In addition to the direct observation of quadrupolar resonances using high-resolution MQMAS methods, further structural information can be obtained by correlating the quadrupolar nucleus to a nearby dipolar (or scalar) coupled spin network. In many biological applications, spin 1/2 nuclei ('H, '3C, '5N, 31P) are found in spatial proximity to the quadrupolar resonance of interest (such as 23Na or 'TO) and may be used to further simplify and refine the structural analysis. We have demonstrated that direct polarization transfer from protons to triple quantum coherence in sodium is an efficient method to obtain MQMAS spectra in which the 23Na resonances are filtered according to their nearest neighbor dipolar 'H-23Na couplings. Experimentally, this triple quantum CP (TQCP) method offers several advantages in comparison to the previously proposed single quantum CP method including broad transfer characteristics at convenient rf field strength and increased sensitivity to large and medium size dipolar couplings. In addition, we have designed adiabatic amplitude modulations that further increase the efficiency of the TQCP methods by more than SO %. [manuscript to be submitted] We have also successfully tested rf pulse schemes for observing correlations within dipolar coupled quadrupolar spin networks. In this case, a slow modulation of the rf amplitude is adjusted to recouple the dipolar interaction under MAS conditions using central transition coherence. We are currently testing modifications of this approach to obtain quadrupolar spin-spin correlations under high-resolution MQMAS conditions. [manuscript in preparation]
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