Conformational analysis and the fine structure of cross peaks in phase‐sensitive homonuclear two‐dimensional correlated NMR spectra of oligosaccharides

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Abstract
Useful structural and conformational information was obtained from a detailed analysis of the fine structure of cross peaks that appears in the phase‐sensitive homonuclear correlated two‐dimensional spectra of complex large glycopeptides. The small chemical shift range observed for the oligosaccharide part results in a variety of cross‐peak patterns. A catalogue of cross‐peak fine structures and patterns expected for common carbohydrates are presented which can help in making some, all‐important and crucial, proton resonance assignments. Analysis of the vicinal coupling constants extracted from the cross peaks corresponding to the hydroxymethyl protons of the inner core carbohydrates of glycopeptides derived from calf fetuin, has yielded information regarding the time‐averaged solution conformation of this triantennary structure. It was shown that in solution the Man(α1–6) arm of the core pentasaccharide unit, which carries a single antenna, adopts for an appreciable part of the time a conformation which brings it close to the arm that is attached to the peptide part. A similar conformation for the Man(α1–6) arm was previously observed for the biantennary oligosaccharide chain. Therefore, we concluded that the presence of an additional antenna on the Man(α1–3) arm of the core unit in the triantennary structure does not alter the basic biantennary conformation.