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 (1)J(NH) and the imino H-1 chemical shift are both dominated by hydrogen bond length. In the absence of molecular alignment, the (1)J(NH) coupling for the imino proton then can be approximated by (1)J(NH) = (1.21Hz/ppm)delta(H) - 103.5 +/- 0.6 Hz, where delta(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 ((1)J(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.