Comparative studies on interactions of bovine serum albumin with cationic gemini and single-chain surfactants

The interactions of bovine serum albumin (BSA) with cationic gemini surfactants alkanediyl-alpha,omega-bis(dodecyldimethylammonium bromide) [C12H25(CH3)2N(CH2)(S)N(CH3)2C12H25]Br2 (designated as C12C(S)C12Br2, S = 3, 6, and 12) and single-chain surfactant dodecyltrimethylammonium bromide (DTAB) have been studied with isothermal titration microcalorimetry, turbidity, fluorescence spectroscopy, and circular dichroism at pH 7.0. Comparing with DTAB, C12C(S)C12Br2 have much stronger binding ability with BSA to induce the denaturation of BSA at very low molar ratio of C12C(S)C12Br2/BSA, and C12C(S)C12Br2 have a much stronger tendency to form insoluble complexes with BSA. The binding of C12C(S)C12Br2 to BSA generates larger endothermic peaks. The first endothermic peak is much stronger than that of the second endothermic peak. The double charges and strong hydrophobicity of the gemini surfactants are the main reasons for these observations. In addition, the spectra results show that the binding of DTAB to BSA only promotes BSA unfolding and aggregation, whereas the secondary structure of BSA is possibly stabilized by a small amount of C12C(S)C12Br2 , even if the small amount of binding C12C(S)C12Br2 could induce the loss of the tertiary structure of BSA. This result may be related to the double tails of gemini surfactants, which may generate the hydrophobic linkages between the nonpolar residues of BSA.