Interactions of bovine serum albumin with ionic surfactants in aqueous solutions

The interactions of bovine serum albumin (BSA) with cationic (DTAB) and anionic (SDS) surfactants in aqueous solution (pH 7.0, 20 mM imidizole, 10 mM NaCl) were studied using isothermal titration calorimetry, differential scanning calorimetry and turbidity measurements. The mixing enthalpy (corrected for demicellization) of DTAB with BSA (0.5 wt%) was endothermic at low surfactant concentrations (0–17 mM), but exothermic at high surfactant concentrations (17–27 mM). BSA was completely denatured when the DTAB concentration exceeded about 4 mM. Insoluble BSA–DTAB complexes formed between 4 and 10 mM DTAB was attributed to charge neutralization of the protein by the surfactant. The corrected mixing enthalpy of SDS with BSA (0.5 wt%) was exothermic at all surfactant concentrations (0–9 mM). BSA became completely denatured when the SDS concentration exceeded about 4 mM. There was no evidence of insoluble protein–surfactant complex formation at any SDS concentration. The corrected mixing enthalpy vs. surfactant concentration profiles could be interpreted in terms of various molecular events, e.g. specific binding, cooperative binding, protein unfolding, protein aggregation and micelle formation. These results have important implications for understanding the influence of surfactants on globular protein functionality in foods.