Interaction between β-lactoglobulin and perfluorooctanoate surfactants: Effect of surfactant counterion

The interaction was investigated between β-lactoglobulin (BLG) and fluorinated surfactants perfluorooctanoate n-C7F15COON(CnH2n+1)4 (n = 0, 1, 2, 3, 4). Circular dichroism and fluorescence were used to monitor the structural changes of protein. Conductivity and 19F NMR were used to observe the binding and aggregation of surfactant on the protein. The fluorinated surfactants showed strong interaction with BLG. A striking influence of fluorinated surfactant counterions was observed. When the size of counterions increased from NH4+ to N(C3H7)4+, the BLG–perfluorooctanoate surfactants interaction was weakened. However, when the counterion was N(C4H9)4+, the interaction of BLG–C7F15COON(C4H9)4 was surprisingly enhanced, and was stronger than those of BLG–C7F15COON(CnH2n+1)4 (n = 1, 2, 3). The unusually strong interaction between BLG and C7F15COON(C4H9)4 was ascribed to the cooperative binding of C7F15COO− and N(C4H9)4+ onto BLG.