Multiple-step relayed correlation spectroscopy: sequential resonance assignments in oligosaccharides.

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Abstract
A general property of the high-resolution proton NMR spectra of oligosaccharides is the appearance of low-field well-resolved resonances corresponding to the anomeric (H1) and H2 protons. The remaining skeletal protons resonate in the region 3-4 ppm, giving rise to an envelope of poorly resolved resonances. Assignments can be made from the H1 and H2 protons to their J-coupled neighbors (H2 and H3) within this main envelope by using 1H-1H correlated spectroscopy. The tight coupling (J .simeq. .delta.) between further protons results in poor spectral dispersion with consequent assignment ambiguities. Three-step 2-dimensional relayed correlation spectroscopy and how it can be used to correlate the resolved anomeric (H1) and H2 protons with remote (H4, H5) protons directly through a linear network of couplings using sequential magnetization transfer around the oligosaccharide rings is described. Resonance assignments are then obtained by inspection of cross-peaks that appear in well-resolved regions of the 2-dimensional spectrum. This offers a general solution to the assignment problem in oligosaccharides and, importantly, these assignments will subsequently allow for the 3-dimensional solution conformation to be determined by using 1-dimensional and 2-dimensional nuclear Overhauser experiments.