OPTIMIZATION OF ROTATION INDUCED A DIABETIC COHERENCE TRANSFERS

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Quadrupolar nuclei may be present in natural systems singly, such as the boron in peptide-boronic-acid alpha-lytic protease inhibitors, or in multiple neighboring sites, such as sodium in telomeric DNA sequences. In the latter case, it is important to develop NMR techniques to accurately measure the relative intensities of the different species for spin counting and assignments. Recently, the rotation-induced adiabatic coherence transfer (RIACT) method for inter-converting central-transition coherence with the highest-order symmetric multiple-quantum coherence was shown to provide accurate relative site intensities in isotropic multiple-quantum magic-angle spinning experiments. In this project, we are optimizing the RIACT performance. Biological macromolecules generally consist only of a few quadrupolar nuclei per molecule, and so have low sensitivity. It is also important to understand the behavior of RIACT under very fast sample spinning, which is required for rotor-synchronized MQMAS as well as for dealing with samples exhibiting very large quadrupole coupling constants. We have shown, through experimental work and numerical simulations, that RIACT transfers are in fact favorable at very fast MAS (10-20 kHz) and that the nutation method of coherence transfers decreases in efficiency at these speeds. The external mechanical modulation of the first order quadrupole coupling via MAS interferes with the nutation process and, as a result, RIACT leads to superior sensitivity at MAS speeds higher than 10 kHz for most samples. This has been demonstrated for both S=3/2 and S=5/2 nuclei. We have also shown, through experimental studies and numerical simulations, that shortening the RIACT pulse leads to improved offset performance, although only on the order of 10-20%. We are currently investigating the role of pulse shapes in sensitivity and offset performance.