Multidimensional Dipolar Exchange-Assisted Recoupling Measurements in Solid-State NMR

摘要

A series of uni- and multidimensional variants of the dipolar exchange-assisted recoupling (DEAR) NMR experiment is described and applied to determinations of ~(13)C-~(14)N dipolar local field spectra in amino acids and dipeptides. The DEAR protocol recouples nearby nuclei by relying on differences in their relative rates of longitudinal relaxation , and has the potential to give quantitative geometric results without requiring radiofrequency pulsing on both members of a coupled spin pair. One- and two-dimensional variants of this recoupling strategy on generic I-S pairs are discussed, and measurements of ~(13)C-~(14)N distances and 2D local field experiments sensitive to the relative orientation of CN vectors with respect to the ~(13)C shielding tensor are presented. Since these measurements did not involve pulsing on the broad nitrogen resonance, their results were independent of the quadrupolar parameters of this nucleus. High-resolution 3D NMR versions of the 2D experiments were also implemented in order to separate their resulting local field patterns according to the isotropic shifts of inequivalent ~(13)C sites. These high-resolution 3D acquisitions involved collecting a series of 2D DEAR NMR data sets on rotating samples as a function of spinning angle, and then subjecting the resulting data to a processing akin to that involved in variable-angle correlation NMR. Once successfully tested on L-alanine this experiment was applied to the analysis of a series of dipeptides, allowing us to extract separate local field ~(13)C-~(14)N spectra from this type of multisite systems.