NMR analysis of conforrnationally dependent nJC,H and nJC,C in the trisaccharide α-L-Rhap-(1 → 2)[α-L-Rhap-(1 → 3)]-α-L-Rhap-OMe and a site-specifically labeled isotopologue thereof

An array of NMR spectroscopy experiments have been carried out to obtain conformationally dependent H-1,C-13- and C-13,C-13-spin-spin coupling cons ants in the trisaccharide alpha-L-Rhap-(1 -> 2)[alpha-L-Rhap-(1 -> 3)]-alpha-L-Rhap-OMe. The trisaccharide was synthesized with C-13 site.specific labeling at C2' and C2 '', i.e. in the rhamnosyl groups in order to alleviate H-1 spectral overlap. This facilitated the measurement of a key trans-glycosidic proton-proton cross-relaxation rate using 1D H-1,H-1-T-ROESY experiments as well as a (3)J(C,H) coupling employing 1D H-1,C-13-long-range experiments, devoid of potential interference from additional J coupling. By means of both the natural abundance compound and the C-13-labeled sample 2D H-1,C-13-J-HMBC and H-1,C-13-HSQC-HECADE NMR experiments, total line-shape analysis of H-1 NMR spectra and 1D C-13 NMR experiments were employed to extract (3)J(C,H), (2)J(C,H), (3)J(C,C), and J(C,C) coupling constants. The C-13 site-specific labeling facilitates straightforward determination of (n)J(C,C) as the splitting of the C-13 natural abundance resonances. This study resulted in eight conformationally dependent coupling conqtants for the trisaccharide and illustrates the use of C-13 site-specific labeling as a valuable approach that extends the 1D and 2D NMR methods in current use to attain both hetero- and homonuclear spin-spin coupling constants that subsequently can be utilized for conformational analysis. Copyright (C) 2011 John Wiley & Sons, Ltd.