A fluorescent probe study of salmine AI.

A fluorescent probe, 1-p-toluidinylnapthalene-8-sulfonate (1,8-TNS), was used to study the nonpolar sites on salmine AI. Fluorescence enhancement resulting from binding between the probe and the protein occurs at a wavelength of maximum emission of 497-500 nm, indicating the existence of moderately nonpolar binding sites on salmine AI. Fluorescence enhancement decreases as the ionic strength of the solvent is increased from 0.002 M to 0.050 M. Fluorescence increases with increasing acidity although this effect is not correlated to the pKa of 1,8-TNS. Positive cooperative binding takes place between 1,8-TNS and salmine AI. Equilibrium dialysis indicates that binding occurs only under conditions resulting in significant fluorescent enhancement. The binding was also studied using thin film dialysis, which is much faster than equilibrium dialysis and avoids the observed changes in probe-protein interaction that occur over long time periods with the latter system.