Interactions of globular proteins with surfactants studied with fluorescence probe methods

Steady-state and dynamic fluorescence measurements were used to investigate the interactions and structures of complexes formed between bovine serum albumin (BSA) and anionic, cationic, and nonionic surfactants: sodium dodecyl sulfate (SDS), N-cetyl-N,N,Ntrimethylammonium bromide (CTAB), and octaoxyethylene glycol n-dodecyl ether (C(12)E(8)) respectively. The lysozyme (Lys)-SDS complex was also studied. The measurements were carried out at different pH's and ionic strengths. In all systems, micellelike clusters adsorbed on protein were evidenced. The average aggregate numbers are smaller than those of free micelles and are not strongly influenced by pH and ionic strength. The fluorescence lifetime of pyrene in BSA/surfactant complexes was constant at low surfactant concentrations, started to decrease at approximately the same protein-surfactant ratio (0.15 mM BSA/1 mM surfactant) regardless of the surfactant type or pH buffer, and, at high surfactant concentrations, merged to the lifetime values corresponding to free micelles. The results of the fluorescence techniques support a "necklace and bead" model of the complex for BSA/surfactant systems, with protein wrapped around the micelles. For the Lys/SDS complex, the model essentially is the same; however, some differences, due to their different sizes appear. Lysozyme is smaller and more rigid and does not wrap up well around the micelle.