Investigation of protein-surfactant interactions in aqueous solutions

It is essential to understand protein-surfactant interactions in order to optimize the use of surfactants as stabilizers in protein formulations. Therefore, interactions between surfactants and human serum albumin (HSA), and the effect of solution environment on the interactions were investigated.^ Interactions between surfactants and HSA were investigated by measuring the tryptophan fluorescence intensity, circular dichroism and maximal bubble pressure of HSA in the presence of the surfactants. The studies showed that sodium dodecyl sulfate (SDS) quenched the tryptophan fluorescence and decreased the $\alpha$-helical content slightly at low concentrations ( (SDS) / (HSA) $\le$ 100), and denatured the protein at high concentrations ( (SDS) / (HSA) $\ge$ 300). The presence of polysorbate 80 did not alter the protein conformation significantly. A blue spectral shift indicated that tryptophan was in a more hydrophobic environment and the interaction between polysorbate 80 and HSA was hydrophobic.^ Changes in solution pH, ionic strength and temperature affected the protein conformation of HSA indicated by changes in the tryptophan fluorescence and the $\alpha$-helical content. The presence of polysorbate 80 (0.002% for 5 $\mu$M HSA) reduced such changes in protein conformation. Irreversible changes in protein conformation during heating and cooling cycle as a result of thermal denaturation were observed. During the thermal cycle, the transition temperature where HSA molecules start to unfold and denature was increased from 55$\sp\circ$C to 60$\sp\circ$C, and the formation of aggregates was hindered by polysorbate 80. Results indicated that polysorbate 80 stabilized HSA against changes in solution environment.^ The maximal bubble pressure surface tension technique is based on the relative mobility of molecules to a newly formed surface, and was used to determine the concentration of free surfactant in HSA-surfactant mixtures. Binding studies showed two classes of high affinity binding sites for SDS on HSA, and one class of binding site with low affinity and high capacity for polysorbate 80. The binding affinity of polysorbate 80 for HSA was much lower than the primary binding affinity of SDS for HSA, indicating that the interaction between SDS and HSA was stronger than that between polysorbate 80 and HSA.