2D EXCHANGE P-31 NMR-SPECTROSCOPY OF BACTERIOPHAGE-M13 AND TOBACCO MOSAIC-VIRUS

Two-dimensional (2D) exchange P-31 nuclear magnetic resonance spectros copy is used to study the slow overall motion of the rod-shaped viruse s M13 and tobacco mosaic virus in concentrated gels. Even for short mi xing times, observed diagonal spectra differ remarkably from projectio n spectra and one-dimensional spectra. Our model readily explains this to be a consequence of the T-2 theta anisotropy caused by slow overal l rotation of the viruses about their length axis. 2D exchange spectra recorded for 30% (w/w) tobacco mosaic virus with mixing times <1 s do not show any off-diagonal broadening, indicating that its overall mot ion occurs in the sub-Hz frequency range. In contrast, the exchange sp ectra obtained for 30% M13 show significant off-diagonal intensity for mixing times of 0.01 s and higher. A log-gaussian distribution around 25 Hz of overall diffusion coefficients mainly spread between 1 and 1 0(3) Hz faithfully reproduces the 2D exchange spectra of 30% M13 recor ded at various mixing times in a consistent way. A small but notable c hange in diagonal spectra at increasing mixing time is not well accoun ted for by our model and is probably caused by P-31 spin diffusion.