Secondary structures of bovine serum albumin in anionic and cationic surfactant solutions

Abstract

Secondary structure changes of bovine serum albumin were examined in solutions of sodium decyl sulfate (SDeS), sodium dodecyl sulfate (SDS), decyltrimethylammonium bromide (DeTAB), dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), and hexadecyltrimethylammonium bromide (HTAB). The relative proportions of α-helix, β-structure, and random coil were estimated by simulating a mixed circular dichroism (CD) spectrum of reference spectra of the corresponding structures to the experimentally obtained spectrum of the protein. The helical proportion of the protein was 66% at neutral pH. This helical proportion of the protein decreased to 52% in SDeS, to 50% in SDS, to 51% in DeTAB, to 49% in DTAB, and to 47% in TTAB and HTAB. All of these surfactants caused changes in the helical proportion from 66 to approximately 50%. However, the shorter the hydrocarbon chain a surfactant had, the higher the concentration necessary to cause the same decrease in the helical proportion. The results also indicate that the longer the hydrocarbon chain of the surfactant the greater the decrease in the helical proportion. In this study the proportion of β-structure increased slightly in every surfactant solution except DeTAB. The well-known complexes AD n (A = BSA, D = SDS, n = 40–55) and AD 2 n are formed when the helical proportion of BSA decreases to 62 and 54%, respectively. The authors speculated, on the basis of Brown's model, which segments of the helix in BSA unfold in surfactant solutions.