Effective charge measurements reveal selective and preferential accumulation of anions, but not cations, at the protein surface


Specific‐ion effects are ubiquitous in nature; however, their underlying mechanisms remain elusive. Although Hofmeister‐ion effects on proteins are observed at higher (>0.3M) salt concentrations, in dilute (<0.1M) salt solutions nonspecific electrostatic screening is considered to be dominant. Here, using effective charge (Q*) measurements of hen‐egg white lysozyme (HEWL) as a direct and differential measure of ion‐association, we experimentally show that anions selectively and preferentially accumulate at the protein surface even at low (<100 mM) salt concentrations. At a given ion normality (50 mN), the HEWL Q* was dependent on anion, but not cation (Li+, Na+, K+, Rb+, Cs+, GdnH+, and Ca2+), identity. The Q* decreased in the order F− > Cl− > Br− > NOurn:x-wiley:09618368:media:PRO591:tex2gif-stack-1 ∼ I− > SCN− > ClOurn:x-wiley:09618368:media:PRO591:tex2gif-stack-2 ≫ SOurn:x-wiley:09618368:media:PRO591:tex2gif-stack-3, demonstrating progressively greater binding of the monovalent anions to HEWL and also show that the SOurn:x-wiley:09618368:media:PRO591:tex2gif-stack-4 anion, despite being strongly hydrated, interacts directly with the HEWL surface. Under our experimental conditions, we observe a remarkable asymmetry between anions and cations in their interactions with the HEWL surface.


Molecular, Cellular and Biomedical Sciences

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Protein Science



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