Gt Rotamer Research Articles

Influence of urea on methyl upbeta-D-glucopyranoside in alkali at different temperatures

The dissolution efficiency plays an important role on the properties of regenerated cellulose-based products. Urea is known to be one of the additives aiding to improve cellulose dissolution in the NaOH(aq) system. The acting mechanism caused by urea has been debated and one of the hypothesis is that urea could induce a conformational change on cellulose, which promotes dissolution. Here we used NMR spectroscopy on a model system for cellulose, namely, methyl upbeta-D-glucopyranoside (upbeta-MeO-Glcp) and compared chemical shifts and J couplings, which both are indicators for conformational changes, as a function of temperature and upon the addition of urea. We found that in NaOH(aq), the hydroxymethyl group changes its conformation in favour of the population of the gt rotamer, while the presence of urea induced temperature dependent conformational changes. Heteronuclear Overhauser effect experiments showed that urea associates with cellulose but in a non-specific manner. This suggests that urea rather than binding to the carbohydrate, changes the chemical environment inducing a change in conformation of upbeta-MeO-Glcp and likely also for cellulose when dissolved in NaOH(aq) with urea.

Exhaustive rotamer search of the 4C1 conformation of α- and β-d-galactopyranose

An exhaustive search approach was used to establish all possible rotamers of α- and β-d-galactopyranose using DFT at the B3LYP/6-311+G** and M06-2X/6-311+G** levels, both in vacuum calculations, and including two variants of continuum solvent models as PCM and SMD to simulate water solutions. Free energies were also calculated. MM3 was used as the starting point for calculations, using a dielectric constant of 1.5 for vacuum modeling, and 80 for water solution modeling. For the vacuum calculations, out of the theoretically possible 729 rotamers, only about a hundred rendered stable minima, highly stabilized by hydrogen bonding and scattered in a ca. 14 kcal/mol span. The rotamer with a clockwise arrangement of hydrogen bonds was the most stable for the α-anomer, whereas that with a counterclockwise arrangement was the most stable for the β-anomer. Free energy calculations, and especially solvent modeling, tend to flatten the potential energy surface. With PCM, the total range of energies was reduced to 9–10 kcal/mol (α-anomer) or 7–8 kcal/mol (β-anomer). These figures fall to 4.5–6 kcal/mol using SMD. At the same time, the total number of possible rotamers increases dramatically to about 300 with PCM, and to 400 with SMD. Both models show a divergent behavior: PCM tends to underestimate the effect of solvent, thus rendering as the most stable many common rotamers with vacuum calculations, and giving underestimations of populations of β-anomers and gt rotamers in the equilibrium. On the other hand, SMD gives a better estimation of the solvent effect, yielding correct populations of gt rotamers, but more β-anomers than expected by the experimental values. The best agreement is observed when the functional M06-2X is combined with SMD. Both DFT models show minimal geometrical differences between the optimized conformers.

Probing crystallinity of never-dried wood cellulose with Raman spectroscopy

The structure of wood cell wall cellulose in its native state remains poorly understood, limiting the progress of research and development in numerous areas, including plant science, biofuels, and nanocellulose based materials. It is generally believed that cellulose in cell wall microfibrils has both crystalline and amorphous regions. However, there is evidence that appears to be contrary to this assumption. Here we show, using 1064-nm FT-Raman spectroscopy, that (1) compared to the crystalline state, cellulose in the never-dried native state is laterally aggregated but in a less-than crystalline state wherein internal chains are water-accessible, (2) hydroxymethyl groups (CH2OH) in cellulose exist not only in the tg conformation but also in the gt rotamer form, and (3) in native-state fibrils, low-frequency Raman bands due to cellulose crystal domains are absent, indicating the lack of crystallinity. Further evidence of the absence of crystallinity of the fibrils was the failure of the normal 64 % H2SO4 hydrolysis procedure to produce nanocellulose crystals from untreated wood. X-ray diffraction data obtained on wood, treated-wood, and wood-cellulose samples were consistent with the new finding and indicated that full-width-at-half-height of the X-ray diffractograms and lateral disorder in samples as measured by Raman were correlated (R2 = 0.95).

