A comparative study of the O-3 reactivity of isomeric N-dimethylmaleoyl-protected D-glucosamine and D-allosamine acceptors

Four isomeric N-dimethylmaleoyl 4,6-O-benzylidene-protected d-hexosamine acceptors (2, 3, 4, and 5) with all possible configurations at C-1 and C-3 (e.g., derived from d-glucosamine and D-allosamine) were prepared, and the assessment of their O-3 relative reactivity through competition experiments using the known per-O-acetylated D-galactopyranosyl trichloroacetimidate donor (15) was then carried out. The reactivities are in the order 4?2>5>3. The analysis of the NMR spectra of 2-5 at different temperature and modeling experiments carried out on analogs of 2-5 (DFT) and on the acceptors themselves (MM) are coincident, and have helped to establish the stability of the different hydrogen bonds, and of the conformers which carry them. The whole results suggest that the electronic effects (hydrogen bonds) are required to explain the observed trend, in spite of the axial conformation of the most reactive hydroxyl group. The steric effects appear only when hydrogen bonds are weak.     

Synthesis of 8-(hydroxyalkyl)adenines

Treatment of methyl 4,6-O-benzylidene-2-O-p-tolylsulfonyl-α-D-ribo-hexopyranosid-3-ulose (1) with triethylamine-methanol at reflux temperature yields methyl 2,3-anhydro-4,6-O-benzylidene-3-methoxy-α-D-allopyranoside (2), a derivative (3) of 3-hydroxy-2-(hydroxymethyl)-4H-pyran-4-one, and methyl 4,6-O-benzylidene-α-D-ribo-hexopyranosid-3-ulose dimethyl acetal (4). The reaction of methyl 4,6-O benzylidene-3-O-p-tolylsulfonyl-α-D-arabino-hexopyranosid-2-ulose (12) with triethylamine-methanol afforded methyl 4,6-O-benzylidene-α-D-ribo-hexopyranosid-2-ulose dimethyl acetal (19) and methyl 2,3-anhydro-4,6-O-benzylidene-2-methoxy-α-D-allopyranoside (20); from the reaction of the β-D anomer (13) of 12, methyl 4,6-O-benzylidene-β-D-ribo-hexopyranosid-2-ulose dimethyl acetal (21) was isolated. Syntheses of the α-keto toluene-p-sulfonates 12 and 13 are described. Mechanisms for the formation of the compounds isolated from the reactions with triethylamine-methanol are proposed.     

Syntheses of branched-chain sugars with push-pull functionality

The reaction of methyl 4,6-O-benzylidene-3(2)-deoxy-- -erythro-hexopyranosid-2(3)-ulose with carbon disulfide, alkyl iodide, and sodium hydride gave methyl 4,6-O-benzylidene-3(2)-[bis(alkylthio)methylene]-3(2)-deoxy-- -erythro-hexopyranosid-2(3)-uloses. Methyl 4,6-O-benzylidene-2-[bis(methylthio)methylene]-2-deoxy-- -erythro-hexopyranosid-3-ulose (5) reacted with aromatic amines to give, in a rearrangement process, N-aryl-2-aryliminomethyl-4,6-O-benzylidene-2-deoxy-- -erythro-hex-1-enopyranosylamin-3-uloses. The reaction of 5 which hydrazine hydrate afforded 5-methylthio-(methyl-4,6-O-benzylidene-2,3-dideoxy-- -erythro-hexopyranosido)[3,2-c]pyrazole.     

Two-dimensional correlation spectroscopy and principal component analysis studies of temperature-dependent IR spectra of cotton-cellulose

