SYNTHESES OF MODEL OLIGOSACCHARIDES OF BIOLOGICAL SIGNIFICANCE .12. SYNTHESIS, NMR, AND CONFORMATIONAL-ANALYSIS OF TRIDEUTERIOMETHYL 4-O-BETA-D-MANNOPYRANOSYL)-2-ACETAMIDO-2-DEOXY-BETA-D-GLUCOPYRANOSIDE- THE USE OF DEPT H-1 TO C-13 TRANSFER FOR T1 MEASUREMENTS AND NOE ASSIGNMENTS OF TIGHTLY COUPLED H-1 NUCLEI

The title disaccharide Manp(beta1-4)GlcpNAcbeta1-OCD3 has been prepared by a short synthetic sequence through the inversion of configuration from gluco to manno involving benzyl Glcp(beta1-4)GlcpNAc. The disaccharide was subjected to detailed high-field H-1 and C-13 NMR study. First, conventional and 2D spectra were run to afford a complete set of assigned spectral parameters. Next, steady-state H-1-{H-1} NOE and H-1, C-13 spin lattice relaxation experiments were performed to infer dynamic spectral data related to molecular conformation. Owing to tight couplings and signal overlaps in the H-1 spectrum, proton relaxation and selected NOE data were obtained via C-13 NMR after transfer of the actual, non-equilibrium, proton magnetization to the C-13 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(beta1-4)GlcpNAcbeta1-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 phi = +50-degrees (psi = 0 +/- 10-degrees) of the glycosidic rotational angle is significantly populated but this global minimum does not represent the only rotational form available to the molecule.