Investigation of protein amide-proton exchange by 1H longitudinal spin relaxation.

The general theory of the Linderstrøm-Lang model, which, in simplified form, is widely used for the interpretation of NMR data on backbone-proton exchange in proteins, is systematically discussed. An experimental protocol for testing the applicability of the customary simplifications is described and experimental data which require the application of the general, rather than of the simplified, theory are presented. The appearance of pH-dependent biexponential magnetization recovery for any amide group in a protein is an unequivocal indication that the conventional simplified versions of the model do not apply.