Residual dipolar coupling TOCSY for direct through space correlations of base protons and phosphorus nuclei in RNA.

Abstract

Established residual dipolar coupling measurements in partially aligned molecules report on the relative orientation of 1H-15N and 1H-13C bond vectors of known lengths with respect to the molecular alignment tensor,1 and are increasingly used in the structure determination of oligonucleotides by NMR.2 Recently, methods for the measurement of homonuclear 1H-1H through space dipolar couplings in anisotropic media were introduced.3 In addition, homonuclear TOCSY sequences can be used to transfer magnetization through dipolar couplings between protons that are not scalar coupled.4 Here we extend the observation of dipolar couplings in partially aligned molecules to heteronuclear through space interactions, specifically 1H-31P residual dipolar couplings, which directly link the phosphate backbone and base protons in oligonucleotides. We introduce the RADAR-TOCSY (residual dipolar coupling total correlation spectroscopy) experiment for the direct observation of through space heteronuclear dipolar couplings in weakly aligned molecules. In analogy to the indirect scalar spin-spin coupling interaction ĤJ, the dipolar coupling interaction between a given pair of unlike spin-/2 nuclei I,S is expressed by the dipolar Hamiltonian ĤD