Assignment Strategy - New and Sensitivity-Optimized Experiments

Since the imino and amino hydrogens are subject to rapid exchange with hydrogens of water, all spectra that are intended to provide information about exchangeable protons 

NOESY in 2HzO provides the assignment of nonexchangeable base and sugar proton resonances, mostly through H6/8-H1 and H6/8-H2 /2 sequential connectivities. 

HETCOR and/or hetero TOCSY proton-phosphorus correlation completes and/or confirms H3, H4 assignments made in the previous step and extend it to H5 /5 . The 

Availability of a sample isotopically enriched with 15N and 13C offers the NMR spectro-scopist additional tools for overcoming the problems encountered in homonuclear experiments. In an obvious extension of the homonuclear experiments, the carbon or nitrogen dimension can be used to improve the resolution in NOESY spectra. However, recent trends clearly favor the through-bond approach where the presence of a cross peak unambiguously proves the existence of a chemical bond between the nuclei involved. Let us review briefly the experiments available for labeled oligonucleotides. 

3D 15N-edited NOESY (15N-NOESY-HMQC) provides imino-imino, imino-amino and aromatic-imino/amino connectivities. The approach often fails in regions such as loops and bulges where bases do not form hydrogen-bonded pairs. In that case, imino assignment can be obtained from 

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