¹H NMR Relaxation and NOEs in Nucleic Acids

The T₁ of protons in macromolecules is not well defined because of extensive spin diffusion, and relaxation is nonexponential. It is therefore not straightforward to calculate the degree of saturation under conditions of rapid pulsing. The recovery of z magnetization in nucleic acids following nonselective and selective pulses has been calculated, from which apparent spin-lattice relaxation rate constants can be determined. These values have been compared with results obtained at different spectrometer frequencies for a DNA tetradecamer. The initial slopes of nonselective experiments show an inverse quadratic dependence on frequency and differ significantly from apparent T₁ values determined by fitting a single exponential to the magnetization-recovery time courses. While many protons give recovery time courses that appear deceptively monoexponential, some protons (especially Ade C2H) show pronounced lags in the magnetization recovery. The influence of methyl-group rotation on proton relaxation is small in nucleic acid fragments of modest size (approximately <20 base pairs), but becomes increasingly important with increasing size. Simulations and experimental NOESY spectra showed that while rapid pulsing substantially diminishes all intensities, most protons have similar saturation factors, so that the normalized cross-peak intensities show a much weaker dependence on the pulse repetition rate.