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
Digital Object Identifier (DOI)
Gokarn YR, Fesinmeyer RM, Salujal, A, Razinkov V, Chase SF, Laue TM and Brems DN (2011) “Effective charge measurements reveal selective and preferential accumulation of anions, but not cations, at the protein surface” Protein Science, 20, 580-587.