A) HDX rates for the c-terminal isoleucine in peptide YPI measured by NMR at various pD. Solid lines represent fits of exponential decay functions. B) Plot of the log of the exchange rate, log(kex), as a function of pD for both YPI (green) and 1,3-dimethylimidazolium (gray). The slopes (m) of the lines are shown next the linear fit. C) pD dependence of HDX at the C-2 proton for 1,3-dimethylimidazolium measured by NMR at various pD. D) General mechanism for HDX at the C-2 proton for 1,3-dimethylimidazolium.