Interaction of alkylpyridinium chlorides with human serum albumin studied by fluorescence techniques

A study has been made on the interaction of cetylpyridium chloride (CPC) and dodecylpyridinium chloride (DPC) with human serum albumin (HSA) in aqueous solution (22 °C, pH = 7.0). The study was performed using techniques based on the effect promoted by the addition of the surfactants on the fluorescence of the protein tryptophan group. From the dependence of the fluorescence quenching with the protein concentration, the binding isotherms of the surfactants with the protein were determined. The results indicate that CPC is considerably more efficient than DPC in quenching the fluorescence of the tryptophan group. However, even for CPC, eight surfactant molecules must be bound (in average) to each HSA molecule to quench half of the protein intrinsic fluorescence, suggesting an association to zones far away from the locus of the fluorescent moiety. Information regarding the locus of CPC and DPC association to HSA was obtained from their effect on the fluorescence of dansyl derivatives bound to the protein. The results show that: (i) CPC binds to site 2 or near it, quenching the fluorescence of the probe bound to this site and increasing the fluorescence of the probe bound to site 1, and (ii) DPC is less efficient in modifying dansyl derivatives fluorescence, being able to interact weakly with compounds located at both sites 1 and 2.