Mesodynamics in the SARS nucleocapsid measured by NMR field cycling

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

Field dependence of R 1 relaxation rates At 500 MHz (11.7 T), R 1 values are roughly constant across the entire SARSN, but R 1 heterogeneity significantly increased as the magnitude of the field was changed in either direction (Fig. 2) . When R 1 was measured at a 17.3 MHz field sampled by the field-cycling apparatus, for several residues R 1 was decreased by as much as 25% relative to the majority of the residues (Fig. 2a) . This reduction is also observed at 7 T, but to a lesser extent. These residues were primarily localized to the b-hairpin (residues 90-108) and a loop region spanning residues 60-64. On the contrary, increasing the field strength above 11.7 T resulted in an enhanced R 1 for the same set of residues relative to the rest of the protein. At the highest J Biomol NMR (2009) 45:217-225 219 field employed in this study, 91.2 MHz, the enhancement was as high as 40% (Fig. 2b) . Qualitatively, this ''R 1 crossover'' implies that these residues have greater flexibility than the remainder of the protein. Lower R 1 values are expected for flexible residues at low field because the internal motion depresses the spectral density in the extreme narrowing limit (xs m ( 1). On the contrary, in the spin-diffusion limit (xs m ) 1), flexible residues are expected to have a greater spectral density and thus a higher R 1 than rigid residues. Moreover, the particular frequency at which the crossover occurs defines an approximate range of internal correlation times for these residues. For a protein of this size, the observed R 1 crossover point implies a motion with an internal correlation time in the range of 0.5-1.5 ns.