Measurement of imino 1H-1H residual dipolar couplings in RNA.

Imino (1)H-(15)N residual dipolar couplings (RDCs) provide additional structural information that complements standard (1)H-(1)H NOEs leading to improvements in both the local and global structure of RNAs. Here, we report measurement of imino (1)H-(1)H RDCs for the Iron Responsive Element (IRE) RNA and native E. coli tRNA(Val) using a BEST-Jcomp-HMQC2 experiment. (1)H-(1)H RDCs are observed between the imino protons in G-U wobble base pairs and between imino protons on neighboring base pairs in both RNAs. These imino (1)H-(1)H RDCs complement standard (1)H-(15)N RDCs because the (1)H-(1)H vectors generally point along the helical axis, roughly perpendicular to (1)H-(15)N RDCs. The use of longitudinal relaxation enhancement increased the signal-to-noise of the spectra by ~3.5-fold over the standard experiment. The ability to measure imino (1)H-(1)H RDCs offers a new restraint, which can be used in NMR domain orientation and structural studies of RNAs.