Anion modulation of the 1H/2H exchange rates in backbone amide protons monitored by NMR spectroscopy

The ability of three anionic cosolutes (sulfate, thiocyanate, and chloride) in modulating the (1)H/(2)H exchange rates for backbone amide protons has been investigated using nuclear magnetic resonance (NMR) for two different proteins: the IGg-binding domain of protein L (ProtL) and the glucose-galactose-binding protein (GGBP). Our results show that moderate anion concentrations (0.2 M-1 M) regulate the exchange rate following the Hofmeister series: Addition of thiocyanate increases the exchange rates for both proteins, while sulfate and chloride (to a less extent) slow down the exchange reaction. In the presence of the salt, no alteration of the protein structure and minimal variations in the number of measurable peaks are observed. Experiments with model compounds revealed that the unfolded state is modulated in an equivalent way by these cosolutes. For ProtL, the estimated values for the local free energy change upon salt addition (m (3,DeltaG )) are consistent with the previously reported free energy contribution from the cosolute's preferential interaction/exclusion term indicating that nonspecific weak interactions between the anion and the amide groups constitute the dominant mechanism for the exchange-rate modulation. The same trend is also found for GGBP in the presence of thiocyanate, underlining the generality of the exchange-rate modulation mechanism, complementary to more investigated effects like the electrostatic interactions or specific anion binding to protein sites.