Dipolar heteronuclear multiple-quantum

W. Sommer, J. Gottwald, D.E. Demco, H.W. Spiess, Dipolar heteronuclear multiple-quantum NMR spectroscopy in rotating sohds, J. Magn. Reson. A 113 (1995) 131-134. 

The D-HMQC (dipolar heteronuclear multiple-quantum coherence) technique is a recently developed NMR pulse sequence particularly suitable for the investigation of spatial proximity between quadrupolar and spin-1/2 nuclei. Compared to the crosspolarisation magic-angle spinning technique applied to a quadrupolar nucleus, D-HMQC does not require time-consuming optimisations and exhibits on the quadrupolar spin a better robustness to irradiation offset and to Cq values and... 

An additional option is the use of relaxation-compensating pulse sequences. This is the domain of the pulse-sequence developers. At the moment, there are two approaches available. One uses heteronuclear multiple-quantum coherences, in which the dipolar relaxation is removed [17]. The achieved gain in intensity is shown on the example of an RNA. 

C. Fernandez, D.P. Lang, J.P. Amoureux, M. Pruski, Measurement of heteronuclear dipolar interactions between quadrupolar and spin-1/2 nuclei in solids by multiple-quantum REDOR NMR, J. Am. Chem. Soc. 120 (1998) 2672-2673. 

In the last decades a series of homonuclear and heteronuclear recoupling experiments have been developed that combine a 2D approach with fast MAS and dipolar multiple-quantum NMR spectroscopy. In this way, the chemical shift resolution of different chemical environments allows sites of interest to be distinguished and fully identified, and then it is possible to obtain structural information since dipole-dipole couplings are sensitive to the distances between the coupled nuclei as well as to the orientation of the internuclear vector. 

M. Pruski, A. Bailly, D. P. Lang, J.-P. Amoureux, C. Fernandez, Studies of heteronuclear dipolar interactions between spin-1/2 and quadrupolar nuclei by using REDOR during multiple quantum evolution, Chem. Phys. Lett., 307, 35 40 (1999). 

All these techniques provide distances which compare very well with those obtained by X-ray diffraction (XRD) if the samples contain isolated spin pairs (typically 0.05 nm and often better). Spiess, Schnell, and coworkers were able to demonstrate the potential of the combined use of advanced SSNMR pulse sequences, (HDOR, heteronuclear dipolar-order rotor encoding and REREDOR, rotor-encoded REDOR), and quantum-chemical calculations for investigating the multiple HB network of N-butylaminocarboxyl-6-tridecylisocitosine in the pyrimidone and pyrimidinol forms (Scheme 3). 

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