Temperature dependence of protein backbone motion from carbonyl 13C and amide 15N NMR relaxation

The NMR spin-lattice relaxation rate (R1) and the rotating-frame spin-lattice relaxation rate (R1rho) of amide 15N and carbonyl 13C (13C') of the uniformly 13C- and 15N-labeled ubiquitin were measured at different temperatures and field strengths to investigate the temperature dependence of overall rotational diffusion and local backbone motion. Correlation between the order parameter of the N-H vector, SNH2, and that of the carbonyl carbon, S2C', was investigated. The effective S2C' was estimated from the direct fit of the experimental relaxation rates and from the slope of 2R2-R1 vs. B2 using Lipari-Szabo formalism. The average SNH2 decreased by 5.9%, while the average S2C' decreased by 4.6% from 15 to 47 degrees C. At the extreme low and high temperatures the difference in the temperature dependence of the order parameters vanishes. At the intermediate temperatures they do not change by the same amount but they follow the same trend. On the same peptide plane along the protein sequence, S2C' and SNH2 are highly correlated. The results suggest that fast local motion experienced at the site of the N-H vector and carbonyl nucleus is more complicated than previously thought and it cannot be easily described by one single type of motion in a broad range of temperature.