NMR analysis of conformationally 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 1H,13C- and 13C,13C-spin–spin coupling constants in the trisaccharide α-L-Rhap-(1 → 2)[α-L-Rhap-(1 → 3)]-α-L-Rhap-OMe. The trisaccharide was synthesized with 13C site-specific labeling at C2′ and C2″, i.e. in the rhamnosyl groups in order to alleviate 1H spectral overlap. This facilitated the measurement of a key trans-glycosidic proton–proton cross-relaxation rate using 1D 1H,1H-T-ROESY experiments as well as a 3JC, H coupling employing 1D 1H,13C-long-range experiments, devoid of potential interference from additional J coupling. By means of both the natural abundance compound and the 13C-labeled sample 2D 1H,13C-J-HMBC and 1H,13C-HSQC-HECADE NMR experiments, total line-shape analysis of 1H NMR spectra and 1D 13C NMR experiments were employed to extract 3JC, H, 2JC, H, 3JC, C, and 1JC, C coupling constants. The 13C site-specific labeling facilitates straightforward determination of nJC, C as the splitting of the 13C natural abundance resonances. This study resulted in eight conformationally dependent coupling constants for the trisaccharide and illustrates the use of 13C 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.

Ämnesord

NATURVETENSKAP  -- Kemi -- Organisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Organic Chemistry (hsv//eng)

Nyckelord

J-HMBC

HSQC-HECADE

HETCOR

oligosaccharide

heteronuclear long-range coupling constants

modified J-doubling

Organic chemistry

Organisk kemi

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