Conformational Transitions in β-Lactoglobulin Induced by Cationic Amphiphiles: Equilibrium Studies

AbstractThe conformational transition from the native state in water (“β-state”) to a state containing a considerable amount of α-helices (“α-state”) was studied for the protein β-lactoglobulin (BLG), from bovine milk, in several colloidal solutions containing mixed micelles or spontaneous vesicles. These aggregates were formed in the bicationic system containing the surfactant dodecyltrimethylammonium chloride (DTAC) and the lipid didodecyldimethylammonium bromide (DDAB). The β→α transition in BLG, investigated by far-ultraviolet circular dichroism spectroscopy, is induced to the same protein α-state by pure and mixed DDAB/DTAC micelles or vesicles. This implies a similar interaction mechanism of BLG with DDAB or DTAC, once the colloidal aggregates are formed. In premicelle DTAC solutions, the fraction of α-helix is lower and increases with the DTAC concentration. DDAB and DTAC also promote conformational changes in the protein tertiary structure that expose the tryptophans to a less constrained environment. These unfolding transitions were investigated by near-ultraviolet circular dichroism and steady-state fluorescence spectroscopies. In equilibrium conditions, it was found that higher DTAC (and, probably, DDAB) concentrations are needed to induce the β→α transition than to unfold the protein. β-Lactoglobulin may therefore be considered as a model for protein–surfactant and protein–lipid interactions