The FTIR spectra were measured for raw Uplands Sicala-V2 cotton fibers over a temperature range of 40-325 degrees C to explore the temperature-dependent changes in the hydrogen bonds of cellulose. These cotton-cellulose spectra exhibited complicated patterns in the 3800-2800 cm(-1) region and thus were analyzed by both the exploratory principal component analysis (PCA) and two-dimensional (2-D) correlation spectroscopy methods. The exploratory PCA showed that the spectra separate into two groups on the basis of thermal degradation of the cotton-cellulose and the consequent breakage of intersheet H-bonds present in its structure. Frequency variables, which strongly contributed to each principal component highlighted in its loadings plot, were linked to the frequencies assigned to vibrations of the OH groups involved in different kinds of H-bonds, as well as to vibrations of the CH groups. Deeper insights into reorganization of the temperature-dependent hydrogen bonding were obtained by 2-D correlation spectroscopy. Synchronous and asynchronous spectra were analyzed in the temperature ranges of 40 to 150 and 250 to 320 degrees C, the ranges indicated by PCA. Detailed band assignments of the OH stretching region and changes in the patterns of the hydrogen bonding network of the cotton-cellulose were proposed with the aid of the 2-D correlation spectroscopy analysis. Below 150 degrees C, distinctly different bands assigned to the less stable Ialpha and the more stable Ibeta interchain H-bonds O-6-H-6...O-3' were observed at about 3230 and 3270 cm(-1), respectively. Evaporation of water entrapped in the cellulose network was examined by means of the band at about 3610 cm(-1). The cooperativity of hydrogen bonds, which play a key role in the cellulose conformation, was monitored by frequencies assigned to intrachain H-bonds. It was possible to separate the frequencies assigned to the O-2-H-2...O-6 and O-3-H-3...O-5 intrachain H-bonds into two separate ranges, the spread of which was controlled by the cooperativity effect. The temperature dependence of the asynchronous spectra indicated that the less stable O-3-H-3...O-5 bonds gave rise to an absorption extending from 3300 to 3384 cm(-1), while the more stable O-2-H-2...O-6 bonds were characterized by the absorption between 3400 and 3470 cm(-1). The final breaking of the inter- and intrachain H-bonds, which occurs at the higher temperatures, was monitored by the asynchronous peaks at 3533 and 3590 cm(-1), respectively. On the basis of both the exploratory PCA and 2-D correlation spectroscopy investigations, it was possible to extract well-defined wavenumber ranges assigned to different kinds of intra- and interchain hydrogen bonds, as well as to the free OH groups of the cotton-cellulose.     

Conformational features of crystal-surface cellulose from higher plants

Native cellulose in higher plants forms crystalline fibrils a few nm across, with a substantial fraction of their glucan chains at the surface. The accepted crystal structures feature a flat-ribbon 21 helical chain conformation with every glucose residue locked to the next by hydrogen bonds from O-3' to O-5 and from O-2 to O-6'. Using solid-state NMR spectroscopy we show that the surface chains have a different C-6 conformation so that O-6 is not in the correct position for the hydrogen bond from O-2. We also present evidence consistent with a model in which alternate glucosyl residues are transiently or permanently twisted away from the flat-ribbon conformation of the chain, weakening the O-3' - 0-5 hydrogen bond. Previous molecular modelling and the modelling studies reported here indicate that this 'translational' chain conformation is energetically feasible and does not preclude binding of the surface chains to the interior chains, because the surface chains share the axial repeat distance of the 21 helix. Reduced intramolecular hydrogen bonding allows the surface chains to form more hydrogen bonds to external molecules in textiles, wood, paper and the living plant.     

Plausible molecular and crystal structures of chitosan/HI type II salt

Chitosan/HI type II salt prepared from crab tendon was investigated by X-ray fiber diffraction. Two polymer chains and 16 iodide ions (I(-)) crystallized in a tetragonal unit cell with lattice parameters of a = b = 10.68(3), c (fiber axis) = 40.77(13) A, and a space group P4(1). Chitosan forms a fourfold helix with a 40.77 A fiber period having a disaccharide as the helical asymmetric unit. One of the O-3... O-5 intramolecular hydrogen bonds at the glycosidic linkage is weakened by interacting with iodide ions, which seems to cause the polymer to take the 4/1-helical symmetry rather than the extended 2/1-helix. The plausible orientations of two O-6 atoms in the helical asymmetric unit were found to be gt and gg. Two chains are running through at the corner and the center of the unit cell along the c-axis. They are linked by hydrogen bonds between N-21 and O-61 atoms. Two out of four independent iodide ions are packed between the corner chains while the other two are packed between the corner and center chains when viewing through the ab-plane. The crystal structure of the salt is stabilized by hydrogen bonds between these iodide ions and N-21, N-22, O-32, O-61, O-62 of the polymer chains.     

