Off-resonance rf fields in heteronuclear NMR: Application to the study of slow motions

The advantages of using off-resonance rf fields in heteronuclear self-relaxation experiments are explored on a fully (15)N-enriched protein. It is firstly shown that in the absence of slow motions the longitudinal and transverse (15)N self-relaxation rate values derived with this method are in agreement with the ones measured by the classical inversion-recovery and Carr-Purcell-Meiboom-Gill (CPMG) sequences, respectively. Secondly, by comparing the (15)N transverse self-relaxation rates obtained by the proposed off-resonance sequence and by the CPMG sequence, 11 residues out of the 61 of toxin alpha are shown to exhibit a chemical exchange phenomenon in water on a time scale ranging from 1 mu s to 100 ms. By varying the effective field amplitude, chemical exchange processes involving these residues an measured and the corresponding correlation times are evaluated without having assumed any motion model. Similar, though less precise, results are given by the analysis of the (15)N off-resonance self-relaxation rates on the basis of the Lipari-Szabo model to describe the fast internal dynamics of toxin alpha.