THE EFFECTS OF DIPOLAR CROSS-CORRELATION ON C-13 METHYL-CARBON T1, T2, AND NOE MEASUREMENTS IN MACROMOLECULES

The effects of dipolar cross correlation on 13C T1, T2, and NOE values are calculated for methyl groups attached to macromolecules. Using the Woessner model to describe methyl-group internal dynamics, it is found that for macromolecules with tumbling times between 5 and 20 ns and fast internal methyl rotation characterized by a correlation time between 15 and 65 ps, the recovery of longitudinal magnetization differs by less than 10% from that of the case with no cross correlation. Therefore for a large range of values of τm and τe, cross-correlation effects on longitudinal relaxation are small, despite the fact that reorientation is highly anisotropic. In contrast, for 13C spin-spin relaxation the effects of dipolar cross correlation are significant for all values of τm and τe in the range examined (2 ns < τm < 40 ns, 0 ps < τe < 100 ps), and differences in relaxation rates by as much as a factor of 3.0 relative to predicted values based on the neglect of cross correlation are obtained for an isolated AX3 spin system attached to a macromolecule. The effect of cross correlation is calculated to change NOE values by no more than 6-7% in the range examined. The influence of neighboring 1H spins on 13C relaxation is assessed by including random-field effects in the calculations. While the effects of cross correlation are attenuated, 13C longitudinal and transverse relaxation rates can, under certain conditions, still be substantially different from rates obtained in the absence of cross correlation. Although a quantitative description of cross-correlation effects on 13C relaxation depends on the details of methyl-group internal dynamics, the results derived here using the Woessner model are qualitatively the same as results obtained for different descriptions of the methyl-carbon internal motions. © 1991.