Association between protein particles and long amphiphilic polymers: Effect of the polymer hydrophobicity on binding isotherms

Association isotherms of bovine serum albumin (BSA) onto hydrophobically modified polyacrylates (HMPA) in aqueous solution were determined by frontal analysis continuous capillary electrophoresis experiments. A set of alkyl-grafted polyacrylates was studied by varying the degree of grafting and the length of the dangling alkyl groups, keeping constant the molar mass and the polydispersity. Binding isotherms indicate that both the concentration of free BSA and the hydrophobicity of the HMPA control the association, which is anti-cooperative. In the presence of a large excess of protein, the number of bound proteins per HMPA chain was found to increase linearly with an increase in the percentage of the dangling groups along the backbone. Although HMPA modified with dodecyl groups required higher protein concentrations to reach the saturation than those with octadecyl grafts, the results suggest that the saturation composition of the complexes correspond to three to four alkyl chains bound per protein. The quantitative analysis of the isotherms reveals that Hill's equation fits well the binding data, leading to an estimate of the anticooperativity index and of the association constant as a function of the polymer structure.