Enhanced measurement of anisotropic NMR parameters using tunable alignment strength and tensor orientation

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Next to established isotropic NMR parameters like chemical shifts, scalar couplings, and NOEs, the in-troduction of weakly oriented samples allows the measurement of anisotropic NMR parameters like re-sidual dipolar couplings (RDCs). The principle technique and its large potential in structure determination of molecules is already known since 1963 but only recent developments in sample preparation made RDCs widely available for complex organic compounds. The various RDCs in organic compounds vary substantially in size, with 2DHH and 1DCH as the largest and nDCC and 1DCN among the smallest (up to 50 times smaller) useful couplings, making an adjustable alignment without change of sample conditions highly desirable. Two techniques for varying the alignment strength, variable angle NMR and an elas-tomer-based stretching apparatus, during the first funding period have been developed to the point where they can be used routinely. The aim of the proposed research is now the development of corre-sponding pulse sequences for efficient measurement of long-range heteronuclear and carbon-carbon couplings. A second objective is pursued with the development of composite alignment media, having a liquid crystalline component fixed inside a gel, which will allow tuning of the alignment tensor orientation of molecules and thereby resolve signal overlap and random degeneracies of RDCs. All techniques de-veloped will first be applied to test molecules and then be used for the structure determination of natural products and other chemically and/or biologically interesting molecules.