Application of maximum bubble pressure surface tensiometer to study protein-surfactant interactions

Binding of a surfactant to proteins can affect their physicochemical stability and solubility in a formulation. The extent of the effect depends on the binding stoichiometry. In this study, we have utilized the technique of maximum bubble pressure surface tensiometry to characterize the binding between human serum albumin (HSA) and surfactants (sodium dodecyl sulfate (SDS) and polysorbate 80) by dynamic surface tension measurements. Results show that two classes of binding sites are present in HSA for SDS, 5 primary binding sites with high binding affinity (Ka = 5.38 × 105 M−1) and 12 secondary binding sites with low affinity (Ka = 6.7 × 104 M−1). The binding is high affinity and limited capacity due to both, ionic and hydrophobic interactions between HSA and SDS. For polysorbate 80, the binding does not follow the Scatchard plot, and is low affinity and high capacity, indicating that polysorbate 80 interacts with HSA through hydrophobic interactions. The results show that maximal bubble pressure surface tensiometry is a fast and convenient technique to determine the concentration of free and bound surfactants in the presence of proteins.