Molecular and crystal structures of chitosan/HI type I salt determined by X-ray fiber diffraction

The three-dimensional structure of chitosan/HI type I salt was determined by the X-ray fiber diffraction technique and linked-atom least-squares refinement method. Two polymer chains and four iodide ions (I(-)) crystallized in a monoclinic unit cell with dimensions a = 9.46(2), b = 9.79(2)], c (fiber axis)=10.33(2)A, beta = 105.2(2) degrees and a space group P2(1). Chitosan chains adopted an extended twofold helical conformation that was stabilized by O-3...O-5 hydrogen bonds, and the O-6 atom adopted nearly gt orientation. Polymer chains zigzag along the b-axis and directly connect to each other by N-2...O-6 hydrogen bonds. Two columns of iodide ions were shown to pack at the bending points of the zigzag sheets, and their locations are closely related to those of water columns in the hydrated chitosan. The iodide ions stabilized the salt structure by forming hydrogen bonds with the N-2 and O-6 atoms of the polymer chains together with an electrostatic interaction between N-2 and the iodide ions.     

Molecular and crystal structures of N-arylglycopyranosylamines formed by reaction between sulfanilamide and D-ribose, D-arabinose and D-mannose

The X-ray crystal structures of three monosaccharide derivatives prepared by the reaction of sulfanilamide with D-ribose, D-arabinose, and D-mannose have been determined. The derivatives are N-(p-sulfamoylphenyl)-alpha-D-ribopyranosylamine (1), N-(p-sulfamoylphenyl)-alpha-D-arabinopyranosylamine (2), and N-(p-sulfamoylphenyl)-beta-D-mannopyranosylamine monohydrate (3). The monosaccharide ring of 1 and 2 has the 1C4 conformation, stabilized in 1 by an intramolecular hydrogen bond from 0-2 to 0-4. Compound 3 has the 4C1 conformation at the monosaccharide ring and the gt conformation at the C-6-O-6 side chain. Occupancy of the water molecule in the crystal of 3 actually examined was 22%. The degree of interaction between sulfamoyl groups and monosaccharide moieties varies from structure to structure. The packing arrangement of 2 involves hydrogen bonding between sulfamoyl groups and monosaccharide hydroxyl groups, but interactions of this type are fewer in 1, and in 3 the hydrogen bonds are either strictly between monosaccharide hydroxyl groups or strictly between sulfamoyl groups. Pairs of hydrogen bonds (two-point contacts) link neighboring molecules in all three structures, between screw-axially related molecules in 1 and 2 and between translationally related molecules in 3. The contact in 3 defined by the O-3-H...O-5 and O-6-H...O-4 hydrogen bonds is found in several other N-aryl-beta-D-mannopyranosylamine crystal structures and is apparently an especially favorable mode of intermolecular interaction in these compounds.     

Chemo-enzymatic supported synthesis of the 3-sulfated Lewis a pentasaccharide on a multimeric polyethylene glycol

The 3-sulfated Lewis(a) pentasaccharide was synthesized on multimeric-based polyethylene glycol support. Coupling of O-(2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl)-(1-->3)-4,6-di-O-acetyl-2-deoxy-2-phthalimido-beta-D-glucopyranosyl trichloroacetimidate with (2,6-di-O-acetyl-beta-D-galactopyranosyl)-(1-->4)-(2,3,6-tri-O-acetyl-beta-D-glucopyranoside) bound onto the polymer afforded lacto-N-tetraose, which was then regioselectively sulfated at the 3-OH position of the terminal galactose using the stannylene procedure. Fucosylation of the sulfated tetrasaccharide was performed using an immobilized fucosyltransferase FucTIII to give the title compound after cleavage.     

Synthetic receptor analogues: the conformation of methyl 4-O-alpha-D-galactopyranosyl-beta-D-galactopyranoside (methyl beta-D-galabioside) and related derivatives, determined by n.m.r. and computational methods

The conformations of galabiose and its methyl and ethyl beta-glycosides as well as the 3-deoxy, 3-O-methyl, 3-deoxy-3-C-methyl, 3-deoxy-3-C-ethyl, and 6-deoxy analogues were investigated by n.m.r. (1H, 13C, n.O.e.) and computational (HSEA) methods. A good correlation was found between the computational data and the n.m.r. data for aqueous solutions. The conformations in aqueous solution were similar, whereas crystalline galabiose or methyl beta-D-galabioside in solution in methyl sulfoxide adopted different conformations that showed intramolecular hydrogen bonds (O-5'. . . O-3 and O-2'. . . O-6, respectively).     

