Measurement of (13)C (α)- (13)CO cross-relaxation rates in (15)N-/ (13)C-labelled proteins.

Internal motions play an important role in the biological function of proteins and NMR relaxation studies may characterize them over a wide range of frequencies. An experimental pulse scheme is proposed for the measurement of the (13)CO-(13)C(α) cross-relaxation rate. For sensitivity reasons, this measurement is performed in an indirect manner through several coherence transfer steps, which should thus be calibrated independently. Contributions of other relaxation pathways can be eliminated by the determination of the initial slope of the buildup curve. The cross-relaxation rates have been determined on a (15)N-/(13)C-labelled 116-residue protein and the significant variations along the sequence have been interpreted as evidence of an increased amount of fast local motion.