Determination of the activation volume of the uncatalyzed hydrogen exchange reaction between N-methylacetamide and water

The amide-water hydrogen exchange rates of the individual backbone amide protons are very sensitive to the local environment of the protons and can be used to probe fluctuations in the local conformations of proteins. Hydrogen exchange between N-methylacetamide (NMA) and water is important in that it serves as a model for the amide-water hydrogen exchange in peptides and proteins. The rate constants for amide-water hydrogen exchange between 16 mol% NMA and water at 66 degrees C were measured at pressures ranging from 1 bar to 5 kbar at three different pH values using the NMR magnetization transfer technique, and the apparent activation volumes of hydrogen exchange were obtained at each pH. The activation volume of the acid-catalyzed hydrogen exchange reaction was calculated to be +1.7 cm(3) mol(-1), and the activation volume of the base-catalyzed hydrogen exchange reaction was calculated to be +11.0 cm(3) mol(-1). The hydrogen exchange rate constants were also measured as a function of concentration and pH at ambient pressure at 66 degrees C so that the rate constants of the acid-, base-, and uncatalyzed hydrogen exchange reactions could be found. With these values, the activation volume of the uncatalyzed hydrogen exchange reaction was calculated to be equal to -9.0 cm(3) mol(-1). This quantity has not been previously determined and is required in hydrogen exchange studies of peptides and proteins at pressures above ambient pressure.