Solution structures of methyl aldopyranosides revealed by vacuum-ultraviolet electronic circular-dichroism spectroscopy

Vacuum-ultraviolet (VUV) electronic circular-dichroism (ECD) spectra of methyl β-D-glucopyranoside (methyl β-D-Glc), methyl α- and β-D-galactopyranosides (methyl α- and β-D-Gal), and methyl α- and β-D-xylopyranosides (methyl α- and β-D-Xyl) were measured down to 163 nm in aqueous solution using a s ynchrotron-radiation VUV-ECD spectrophotometer. Five methyl aldopyranosides exhibited characteristic ECD spectra depending on the gauche (G) and trans (T) conformations of the hydroxymethyl group at C-5 and the α-/β-anomer configurations of the methoxy group at C-1. To elucidate the contributions of these structures to the spectrum, the ECD spectra of three rotamers (GT, GG and TG) of methyl β-D-Glc, methyl α-D-Gal and methyl β-D-Gal were calculated using a time-dependent density functional theory and molecular dynamics simulations. These theoretical spectra were very similar to those observed experimentally, indicating that the GT and GG rotamers show negative and positive ECD around 170 nm, respectively, and that the α- and β-anomers exhibit negative and positive ECD around 160 nm, respectively. These spectral differences between the two rotamers and between the two anomers were attributable to changes in steric configurations, including the intramolecular hydrogen bond around the ring oxygen and the methoxy oxygen, respectively. These relationships were supported by the ECD spectra experimentally observed for methyl α-D-Xyl and methyl β-D-Xyl, demonstrating that the VUV-ECD spectroscopy is a powerful technique for characterizing the structures of saccharides in aqueous solution.

Cyclic and acyclic fructose conformers in the gas phase: a large-scale second-order perturbation theory study.

We performed large-scale second-order perturbation theory gas-phase calculations to study about five hundred structures of D-fructose. The two lowest energy fructose structures identified are β-pyranoses possessing (2)C5 chair, with ΔG(298 K) of 6 kJ/mol, differing in orientation of the equatorially positioned hydroxymethyl group, gt and g'g, where the gt rotamer is the global minimum, consistent with the recent microwave spectroscopy study. We have found that interconversions from the fructose global minimum to the second and third most stable β-pyranose rotamers involve the energy barriers of ca. 30 kJ/mol. Among numerous fructofuranose conformers discovered (about 250), a pair of the ((3)T2) α- and (E3) β-anomers are energetically most preferred and lie at least 12 kJ/mol above the global minimum. We also found that the fructose open-chain structures lie significantly higher in energy than the most stable cyclic species. The commonly used M06-2X density functional performs well compared to MP2 and G4 theory at identifying the low-energy fructose minima, including the global one, and at reproducing their intramolecular H-bond geometric parameters. The lowest-energy gas-phase pyranose and furanose structures of fructose benefit from stabilization due to the cooperative or quasi-linear H-bonding and both endo and exo anomeric effects.

Conformational properties of glucosyl-thioglucosides and their S-oxides in solution

A stereochemical study of a series of alkyl glucosyl-β-(1→6)-thioglucosides and their S-oxides by means of nuclear magnetic resonance and circular dichroism revealed that the populations around the thioglucosidic bond (ring I) as well as those of the interglycosidic linkage ω depend on the aglycone, the solvent and, in the S-oxides, on the absolute configuration of the sulfur atom. The results for the thio-disaccharides showed the strong influence of the solvent polarity on the conformational preferences of the interglycosidic bond. In polar solvents, the magnitudes of the rotamer populations, Pgg and Pgt, remained practically constant through the series, while in non-polar solvents a clear predominance of gt conformation was observed as well as the influence of the aglycone on the conformational equilibrium. The results for both (SS)- and (RS)-alkyl thiogentiobiosyl S-oxide series showed a clear predominance of the gt rotamer, Pgt always having a higher magnitude in the latter series than in the former. Both series exhibited linear correlations between interglycosidic Pgg and Pgt and Taft’s steric parameter (ES) for the alkyl group attached to the sulfinyl group, especially in non-polar solvents. The stereochemical study around the C1–S bond established that the flexibility around this linkage depends on aglycone size, solvent polarity, and the absolute configuration of the sulfur, derivatives of the SS series showing higher flexibility in both polar and non-polar media. The conformational properties of these compounds in solution are explained in terms of stereoelectronic, steric, and solvent effects.

Direct Synthesis of Maradolipids and Other Trehalose 6-Monoesters and 6,6′-Diesters

It was shown that reaction of trehalose with 1 equiv of a fatty acid in pyridine promoted by 1 equiv of the uronium-based coupling agent 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate (TBTU) at room temperature gives a good yield of the primary ester accompanied by small amounts of the diprimary ester using hexanoic, palmitic, and oleic acids as examples. Reactions using 2 equiv of the fatty acids gave the symmetrical diesters. The monoesters were reacted with different fatty acids to give nonsymmetric 6,6'-diesters in very good yields. Compounds synthesized include the most abundant component of the very complex maradolipid mixture, 6-O-(13-methyltetradecanoyl)-6'-O-oleoyltrehalose, and a component potentially present in this mixture, 6-O-(12-methyltetradecanoyl)-6'-O-oleoyltrehalose, a derivative of an ante fatty acid. The C5-C6 rotameric populations of 6-O-monoesters, symmetrical 6,6'-diesters, and 2,6,6'-triesters of fatty acids were calculated from the values of the H5-H6R and H5-H6S coupling constants and found to be similar to those found for glucose. The rotameric populations of the monosubstituted glucose residues in the 2,6,6'-triesters was altered considerably to favor the gt rotamer, presumably because of attraction between the 2- and 6'-fatty acid chains.

