Heteronuclear correlation 2D NMR spectroscopy of macromolecules

The relaxation properties of macromolecules present problems which require modification of existing experimental procedures for 2D heteronuclear cross-correlation NMR of small molecules. In macromolecules, the large ¹H linewidths for hydrogens bound directly to ¹³C strongly limit achievable resolution for correlations over one bond. For correlations utilizing ⁿJ, n > 1, resolution is improved at the expense of sensitivity, but explicit consideration must be given to the effect of transverse relaxation on the efficiency of heteronuclear magnetization transfer. For macromolecules with correlation times in the spin-diffusion regime, long-range correlations were obtained via ¹H¹H cross relaxation followed by transfer to ¹³C. This experiment shows improved resolution and sensitivity, permits the observation of ¹H¹H cross relaxation in some magnetically equivalent spin systems and may be useful for indirect detection of ¹H¹H cross relaxation in crowded ¹H NMR spectra.