Analysis of Hofmeister effects on the density profile of protein adsorbates: a neutron reflectivity study

The effects of various salts on the adsorption of ribonuclease A (RNase) at a hydrophobic poly(styrene) film was analyzed in this study applying neutron reflectometry. It has been found that both the kosmotropic salts, (NH4)2SO4 and Na2SO4, and the chaotropic salts, NaSCN and Ca(SCN)2, significantly reduce the amount of adsorbed protein. Maximum adsorption is observed in the presence of NaCl. Apparently, there is no single Hofmeister effect on the degree of protein adsorption at an aqueous-solid interface that ranges from kosmotropic to chaotropic ions. The observed variations in the adsorbed amount can be attributed to variations in the packing density of the adsorbed protein molecules. The results suggest that kosmotropic salts reduce the degree of protein adsorption by disfavoring a conformational adaptation and a dehydration of the protein molecules at a hydrophobic poly(styrene) film. On the other hand, chaotropic salts shield hydrophobic interactions between the protein molecules and the substrate by saturating hydrophobic patches on the protein surface. Remarkably, the results of this study corroborate earlier findings on the effects of nonionic cosolvents on protein adsorption (Koo, J.; Gutberlet, T.; Czeslik, C. J. Phys. Chem. B 2008, 112, 6292-6295).