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

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 ( Br− > NO 3− ∼ I− > SCN− > ClO 4− ≫ SO 42−, demonstrating progressively greater binding of the monovalent anions to HEWL and also show that the SO 42− 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.