Magnetic field induced residual dipolar couplings of imino groups in nucleic acids from measurements at a single magnetic field

For base-paired nucleic acids, variations in ¹J_NH and the imino ¹H chemical shift are both dominated by hydrogen bond length. In the absence of molecular alignment, the ¹J_NH coupling for the imino proton then can be approximated by ¹J_NH = (1.21Hz/ppm) δ_H - 103.5 ± 0.6 Hz, where δ_H represents the chemical shift of the imino proton in ppm. This relation permits imino residual dipolar couplings (RDCs)resulting from magnetic susceptibility anisotropy (MSA) to be extracted from measurement of (¹J_NH + RDC) splittings at a single magnetic field strength. Magnetic field-induced RDCs were measured for tRNA^Val and the alignment tensor determined from magnetic-field alignment of tRNA^Val agrees well with the tensor calculated by summation of the MSA tensors of the individual nucleobases.