Crystal structure of beta-cyclodextrin-benzoic acid inclusion complex

The inclusion complex of beta-cyclodextrin (beta-CD) with benzoic acid (BA) has been characterized crystallographically. Two beta-CDs cocrystallize with two BAs, 0.7 ethanol and 20.65 water molecules [2(C(6)H(10)O(5))(7).2(C(7)H(6)O(2)).0.7(C(2)H(6)O).20.65H(2)O] in the triclinic space group P1 with unit cell constants: a=15.210(1), b=15.678(1), c=15.687(1) A, alpha=89.13(1), beta=74.64(1), gamma=76.40(1) degrees. The anisotropic refinement of 1840 atomic parameters against 16,201 X-ray diffraction data converged at R=0.078. In the crystal lattice, beta-CD forms dimers stabilized by direct O-2(m)_1/O-3(m)_1...O-2(n)_2/O-3(n)_2 hydrogen bonds (intradimer) and by indirect O-6(m)_1...,O-6(n)_2 hydrogen bonds with one or two bridging water molecules joined in between (interdimer). These dimers are stacked like coins in a roll constructing endless channels where the guest molecules are included. The BA molecules protrude with their COOH groups at the beta-CD O-6-sides and are maintained in positions by hydrogen bonding to the surrounding O-6-H groups and water molecules. Water molecules (20.65) are distributed over 30 positions in the interstices between beta-CD molecules, except the water sites W-1, W-2 that are located in the channel of the beta-CD dimer. Water site W-2 is hydrogen bonded to the disordered ethanol molecule (occupancy 0.7).     

A comparative study of the O-3 reactivity of isomeric N-dimethylmaleoyl-protected d-glucosamine and d-allosamine acceptors

Four isomeric N-dimethylmaleoyl 4,6-O-benzylidene-protected d-hexosamine acceptors (2, 3, 4, and 5) with all possible configurations at C-1 and C-3 (e.g., derived from d-glucosamine and d-allosamine) were prepared, and the assessment of their O-3 relative reactivity through competition experiments using the known per-O-acetylated d-galactopyranosyl trichloroacetimidate donor (15) was then carried out. The reactivities are in the order 4 ? 2 > 5 > 3. The analysis of the NMR spectra of 2–5 at different temperature and modeling experiments carried out on analogs of 2–5 (DFT) and on the acceptors themselves (MM) are coincident, and have helped to establish the stability of the different hydrogen bonds, and of the conformers which carry them. The whole results suggest that the electronic effects (hydrogen bonds) are required to explain the observed trend, in spite of the axial conformation of the most reactive hydroxyl group. The steric effects appear only when hydrogen bonds are weak.     

A new crystal form of beta-cyclodextrin-ethanol inclusion complex: channel-type structure without long guest molecules

A new crystal form of beta-cyclodextrin (beta-CD)[bond]ethanol[bond]dodecahydrate inclusion complex [(C(6)H(10)O(5))(7).0.3C(2)H(5)OH.12H(2)O] belongs to monoclinic space group C2 (form II) with unit cell constants a=19.292(1), b=24.691(1), c=15.884(1) A, beta=109.35(1) degrees. The beta-CD macrocycle is more circular than that of the complex in space group P2(1) [form I: J. Am. Chem. Soc. 113 (1991) 5676]. In form II, a disordered ethanol molecule (occupancy 0.3) is placed in the upper part of beta-CD cavity (above the O-4 plane) and is sustained by hydrogen bonding to water site W-2. In form I, an ethanol molecule located below the O-4-plane is well ordered because it hydrogen bonds to surrounding O-3[bond]H, O-6[bond]H groups of the symmetry-related beta-CD molecules. In the crystal lattice of form I, beta-CD macrocycles are stacked in a typical herringbone cage structure. By contrast, the packing structure of form II is a head-to-head channel that is stabilized at both O-2/O-3 and O-6 sides of each beta-CD by direct O(CD)...O(CD) and indirect O(CD)...O(W)...(O(W))...O(CD) hydrogen bonds. The 12 water molecules are disordered in 18 positions both inside the channel-like cavity of beta-CD dimer (W-1[bond]W-6) and in the interstices between the beta-CD macrocycles (W-7[bond]W-18). The latter forms a cluster that is hydrogen bonded together and to the neighboring beta-CD O[bond]H groups.     

