title: 4J(COCCH) and 4J(CCCCH) as probes of exocyclic hydroxymethyl group conformation in saccharides.
abstract: [structure: see text] 1H NMR spectra of aldohexopyranosyl rings containing 13C-enrichment at either C1 or C3 reveal the presence of long-range 4J(C1,H6R/S) and 4J(C3,H6R/S) whose magnitudes depend mainly on the O5-C5-C6-O6 torsion angle. Using theoretical calculations (density functional theory, DFT; B3LYP/6-31G*) and conformationally constrained experimental model compounds, the magnitudes and signs of 4J(C1,H6R/S) and 4J(C3,H6R/S) have been established, and their dependencies on the geometry of the C1-O5-C5-C6-H6R/S and C3-C4-C5-C6-H6R/S coupling pathways, respectively, were determined. The latter dependencies mimic that observed previously for 4J(HH) in aliphatic compounds such as propane. DFT calculations also showed that inclusion of non-Fermi contact terms is important for accurate predictions of 4J(CH) values. Application to methyl alpha- and beta-D-glucopyranosides reveals different rotameric distributions about their hydroxymethyl groups, with the beta-anomer enriched in the gt rotamer, in agreement with recent multi-J redundant coupling analyses. 4J(C1,H6R/S) and 4J(C3,H6R/S) are expected to complement other recently developed J-couplings for the assignment of hydroxymethyl group conformation in oligosaccharides containing 1,6-glycosidic linkages.
authors.affiliation: Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, USA.
authors.forename: Qingfeng
authors.initials: Q
authors.lastname: Pan
authors.forename: Thomas
authors.initials: T
authors.lastname: Klepach
authors.forename: Ian
authors.initials: I
authors.lastname: Carmichael
authors.forename: Meredith
authors.initials: M
authors.lastname: Reed
authors.forename: Anthony S
authors.initials: AS
authors.lastname: Serianni
mesh_terms: D002236:Carbohydrate Conformation; D009682:Magnetic Resonance Spectroscopy