Binding of sodium dodecyl sulphate and dodecyl trimethyl ammonium chloride to β-lactoglobulin: A calorimetric study

Thermally induced unfolding of β-lactoglobulin in the presence of surfactants was studied by differential scanning calorimetry (DSC). Anionic surfactants, sodium dodecyl sulphate (SDS), and cationics, dodecyl trimethyl ammonium chloride (DOTAC), were used. In a solution containing a 1:1 molar ratio of SDS and β-lactoglobulin, the protein was stabilised as evidenced by a substantial increase in the unfolding temperature. Further increase of the SDS concentration causes unfolding of the protein. As opposed to these effects of the anionic surfactant, a slight decrease in the unfolding temperature was observed in the presence of DOTAC under similar conditions. An increase of the DOTAC/β-lactoglobulin molar ratios above 1:1 causes precipitation of the protein. The cationic surfactant could be fairly easily removed from a mixed β-lactoglobulin/DOTAC solution by dialysis. The anionic surfactant appeared to interact more strongly with the protein and the 1:1 molar interaction with SDS was impossible to separate by dialysis. The experimental findings are discussed in terms of possible binding sites for the surfactant and connected to micelle formation of the surfactants which is related in a predictable scheme to temperature and ionic strength effects.