Mesodynamics in the SARS nucleocapsid measured by NMR field cycling

Clarkson, Michael W.; Lei, Ming; Eisenmesser, Elan Z.; Labeikovsky, Wladimir; Redfield, Alfred; Kern, Dorothee

While the majority of residues have high order parameters and short internal correlation times typical for a stable, folded structure, residues with the above described unusual R 1 and NOE dispersions fit to model 5. These model 5 residues are characterized by high S 2 f and low S 2 s values, indicative of local order combined with largeramplitude slower fluctuations. Critically, the field dependence of the R 1 and NOE data enabled us to robustly determine the correlation time s s of these slower fluctuations. The striking similarity of s s of about 0.8 ns for the entire b-hairpin and adjacent loops (Figs. 1, 5) suggests that these residues undergo a correlated motion as a stable structural unit. This is consistent with the structure showing a regular hydrogen-bonded b-hairpin as well the chemical shifts in this region (Cornilescu et al. 1999) . The high accuracy of the determined s s values arises from the crossover in the R 1 data and the pronounced NOE dispersion data as illustrated by simulated relaxation parameters (Fig. 6) . These theoretical curves emphasize the value of field-cycling data in identifying the R 1 crossover and thereby defining the timescale of the collective mesodynamics.