Synergy between Hofmeister effect and coupled water in proteins: Unusual dilational moduli of BSA at air/solution interface

This study relates interfacial interactions of bovine serum albumin (BSA) molecules in dilute solutions with its dilatational rheology. Dynamic surface tension and the associated dilational elastic modulus and viscosity for BSA and mixtures of BSA with Hofmeister electrolytes—NaCl, NaClO₄, Na₂SO₄, NaF and Na₂HPO₄ have been studied using a sinusoidal surface compression and expansion for frequencies ranging from 0.01 to 0.4 Hz. at solution/air interface. In all the BSA + electrolyte systems, both the elastic modulus and viscosity show unusually high values compared with pure BSA or pure electrolytes. In the presence of NaF and Na₂SO₄ the viscosity of protein increases almost by 50–80-fold and the corresponding elastic modulus also changes by 30–50-fold. Hydrated Hofmeister ions surely influence the measured rheological properties. In addition, the synergistic effect of the hydrated protein and the vicinal hydrated electrolytes possibly contribute to the high viscosity and elasticity due to change in dynamics of these assemblies. Thus the behavior of BSA is effected by salts in different ways, especially due to the dynamics and strength of the water molecules in the assembly.