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 13C/15N-enriched ribonuclease T1 have beeninvestigated using carbonyl carbon relaxation times recorded at three different spectrometerfrequencies. Pulse sequences for the determination of the longitudinal (T1) and transverse (T2)relaxation times are presented. The relaxation behaviour was analysed in terms of a multispinsystem. Although the chemical shift anisotropy relaxation mechanism dominates at highmagnetic field strength, the contributions of the dipole-dipole interactions and thecross-correlation between these two relaxation mechanisms have also been considered.Information about internal motions has been extracted from the relaxation data using themodel-free approach of Lipari and Szabo in order to determine order parameters (S2) andeffective internal correlation times (taui). Using a relatively simple relation between themeasured relaxation rates and the spectral density function, an analytical expression for themicrodynamical parameters in dependence of T1 and T2 has been derived. The spectraldensity mapping technique has been applied in order to study the behaviour of the carbonylcarbon resonances in more detail.