Syntheses of model oligosaccharides of biological significance. XII. Synthesis, NMR, and conformational analysis of trideuteriomethyl 4-O-(β-D-mannopyranosyl)- 2-acetamido-2-deoxy-β-D-glucopyranoside: the use of DEPT 1H to 13C transfer for T1 measurements and NOE assignments of tightly coupled 1H nuclei

Abstract

The title disaccharide Manp(β1–4)GlcpNAcβ1-OCD3 has been prepared by a short synthetic sequence through the inversion of configuration from gluco to manno involving benzyl Glcp(β1–4)GlcpNAc. The disaccharide was subjected to detailed high-field 1H and 13C NMR study. First, conventional and 2D spectra were run to afford a complete set of assigned spectral parameters. Next, steady-state 1H–{1H} NOE and 1H, 13C spin lattice relaxation experiments were performed to infer dynamic spectral data related to molecular conformation. Owing to tight couplings and signal overlaps in the 1H spectrum, proton relaxation and selected NOE data were obtained via 13C NMR after transfer of the actual, non-equilibrium, proton magnetization to the 13C frequency domain. Using this experimental approach it has been found that T1's for Manp H-4 and GlcpNAc H-2 were substantially longer than those of the other sugar ring protons, and that the principal interresidue dipolar contact in Manp(β1-4)GlcpNAcβ1-OCD3 takes place between Manp H-1 and GlcpNAc H-4. Subsequently, a conformational analysis of the disaccharide was executed by means of a semiempirical method comparing Boltzman-averaged computed observables with experimental T1 and NOE values. The results suggest that the disaccharide has substantial conformational flexibility. Its exo-anomeric-stabilized conformational minimum at approximately [Formula: see text] of the glycosidic rotational angle is significantly populated but this global minimum does not represent the only rotational form available to the molecule.