Use of 15N-NMR to resolve molecular details in isotopically-enriched carbohydrates:: sequence-specific observations in hyaluronan oligomers up to decasaccharides

The glycosaminoglycan hyaluronan is a vital structural component of extracellular matrices with diverse biological functions, a molecular understanding of which requires a detailed description of secondary and tertiary solution structures. Various models of these structures have been proposed on the basis of H-1 and C-13 natural-abundance nuclear magnetic resonance (NMR) experiments, but resonance overlap limits further progress with these techniques. We have therefore produced N-15- and C-13- isotopically-labeled hyaluronan oligosaccharides and applied triple-resonance and 3D experiments to overcome this restriction. Spectra recorded on oligosaccharides (of lengths 4, 6, 8, 10, and 12 sugar rings), reveal that the N-15 nucleus allows resolution of the amide groups in a decamer at high magnetic field, whereas C-13 natural-abundance NMR can only resolve internal groups up to hexamers. Complete N-15 sequence- specific assignments of these oligosaccharides indicate that the chemical shift dispersion can be explained by end-effects, which are seen even in the middle of octamers. Triple- resonance and N-15-edited 3D experiments, among the first of their kind in oligosaccharides, have been used to achieve resolution of ring H-1 and C-13 nuclei where not possible previously. The subtle chemical shift perturbations resolved suggest that different conformations and dynamics occur at the ends, which may contribute to the range of biological activities displayed by varying lengths of hyaluronan. N-15-NMR in carbohydrates has not received much attention before, however, this study demonstrates it has clear advantages for achieving resolution and assessing dynamic motion. These conclusions are likely to be applicable to the study of the structure and dynamics of other nitrogen-containing carbohydrates.