Magnetic field induced residual dipolar couplings of imino groups in nucleic acids from measurements at a single magnetic field
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For base-paired nucleic acids, variations in 1 J NH and the imino 1H 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)δH − 103.5 ± 0.6 Hz, where δH 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 tRNAVal and the alignment tensor determined from magnetic-field alignment of tRNAVal agrees well with the tensor calculated by summation of the MSA tensors of the individual nucleobases.
KeywordsAlignment Chemical shift Dipolar coupling Magnetic susceptibility anisotropy RDC tRNAVal
This work was supported in part by the Intramural Research Program of the NIDDK, NIH, and by the Intramural AIDS-Targeted Antiviral Program of the Office of the Director, NIH, and NIH grant AI33098 (AP); MPL was supported in part by an NIH Training Grant T32 GM65103. We thank Dr. Sam E. Butcher and Dipa Sashital for providing us with the15N-enriched U2-U6 sample.