13 C- 13 C spin-coupling constants in crystalline 13 C-labeled saccharides: conformational effects interrogated by solid-state 13 C NMR spectroscopy.

Solid-state C-13 NMR spectroscopy has been used in conjunction with selectively C-13-labeled mono- and disaccharides to measure C-13-C-13 spin-couplings (J(CC)) in crystalline samples. This experimental approach allows direct correlation of J(CC) values with specific molecular conformations since, in crystalline samples, molecular conformation is essentially static and can be determined by X-ray crystallography. J(CC) values measured in the solid-state in known molecular conformations can then be compared to corresponding J(CC) 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 (1)J(CC), (2)J(CCC) and (3)J(COCC) 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 J(CC) values can be measured in crystalline saccharides that have been suitably labeled with C-13, and (b) DFT-calculated J(CC) values compare favorably with those determined by solid-state C-13 NMR when molecular conformation is a constant in both determinations.