Mesodynamics in the SARS nucleocapsid measured by NMR field cycling

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

We want to note that although the observed relaxation rates at 17.3 and 30.2 MHz, obtained by field-cycling, qualitatively agree with high-field observations, the lowfield R 1 values are 45 and 30% lower than predicted from model-free parameters using high-field data, respectively. Error from interference between CSA and dipolar relaxation Model-free parameters derived from fits of all relaxation data acquired at 500 MHz and above fit to various model-free models (Lipari and Szabo 1982; Clore et al. 1990 ). a Order parameters S 2 (open circle), S 2 s (red filled inverted triangle), S 2 f (orange filled triangle). b Corresponding internal correlation times s e (open circle) or s s (red filled inverted triangle). Error bars are derived from 150 Monte Carlo simulations mechanisms is of particular concern here, due to the inability to regularly invert proton polarization during the relaxation period in the field-cycled experiments. While this effect certainly contributes to the observed bias towards too low R 1 values, it is not possible to account for the entire effect at 17.3 and 30.2 MHz using analytical calculations of g z , implying that the observed bias cannot be attributed to any single source of interference.