Nuclear magnetic resonance studies of exchangeable protons. II. The solvent exchange rate of the indole nitrogen proton of tryptophan derivatives.

The exchange rate of the indole nitrogen proton with solvent water protons was measured as a function of pH and temperature for tryptophan, N-methyl tryptophan, glycyl tryptophan, tryptophanamide, tryptophylglycine, and tryptophylglycyl glycine. The nmr observation was by long-pulse Fourier transform methods, and kinetics were inferred from saturation recovery, H₂O transfer of saturation, and linewidth. There are observable differences between the rates of these compounds, but all are describable within a factor of two by specific acid and base-catalyzed rates k_H = 100 and k_OH = 10⁸ 1/mol-sec at 27°C. It is concluded that this behaviour is representative of this proton on the indole side chain in a random-chain peptide exposed to water.