Hofmeister Anion Effects on Protein Adsorption at an Air–Water Interface

Abstract

Hofmeister anion effects on adsorption kinetics of the positively charged lysozyme (pH < pI) at an air-water interface were studied by surface tension measurements and time-resolved X-ray reflectometry. In the salt-free solution, the protein adsorption rate increases with decreasing the net positive charge of lysozyme. When salt ions are dissolved in water, the protein adsorption rate drastically increases, and the rate is following an inverse Hoffmeister series (Br(-) > Cl(-) > F(-)). This is the result of the strongly polarized halide anion Br(-) being attracted to the adsorbed protein layer due to strong interaction with local electric field, while weakly polarized anion F(-) having no ability to penetrate the protein layer. In X-ray reflection studies, we observed that the lysozyme molecules initially adsorbed on the air-water interface have a flat unfolded structure as previously reported in the salt-free solution. In contrast, in the concentrated salt solutions, the lysozyme molecules begin to refold during adsorption. This protein refolding as a result of protein-protein rearrangements may be a precursor phenomenon of crystallization. The refolding is most significant for Cl(-), which is a good crystallization agent, whereas it is less observed for the strongly hydrated F(-). It is widely known in the bulk state that kosmotropic anions tend to precipitate proteins but at the same time stabilize proteins against denaturing. On the other hand, at the air-water interface where adsorbed proteins usually unfold, we observed chaotropic anions strongly bound to proteins that reduce electrostatic repulsion between protein molecules, and subsequently they induce protein refolding whereas the kosmotropic anions do not.

Salting Up of Proteins at the Air/Water Interface

“…Most previous studies used buffer solutions, which are at relatively low ionic strength. 15,17,[24][25][26]…”

“…VSFG, as a second order nonlinear optical technique, has inherent interface selectivity, and has become a useful spectroscopic tool to detect proteins at air/liquid interface in situ. ,− Most of the previous VSFG experiments have focused on high protein concentrations (∼1000 ppm), and not at low environmentally relevant concentration (∼2 ppm). , Additionally, although the effects of salts on protein behavior in bulk solutions have been extensively studied, − the influence of relatively high concentrations of salts on proteins at the air/water interface has hitherto not been demonstrated. Most previous studies used buffer solutions, which are at relatively low ionic strength. ,,− …”

The Hofmeister Anion Effect on Ionophore‐based Ion‐selective Nanospheres Containing Solvatochromic Dyes

“…The positive nature of the SDs ions, however, brings possible interaction between various anions and the SDs. In fact, Hofmeister series was used to describe the specific‐ion effect regularly observed in systems such as latex dispersion , chromatographic separation , and protein adsorption . Adsorption of anions on metal surface to form electric double layers was previously documented in electrochemistry textbooks .…”

Evaluating polymeric biomaterial–environment interfaces by Langmuir monolayer techniques

“…Lysozymes form stable and homogeneous layers at the air -water interface, which can be adjusted by the pH value and the salt concentration [140]. The protein adsorption rate decreases with increasing the positive charge of lysozyme [141]. The equilibrium time period depends on the subphase ion in the following order: buffer , water , saponite dispersion.…”

“…The effect of Hofmeister anion (NaX, X ¼ I, Br, Cl, F) on the adsorption kinetics of the positively charged lysozyme (below the isoelectric point of 11.35) is evaluated by surface tension measurements and time-resolved X-ray reflectometry [141]. The presence of salt increases the protein adsorption rate following an inverse Hofmeister series caused by the interaction of strongly polarized halide anion Br-with the local electric field of the layer which is not the case for F 2 .…”