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

Abstract

Solid-state ¹³C NMR spectroscopy has been used in conjunction with selectively ¹³C-labeled mono- and disaccharides to measure ¹³C–¹³C 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 ¹J_CC, ²J_CCC and ³J_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 ¹³C, and (b) DFT-calculated J_CC values compare favorably with those determined by solid-state ¹³C NMR when molecular conformation is a constant in both determinations.