence spectra were also used to determine the pH dependent structural changes of α-lactalbumin.
It was found that there is a close relationship between the binding affinity and the extent of the formation of the "molten globular structure" of α-lactalbumin.