Mesodynamics in the SARS nucleocapsid measured by NMR field cycling

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

Fringe field strengths versus height were measured with magnetometers as described (Redfield 2003) and checked with a commercial Gaussmeter (Lake Shore). The position of the sample in the fringe field was controlled by digital input to the stepper motor. The field strengths reported in the results reflect the measured field at the center of the sample. R 1 measurements at low field used a standard experiment (Farrow et al. 1994 ) with the addition of commands to control the field-cycling apparatus, and 100-150 ms delays to accommodate the raising and lowering of the sample. This transit delay was adjusted to account for the greater distance necessary to sample low fields. In order to compensate for signal attenuation due to relaxation in transit, 4-8 times as many transients were acquired for these experiments as at high field. Fieldcycling did not affect the lock to a greater extent than did a strong gradient pulse, and no degradation of the lock was observed over the course of the experiment. Proton inversion pulses during the relaxation period were not feasible because of the inhomogeneity of the fringe field and absence of a transmitter coil at the relevant height.