Abstract
Due to the fact that surfactant molecules are known to alter the structure (and consequently the function) of a protein, protein–surfactant interactions are very important in the biological, pharmaceutical, and cosmetic industries. Although there are numerous studies on the interactions of albumins with surfactants, the investigations are often performed at fixed environmental conditions and limited to separate surface-active agents and consequently do not present an appropriate comparison between their different types and structures. In the present paper, the interactions between selected cationic, anionic, and nonionic surfactants, namely hexadecylpyridinium chloride (CPC), hexadecyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), polyethylene glycol sorbitan monolaurate, monopalmitate, and monooleate (TWEEN 20, TWEEN 40, and TWEEN 80, respectively) with bovine serum albumin (BSA) were studied qualitatively and quantitatively in an aqueous solution (10 mM cacodylate buffer; pH 5.0 and 7.0) by steady-state fluorescence spectroscopy supported by UV spectrophotometry and CD spectroscopy. Since in the case of all studied systems, the fluorescence intensity of BSA decreased regularly and significantly under the action of the surfactants added, the fluorescence quenching mechanism was analyzed thoroughly with the use of the Stern–Volmer equation (and its modification) and attributed to the formation of BSA–surfactant complexes. The binding efficiency and mode of interactions were evaluated among others by the determination, comparison, and discussion of the values of binding (association) constants of the newly formed complexes and the corresponding thermodynamic parameters (ΔG, ΔH, ΔS). Furthermore, the influence of the structure of the chosen surfactants (charge of hydrophilic head and length of hydrophobic chain) as well as different environmental conditions (pH, temperature) on the binding mode and the strength of the interaction has been investigated and elucidated.
Highlights
We report the interactions between selected surfactants with bovine serum albumin studied under various experimental conditions
Fluorescence quenching experiments supported by UV spectrophotometry have been applied to unravel and compare the nature of binding of selected cationic, anionic, and nonionic surfactants with different head groups and chains to the circulatory protein bovine serum albumin (BSA)
The quenching mechanism of the albumin induced by sodium dodecyl sulfate (SDS), CPC, CTAB, TWEEN 20, TWEEN 40, and TWEEN 80 has been attributed to a static quenching process
Summary
The macromolecule–surfactant interactions depend on many factors, among which the type of surfactant species (monomer, micelles) presented in the system under study [18,19], the chemical structure of a surfactant (anionic, cationic, amphoteric, or nonionic) [20], its architecture [21,22], as well as experimental conditions (pH, temperature) [23,24] play crucial roles. We report the interactions between selected surfactants (the structures are presented in Figure 1) with bovine serum albumin studied under various experimental conditions. Its relatively wide useful buffering range enables performing the measurements in the same buffer solution and at different pH (5.0 and 7.0)
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