Synthesis and NMR of RNA with selective isotopic enrichment in the bases

Efficient syntheses of pyrimidine and purine nucleosides and nucleotides with selective C-13 enrichment in the base moieties are described. Uridine and cytidine are labeled at position C6 and adenosine and guanosine are labeled at position C8. The selectively labeled nucleosides were converted to nucleoside triphosphates and used with in vitro transcription to synthesize labeled RNA. Isotope-edited C-12 and C-13 sub-spectra of a omega 1-1/2-X-filtered NOESY experiment are demonstrated to be useful for making resonance assignments and for deriving structural information in large (>20 nt) RNA molecules. The labeled RNAs also allow heteronuclear J-couplings and relaxation parameters to be measured without complications from C-13-C-13 J-couplings.