Conformational behavior of α- d-mannopyranosyl-(1→6)-α,β- d-mannose complexed with two mannose-binding plant lectins, Allium sativam agglutinin I and concanavalin A, using NMR and molecular modeling techniques

Herein, we report the intrinsic conformational preferences of α- d-Man p-(1→6)-α,β- d-Man p, ( 1) in the free state and as two (ASAI and ConA) lectin-bound forms. NMR spectroscopy and molecular dynamics techniques are used as 3D-structural determination tools. In free form disaccharide 1 displays a fair amount of conformational freedom, with one major ( ϕ/ ψ 95 ± 30°/195 ± 20°) and one minor (95 ± 30°/70 ± 20°) conformations around the glycosidic linkage and around the ω angle, both the gg and gt rotamers are almost equally populated. This is a first report of a three-dimensional structure of 1 bound with ASAI. Both lectins recognize a major ϕ/ ψ 95 ± 30°/200 ± 30° conformer with the ligand showing more flexibility in the binding site of ConA. Comparison of the mode of binding of the two lectins explains the differences in observed specificities.

Conformational Analysis of 1-Butyl-3-methylimidazolium by CCSD(T) Level Ab Initio Calculations: Effects of Neighboring Anions

Conformational energies for the butyl group of 1-butyl-3-methylimidazolium (bmim) were calculated by high-level ab initio methods. Estimated relative energies for the TT, GT and G'T rotamers of an isolated bmim cation at the CCSD(T)/cc-pVTZ level are 0.0 -0.02 and -0.50 kcal/mol, respectively. The close contact of a Cl anion to theC(2)-H of imidazolium considerably increases the relative stability of the GT rotamer. Estimated relative energies for the three rotamers of the [bmim]Cl complex, in which the Cl anion exists close to the C(2)-H, are 0.0, -1.61 and -0.25 kcal/mol, respectively. The GT rotamer is favored by the strong attractive electrostatic interaction between the bmim cation and Cl anion. The C(2)-H group in the GT rotamer has a larger positive charge compared with those in the TT and G'T rotamers. The contact of a Br anion to the C(2)-H also stabilizes the GT rotamer. The effects of the Cl anion close to the C(4)-Hand C(5)-Hare small. The anion effects suggest that the GT rotamer is the most stable in ionic liquids. The positive charge on imidazolium ring does not largely change the conformational energies. Estimated relative energies for the three rotamers of N-butylimidazole (0.0, -0.29 and -0.75 kcal/mol, respectively) are not largely different from those for isolated bmim. Calculated MP2/cc-pVTZ level torsional potential for the C im-N im-C-C bond has a minimum when the torsional angle is close to 90 degrees. Coplanar conformation is not a stable structure. Calculated torsional barrier height between the two nonplanar minima is less than 1 kcal/mol.

Pathways and populations: stereoelectronic insights into the exocyclic torsion of 5-(hydroxymethyl)tetrahydropyran.

High level ab initio computations in vacuum and with the IEFPCM implicit solvent model are carried out on 5-(hydroxymethyl)tetrahydropyran to investigate the effects of water on the exocyclic torsional surface. Rotamer populations evaluated from the omega(C-C-C-O), theta(C-C-C-O) solvent surface agree almost quantitatively with experimental values for the closely related methyl 4-deoxy-alpha-D-xylohexopyranoside. Potentials of mean force obtained from the two surfaces show substantial solvent stabilization of the TG (omega = 180 +/- 60 degrees) rotamer and the barriers at omega= 120 and 240 degrees but solvent destabilization at the cis barrier (omega = 0 degrees). Natural bond orbital analyses indicate that energetics of these effects are largely explained by overstabilization of the vacuum GT (omega= 60 +/- 60 degrees) and GG (omega = 300 +/- 60 degrees) rotamers. Solvent stabilization of theta conformations provides entropic stabilization.