Determining the crystal structure of cellulose III(I) by modeling

Recently, a one-chain monoclinic unit cell for cellulose III(I) having P2(1) symmetry and a single glucose in the asymmetric unit was proposed, based on high-resolution diffraction patterns. The new work challenged a two-chain structure that was published 25 years earlier, although it did not provide new three-dimensional coordinates. Our goals were to solve the structure by modeling, find whether modeling would reject the previously determined two-chain unit cell, and compare the model with the anticipated experimental structure. Combinations of three rotamers of the O-2, O-3, and O-6 hydroxyl groups produced 27 'up' and 27 'down' starting structures. Clusters ('minicrystals') of 13 cellotetraose chains terminated by methyl groups for each of the 54 starting structures were optimized with MM3(96). Hydroxyl groups on 16 of these 54 structures reoriented to give very similar hydrogen-bonding schemes in the interiors, along with the lowest energies. Hydrogen bonds included the usual intramolecular O-3H...O-5' linkage, with O-6' also accepting from O-3H. Interchain hydrogen bonds form an infinite, cooperative O-6H...O-2H...O-6 network. Direct comparison of total minicrystal energies for the one- and two-chain unit cell was inappropriate because the two-chain cell's alternate chains are shifted 0.9 A along the z-axis. To get comparable energy values, models were built with both cellotetraose and cellohexaose chains. The differences in their energies represent the energies for the central layers of cellobiose units. The one-chain cell models had much lower energy. The eight best 'up' one-chain models agree reasonably well with the structure newly determined by experiment.     

The synthesis of racemic purpurosamine B

Starting from methyl 4,6-dichloro-4,6-dideoxy-α-D-galactopyranoside (1), D-chalcose (4,6-dideoxy-3-O-methyl-D-xcylo-hexopyranose) (5) was prepared by dechlorination with tributyltin hydride, selective benzoylation with benzoyl cyanide at O-2, methylation at O-3, and acid hydrolysis. D-Chalcose (5) was obtained as well by direct methylation of 1 with diazomethane at O-3, reduction with tin hydride, and hydrolysis. Chalcosyl bromide prepared from 5 was not very suitable for β-glycoside synthesis under Koenigs-Knorr conditions, and better results were obtained with 2- O-acetyl-4,6-dichloro-4,6-dideoxy-3-O-methyl-α-D-galactopyranosyl bromide, which gave β-glycosides with methanol, cyclohexanol, benzyl alcohol, 1,2:3,4-di-O-isopropylidene-α-D-galactopyranose, and methyl 2,3-di-O-benzyl-6-deoxy-α-D-glucopyranoside. After dechlorination with tributyltin hydride, the corresponding β-glycosides of D-chalcose were obtained in good yield.     

Synthesis of 2-acetamido-2-deoxy-3-O-β-d-mannopyranosyl-d-glucose

Condensation of 4,6-di-O-acetyl-2,3-O-carbonyl-α-d-mannopyranosyl bromide with benzyl 2-acetamido-4,6-O-benzylidene-2-deoxy-α-d-glucopyranoside (2) gave an α-d-linked disaccharide, further transformed by removal of the carbonyl and benzylidene groups and acetylation into the previously reported benzyl 2-acetamido-4,6-O-benzylidene-2-deoxy-3-O-(2,3,4,6-tetra-O-acetyl-α-d-mannopyranosyl)-α-d-glucopyranoside. Condensation of 3,4,6-tri-O-benzyl-1,2-O-(1-ethoxyethylidene)-α-d-glucopyranose or 2-O-acetyl-3,4,6-tri-O-benzyl-α-d-glucopyranosyl bromide with 2 gave benzyl 2-acetamido-3-O-(2-O-acetyl-3,4,6-tri-O-benzyl-β-d-glucopyranosyl)-4,6-O-benzylidene-2-deoxy-α-d-glucopyranoside. Removal of the acetyl group at O-2, followed by oxidation with acetic anhydride-dimethyl sulfoxide, gave the β-d-arabino-hexosid-2-ulose 14. Reduction with sodium borohydride, and removal of the protective groups, gave 2-acetamido-2-deoxy-3-O-β-d-mannopyranosyl-d-glucose, which was characterized as the heptaacetate. The anomeric configuration of the glycosidic linkage was ascertained by comparison with the α-d-linked analog.     

