The effect of araC incorporation into the dodecamer duplex [d(CGCGAATT) (araC)d(GCG)]2 was examined by comparing its nuclear magnetic resonance (NMR)-determined solution structure with that of the control duplex d[(CGCGAATTCGCG)]2. 1H and 31P resonances in both duplexes were assigned using a combination of 2-D 1H NMR and a 3-D 31P-1H heteroTOCSY-NOESY experiment. Proton-proton distances (determined from NOESY data) and sugar dihedral angles (from NOESY and COSY data) were used in restrained molecular dynamics simulations starting from canonical A- or B-form DNA models. Both the control and araC sets of simulations converged to B-type structures. These structures were subjected to full relaxation matrix refinement to produce final structures which were in excellent agreement (R1/6 < 0.05) with the observed NOE intensities. A detailed comparison of the final control and araC structures revealed a global similarity (overall RMSD approximately 1.3 A), with significant differences localized at the araC site and neighboring bases. These included changes in sugar pucker, backbone torsion angles, base stacking, and other helical parameters. These findings are in general agreement with the previously published X-ray structure of a decamer duplex containing araC. One intriguing feature of the NMR solution structure not found in the crystal structure is the presence of an intramolecular hydrogen bond between the 2' hydroxyl on the araC sugar and the 3' phosphate group.

We describe a strategy for sequential assignment of 31P and deoxyribose 1H NMR resonances in oligode-oxyribonucleotides. The approach is based on 31P-1H J-cross-polarization (hetero TOCSY) experiments, recently demonstrated for the assignment of resonances in RNA [Kellogg, G.W. (1992) J. Magn. Reson., 98, 176; Kellogg, G.W. et al. (1992) J. Am. Chem. Soc., 114, 2727]. Two-dimensional heteroTOCSY and heteroTOCSY-NOESY experiments are used to connect proton spin systems from adjacent nucleotides in the dodecamer d(CGCGAATTCGCG)2 entirely on the basis of through-bond scalar connectivities. All phosphorus resonances of the dodecamer are assigned by this method, and many deoxyribose 1H resonances can be assigned as well. A new three-dimensional heteroTOCSY-NOESY experiment is used for backbone proton 4', 5' and 5" resonance assignments, completing assignments begun on this molecule in 1983 [Hare, D.R. et al. (1983) J. Mol. Biol., 171, 319]. Numerical simulations of the time dependence of coherence transfer aid in the interpretation of heteroTOCSY spectra of oligonucleotides and address the dependence of heteroTOCSY and related spectra on structural features of nucleic acids. The possibility of a generalized backbone-driven 1H and 31P resonance-assignment strategy for oligonucleotides is discussed.

It is well known that 2D 15N‐1H correlation spectra can resolve overlapped imino proton resonances in the downfleld NMR spectra of nucleic acids according to their 15N chemical shifts, and that these resonances can be assigned by base type on that basis, independent of conformation. This information can be extremely important in determining the solution structure of a nucleic acid by NMR, but previously could only be obtained using 15N‐labeled, or very concentrated samples. Here we report the design of a gradient‐enhanced, jump‐return spin echo version of an 15N‐1H HMQC experiment (GE‐JRSE HMQC) that is sensitive enough to work on unlabeled nucleic acid samples at normal NMR concentrations. This experiment has led to the assignment of imino proton resonances with non‐Watson‐Crick chemical shifts in the spectrum of a 29 residue oligoribonucleotide that models the sarcin/ricin loop from 28S rRNA.

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