Differentiation between unlabeled and very-low-level fully 15N,13C-labeled nucleotides for resonance assignments in nucleic acids.

Based on different characteristics between unlabeled and fully 15N,13C-labeled nucleotides, we develop a method for unambiguous resonance assignments in nucleic acids following site-specific fully 15N,13C isotope incorporation at very low levels. The J-couplings between heteronuclei provide for distinction between the NMR signals of the fully labeled nucleotides and those of the natural abundance nucleotides. The method is demonstrated for DNA oligonucleotides, in the dimeric G-quadruplex [d(GGGTTCAGG)]2 and in the 22-nucleotide human telomeric fragment d[AG3(TTAG3)3]. We expect this approach to be useful for selective monitoring of important functional domains and of their interactions in large nucleic acids.