13C–13C spin-coupling constants in crystalline 13C-labeled saccharides: conformational effects interrogated by solid-state 13C NMR spectroscopy†
Solid-state 13C NMR spectroscopy has been used in conjunction with selectively 13C-labeled mono- and disaccharides to measure 13C–13C spin-couplings (JCC) in crystalline samples. This experimental approach allows direct correlation of JCC values with specific molecular conformations since, in crystalline samples, molecular conformation is essentially static and can be determined by X-ray crystallography. JCC values measured in the solid-state in known molecular conformations can then be compared to corresponding JCC values calculated in the same conformations using density functional theory (DFT). The latter comparisons provide important validation of DFT-calculated J-couplings, which is not easily obtained by other approaches and is fundamental to obtaining reliable experiment-based conformational models from redundant J-couplings by MA′AT analysis. In this study, representative 1JCC, 2JCCC and 3JCOCC values were studied as either intra-residue couplings in the aldohexopyranosyl rings of monosaccharides or inter-residue (trans-glycoside) couplings in disaccharides. The results demonstrate that (a) accurate JCC values can be measured in crystalline saccharides that have been suitably labeled with 13C, and (b) DFT-calculated JCC values compare favorably with those determined by solid-state 13C NMR when molecular conformation is a constant in both determinations.