The crystal structures of 2,3,4,6-tetra- O -benzoyl-β-D-galactopyranosyl-(1→4)-1,2,6-tri- O -benzoyl-β-D-glucopyranose ethyl acetate hemisolvate, C 61 H 50 O 18 ·0.5C 4 H 8 O 2 , and 1,2,4,6-tetra- O -benzoyl-β-D-glucopyranose acetone monosolvate, C 34 H 28 O 10 ·C 3 H 6 O, were determined and compared to those of methyl β-D-galactopyranosyl-(1→4)-β-D-glucopyranoside (methyl β-lactoside) and methyl β-D-glucopyranoside hemihydrate, C 7 H 14 O 6 ·0.5H 2 O, to evaluate the effects of O -benzoylation on bond lengths, bond angles and torsion angles. In general, O -benzoylation exerts little effect on exo- and endocyclic C—C and endocyclic C—O bond lengths, but exocyclic C—O bonds involved in O -benzoylation are lengthened by 0.02–0.04 Å depending on the site of substitution. The conformation of the O -benzoyl side-chains is highly conserved, with the carbonyl O atom either eclipsing the H atom attached to a 2°-alcoholic C atom or bisecting the H—C—H bond angle of an 1°-alcoholic C atom. Of the three bonds that determine the side-chain geometry, the C—O bond involving the alcoholic C atom exhibits greater rotational variability than the remaining C—O and C—C bonds involving the carbonyl C atom. These findings are in good agreement with recent solution NMR studies of the O -acetyl side-chain conformation in saccharides.
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Glycosidic linkage, N -acetyl side-chain, and other structural properties of methyl 2-acetamido-2-deoxy-β- D -glucopyranosyl-(1→4)-β- D -mannopyranoside monohydrate and related compounds
The crystal structure of methyl 2-acetamido-2-deoxy-β-D-glycopyranosyl-(1→4)-β-D-mannopyranoside monohydrate, C 15 H 27 NO 11 ·H 2 O, was determined and its structural properties compared to those in a set of mono- and disaccharides bearing N -acetyl side-chains in βGlcNAc aldohexopyranosyl rings. Valence bond angles and torsion angles in these side chains are relatively uniform, but C—N (amide) and C—O (carbonyl) bond lengths depend on the state of hydrogen bonding to the carbonyl O atom and N—H hydrogen. Relative to N -acetyl side chains devoid of hydrogen bonding, those in which the carbonyl O atom serves as a hydrogen-bond acceptor display elongated C—O and shortened C—N bonds. This behavior is reproduced by density functional theory (DFT) calculations, indicating that the relative contributions of amide resonance forms to experimental C—N and C—O bond lengths depend on the solvation state, leading to expectations that activation barriers to amide cis – trans isomerization will depend on the polarity of the environment. DFT calculations also revealed useful predictive information on the dependencies of inter-residue hydrogen bonding and some bond angles in or proximal to β-(1→4) O -glycosidic linkages on linkage torsion angles ϕ and ψ. Hypersurfaces correlating ϕ and ψ with the linkage C—O—C bond angle and total energy are sufficiently similar to render the former a proxy of the latter.
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- Award ID(s):
- 2002625
- PAR ID:
- 10247235
- Date Published:
- Journal Name:
- Acta Crystallographica Section C Structural Chemistry
- Volume:
- 76
- Issue:
- 3
- ISSN:
- 2053-2296
- Page Range / eLocation ID:
- 287 to 297
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Methyl 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→4)-2-acetamido-2-deoxy-β-D-glucopyranoside (methyl β-chitobioside), (IV), crystallizes from aqueous methanol at room temperature to give a structure (C17H30N2O22·CH3OH) containing conformational disorder in the exocyclic hydroxymethyl group of one of its βGlcNAc residues. As observed in other X-ray structures of disaccharides containing β-(1→4)O-glycosidic linkages, inter-residue hydrogen bonding between O3H of the βGlcNAc bearing the OCH3aglycone and O5 of the adjacent βGlcNAc is observed based on the 2.79 Å internuclear distance between the O atoms. The structure of (IV) was compared to that determined previously for 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→4)-2-acetamido-2-deoxy-β-D-glucopyranose (β-chitobiose), (III). TheO-glycosidic linkage torsion angles,phi(ϕ) andpsi(ψ), in (III) and (IV) differ by 6–8°. TheN-acetyl side chain conformation in (III) and (IV) shows some context dependence, with the C1—C2—N—Ccartorsion angle 10–15° smaller for the βGlcNAc residue involved in the internalO-glycosidic linkage. In (IV), conformational disorder is observed in the exocyclic hydroxymethyl (–CH2OH) group in the βGlcNAc residue bearing the OCH3aglycone, and a fitting of the electron density indicates an approximate 50:50 distribution of thegauche–gauche(gg) andgauche–trans(gt) conformers in the lattice. Similar behavior is not observed in (III), presumably due to the different packing structure in the vicinity of the –CH2OH substituent that affects its ability to hydrogen bond to proximal donors/acceptors. Unlike (IV), a re-examination of the previously reported electron density of (III) revealed conformational disorder in theN-acetyl side chain attached to the reducing-end βGlcNAc residue caused by rotation about the C2—N bond.more » « less
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The crystal structure of methyl α-D-mannopyranosyl-(1→3)-2- O -acetyl-β-D-mannopyranoside monohydrate, C 15 H 26 O 12 ·H 2 O, ( II ), has been determined and the structural parameters for its constituent α-D-mannopyranosyl residue compared with those for methyl α-D-mannopyranoside. Mono- O -acetylation appears to promote the crystallization of ( II ), inferred from the difficulty in crystallizing methyl α-D-mannopyranosyl-(1→3)-β-D-mannopyranoside despite repeated attempts. The conformational properties of the O -acetyl side chain in ( II ) are similar to those observed in recent studies of peracetylated mannose-containing oligosaccharides, having a preferred geometry in which the C2—H2 bond eclipses the C=O bond of the acetyl group. The C2—O2 bond in ( II ) elongates by ∼0.02 Å upon O -acetylation. The phi (φ) and psi (ψ) torsion angles that dictate the conformation of the internal O -glycosidic linkage in ( II ) are similar to those determined recently in aqueous solution by NMR spectroscopy for unacetylated ( II ) using the statistical program MA′AT , with a greater disparity found for ψ (Δ = ∼16°) than for φ (Δ = ∼6°).more » « less
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