Dipolar recoupling in magic-angle-spinning nuclear magnetic resonance

Abstract: This thesis concerns the development of radio-frequency pulse sequences in magic-angle-spinning solid-state nuclear magnetic resonance.First, two classes of pulse sequences are presented which are synchronized with the sample rotation. Symmetry theorems are described which link the symmetry of the pulse sequences to selection rules for the recoupling and/or decoupling of certain spin interactions. Pulse sequences are demonstrated which recouple direct homonuclear dipolar interactions at high sample spinning frequencies. Several applications are shown, including the efficient excitation of double-quantum coherences, two-dimensional double-quantum spectroscopy, transfer of longitudinal magnetization and two-dimensional correlation spectroscopy. In addition, generalized Hartmann-Hahn sequences are demonstrated in which radio-frequency irradiation is applied simultaneously to two isotopic spin species. These sequences selectively recouple direct heteronuclear dipolar interactions and suppress all homonuclear interactions for both spin species. Experimental demonstrations are given of heteronuclear two-dimensional correlation spectroscopy, heteronuclear multiple-quantum spectroscopy and the estimation of heteronuclear dipolar couplings.Second, a magic-angle-spinning nuclear magnetic resonance method is developed which directly estimates the backbone torsional angle Psi. in peptides and proteins. The method exploits multiple-quantum 13C coherence evolving under heteronuclear 13C-15N dipolar interactions. Single torsional angles Psi are determined with an accuracy of 5-10 degrees in the tripeptides gly-gly-gly and ala-gly-gly by exploiting double-quantum and triple-quantum coherences respectively.