Measurement of imino 1H–1H residual dipolar couplings in RNA

Imino 1H–15N residual dipolar couplings (RDCs) provide additional structural information that complements standard 1H–1H NOEs leading to improvements in both the local and global structure of RNAs. Here, we report measurement of imino 1H–1H RDCs for the Iron Responsive Element (IRE) RNA and native E. coli tRNAVal using a BEST-Jcomp-HMQC2 experiment. 1H–1H 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 1H–1H RDCs complement standard 1H–15N RDCs because the 1H–1H vectors generally point along the helical axis, roughly perpendicular to 1H–15N 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 1H–1H RDCs offers a new restraint, which can be used in NMR domain orientation and structural studies of RNAs.