Ion-Pairing Propensity in Guanidinium Salts Dictates Their Protein (De)stabilization Behavior

Since the proposition of the Hofmeister series, guanidinium (Gdm) salts hold a special mention in protein science owing to their contrasting effect on protein(s) depending on the counteranion(s). For example, while GdmCl is known to act as a potential protein denaturant, Gdm₂SO₄ offers minimal effect on protein structure. Despite the fact that theoretical studies reckon the formation of ion-pairing to be responsible for such behavior, experimental validation of this hypothesis is still in sparse. In this study, we combine electrochemical impedance spectroscopy (EIS) and THz spectroscopy to underline the effect of GdmCl and Gdm₂SO₄ on a model amide molecule N-methylacetamide (NMA). Molecular dynamics (MD) simulation studies predict that Gdm₂SO₄ forms heteroion pairing in water, which inhibits Gdm⁺ ions to approach NMA molecules, while in case of GdmCl, Gdm⁺ ions directly interact with NMA. The experimental findings on ion hydration, specifically the detailed analysis of the ion-water rattling mode, which appears in the THz frequency domain, unambiguously endorse this hypothesis. Our study establishes the fact that the propensity of ion-pairing in Gdm salts dictates their (de)stabilization effect on proteins.