Quenching of BSA intrinsic fluorescence by alkylpyridinium cations: Its relationship to surfactant-protein association

Alkyl pyridinium ions readily quench the fluorescence arising from Trp groups incorporated to a dipeptide (Ala–Trp) or protein (BSA). The quenching of the dipeptide fluorescence takes place with a Stern–Volmer constant of 730 M−1 in aqueous solution, irrespective of the length of the alkyl chain (octyl, dodecyl or hexadecyl). On the other hand, the quenching of BSA fluorescence strongly depends upon the length of the alkyl chain. Octyl pyridinium ion presents a normal behavior, with KSV values of 310 and 280 M−1 in aqueous solution and in presence of 8 M urea, respectively. These values are independent of the protein concentration. Dodecyl and hexadecyl pyridinium ions are considerably more efficient as quenchers. Stern–Volmer plots show a strong upward curvature, the quenching efficiency decreases when the protein concentration increases, and is considerably reduced in presence of salt (phosphate) or urea. These results are explained in terms of a static quenching between the adsorbed alkyl pyridinium and the excited Trp groups. From the dependence of the quenching efficiency with the protein concentration it is obtained the alkyl pyridinium/BSA association constant. The values obtained indicate that the binding is dominated by hydrophobic (dependence with the alkyl chain length and urea concentration) effects and, in minor degree, by electrostatic interactions.