Comparative (1)H NMR and molecular modeling study of hydroxy protons of beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->2)-alpha-D-Manp-(1-->O)(CH(2))(7)CH(3) analogues in aqueous solution

The (1)H chemical shifts, coupling constants, temperature coefficients, exchange rates, and inter-residual ROEs have been measured, in aqueous solution, for the hydroxy and amine/amide proton resonances of a set of beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->2)-alpha-D-Manp-(1-->O)(CH(2))(7)CH(3) analogues. From the structural data, a few significant structural features could be ascertained, such as a preferential anti-conformation for the amide protons of the N-acetyl and N-propionyl groups. The introduction of systematic modifications at Gal 2-C and Gal 6-C resulted in alterations of the Gal 4-OH, Gal 3-OH, and GlcNAc 3-OH areas, since variations in chemical shifts and temperature coefficient were observed. In order to verify the possibility of hydrogen bonds, molecular dynamics simulations in the gas phase and explicit solvent were performed and correlated with the experimental data. A network of hydrogen bonds to solvent molecules was observed, but no strong intramolecular hydrogen bonding was observed.     

Regioselective reductive openings of 4,6-benzylidene acetals: synthetic and mechanistic aspects

The use of benzylidene acetals as protecting groups in carbohydrate chemistry is utterly important. The main advantage of benzylidene acetal is the ability for regioselective openings. 4,6-benzylidene acetal can be opened selectively under reductive conditions to yield either free 4-OH or 6-OH. There are a plethora of methods available for regioselective openings, but only a few of these are widely used. In recent years, the mechanism has been investigated for borane mediated openings and it seems likely that the regioselectivity is determined by borane, rather than Lewis acid. When borane is activated by Lewis acids, borane is the most electrophilic species that consequently coordinates to the most nucleophilic oxygen of the acetals, usually O-6. This results in the formation of 6-O-benzyl ethers. If borane is not activated, Lewis acid is the most electrophilic species that thus adds to O-6 and hence generates the 4-O-benzyl ether.     

Structures for polyinosinic acid and polyguanylic acid

X-ray-diffraction analysis of oriented, partially crystalline fibres of polyinosinic acid has resulted in a new molecular model. This model consists of four identical polynucleotide chains related to one another by a fourfold rotation axis. The coaxial helices are righthanded (screw symmetry 23(2)) and have an axial translation per residue h=0.341nm and a rotation per residue t=31.3 degrees . Incorporated in the model are standard bond lengths, bond angles and C-2-endo furanose rings. The nucleotide conformation angles, determined by linked-atom least-squares methods, are orthodox and the fit with the X-ray intensities is good. Each hypoxanthine base is linked to two others by hydrogen bonds involving O-6 and N-1. Further stability may arise from intrachain hydrogen bonds between each ribose hydroxyl group and the phosphate oxygen O-3. If guanine were to be substituted for hypoxanthine in an isogeometrical molecular structure, additional hydrogen bonds could be made between every N-2 and N-7.     

The crystal structure of the alpha-cellobiose.2 NaI.2 H(2)O complex in the context of related structures and conformational analysis

The crystal structure of beta-D-glucopyranosyl-(1-->4)-alpha-D-glucopyranose (alpha-cellobiose) in a complex with water and NaI was determined with Mo K(alpha) radiation at 150 K to R=0.027. The space group is P2(1) and unit cell dimensions are a=9.0188, b=12.2536, c=10.9016 A, beta=97.162 degrees. There are no direct hydrogen bonds among cellobiose molecules, and the usual intramolecular hydrogen bond between O-3 and O-5' is replaced by a bridge involving Na+, O-3, O-5', and O-6'. Both Na+ have sixfold coordination. One I(-) accepts six donor hydroxyl groups and three C-H***I(-) hydrogen bonds. The other accepts three hydroxyls, one Na+, and five C-H***I(-) hydrogen bonds. Linkage torsion angles phi(O-5) and psi(C-5) are -73.6 and -105.3 degrees, respectively (phi(H)=47.1 degrees and psi(H)=14.6 degrees ), probably induced by the Na+ bridge. This conformation is in a separate cluster in phi,psi space from most similar linkages. Both C-6-O-H and C-6'-O-H are gg, while the C-6'-O-H groups from molecules not in the cluster have gt conformations. Hybrid molecular mechanics/quantum mechanics calculations show <1.2 kcal/mol strain for any of the small-molecule structures. Extrapolation of the NaI cellobiose geometry to a cellulose molecule gives a left-handed helix with 2.9 residues per turn. The energy map and small-molecule crystal structures imply that cellulose helices having 2.5 and 3.0 residues per turn are left-handed.