Backbone dynamics of proteins derived from carbonyl carbon relaxation times at 500, 600 and 800 MHz: Application to ribonuclease T1

The backbone dynamics of uniformly C-13/N-15-enriched ribonuclease T1 have been investigated using carbonyl carbon relaxation times recorded at three different spectrometer frequencies. Pulse sequences for the determination of the longitudinal (T-1) and transverse (T-2) relaxation times are presented. The relaxation behaviour was analysed in terms of a multispin system. Although the chemical shift anisotropy relaxation mechanism dominates at high magnetic field strength, the contributions of the dipole-dipole interactions and the cross-correlation between these two relaxation mechanisms have also been considered. Information about internal motions has been extracted from the relaxation data using the model-free approach of Lipari and Szabo in order to determine order parameters (S-2) and effective internal correlation times (tau(i)). Using a relatively simple relation between the measured relaxation rates and the spectral density function, an analytical expression for the microdynamical parameters in dependence of T-1 and T-2 has been derived. The spectral density mapping technique has been applied in order to study the behaviour of the carbonyl carbon resonances in more detail.