Involvement of the S-aglycon in the conformational preferences of thioglucosides

The conformational preferences of two series of alkyl β- d-thioglucopyranosides in solution were investigated by NMR and CD. The rotamer populations of the hydroxymethyl group were found to depend on the structural nature of the S-aglycon. The population of the gt rotamer increased and that of the gg rotamer decreased as the alkyl group attached to the S atom increased in size. These rotamer populations have a linear correlation with the Taft’ steric parameters, the n S – σ C – O ∗ exo-anomeric interaction may express these rotational preferences. Comparative analysis of the hydroxymethyl populations between alkyl O- and S-glucosides revealed identical or slightly higher gt and smaller gg populations for the latter compounds.

The exo-deoxoanomeric effect in the conformational preferences of C-glycosides

Rotational studies of a series of β- d- C-glycopyranosides were carried out by CD and NMR spectroscopy. The populations around the C-glycosidic bond were strongly dependent on the structure of the C-aglycon, the exo– syn rotamer population increasing with the degree of substitution on the C-aglycon. The hydroxymethyl group populations also showed dependence on the aglycon, although to a lesser degree; its gt rotamer smoothly increases with the substitution on the aglycon. These rotational preferences, together with the experimentally observed correlations between 1H and 13C NMR chemical shifts and the structural nature of the C-aglycon, point to a stereoelectronic σ CH – σ CO ∗ effect (hyperconjugation) directly involved in the rotation around the pseudo-glycosidic bond and indirectly around the C5–C6 bond (hydroxymethyl group).

4JCOCCH and 4JCCCCH as Probes of Exocyclic Hydroxymethyl Group Conformation in Saccharides

[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.

Experimental evidence on the hydroxymethyl group conformation in alkyl β- d-mannopyranosides

A rotational population study of the hydroxymethyl group of alkyl β- d-mannopyranosides was performed by means of CD and NMR spectroscopy. Three different benzyl, acetyl, and p-bromobenzoyl series of alkyl β- d-mannopyranosides with different chiral and nonchiral aglycons were synthesized and analyzed. Different rotational populations were observed for each series by changing the structure of the aglycon. The results showed a clear correlation between the rotational population of the hydroxymethyl group around the C5–C6 bond and the p K a of the bonded alcohol (aglycon). The population of the gt rotamer gradually increased as the p K a increased while that of the gg rotamer decreased and the population of the tg rotamer remained almost constant. This is explained by the exo-anomeric effect. For chiral alkyl derivatives, the results also showed a close dependence on the absolute configuration of the aglycon. Comparison of rotational population anomers revealed the dependence of the hydroxymethyl group on the anomeric configuration and a greater dependence on the aglycon structure in the β anomers.

NMR and Monte Carlo Studies on the Solution Conformation of a Linear Capsular Polysaccharide from a Soybean‐Nodulating Bacterium (Strain B33) †

The conformational behavior of the capsular polysaccharide obtained from a fast‐growing soybean‐nodulating rhizobia (strain B33) isolated from Xinjiang Autonomous Region (Eastern China) has been analyzed by NMR and molecular mechanics simulations. This polysaccharide has the repeating unit →6)‐4‐O‐Me‐α‐d‐Glcp‐(1→4)‐3‐O‐Me‐β‐d‐GlcpA‐(1→. The NMR results indicate that the α‐(1→4) linkage may adopt a variety of conformations, and that at least two of the resulting minima must exist in solution. NOE data agree with an 85:10:5 ratio for the lowest‐energy conformations. In the case of the β‐(1→6) linkage, NMR indicates that the rotamer gg is highly populated. Experimental and calculated NOE intensities match well when the global energy minimum conformation for this linkage has exclusively the gg orientation. The influence of the adjacent methyl group on the glycosyloxymethyl population has been evaluated by simulation of a disaccharide without this group. A relative destabilization of gt rotamer has been found. Long chains have been simulated using a Metropolis algorithm at different ratios of the gg and gt rotamers in the glucose moiety. It was observed that the increase in population of the gt rotamer yielded more close contacts in the chain. †This paper is dedicated to Professor Gérard Descotes on the occasion of his 70th birthday.

Conformational study of the hydroxymethyl group in α-d-mannose derivatives

A study of the dependence of the hydroxymethyl group in α-d-mannose derivatives on the aglycon and its absolute configuration was performed by means of circular dichroism (CD) and NMR data. Depending mainly on the aglycon present, the gg or the gt rotamer was the most populated, the tg rotamer having a small population. In addition, the study showed a correlation between the rotational populations and the aglycon, the population of the gt rotamer increasing as the pKa of the bonded alcohol (aglycon) increased. Furthermore, the results revealed a strong dependence on the absolute configuration of the aglycon and point to the stereoelectronic exo-anomeric effect being responsible for these rotational dependencies besides nonbonding interactions.

Hydroxymethyl Rotamer Populations in Disaccharides.

Sixteen methyl glucopyranosyl glucopyranoside disaccharides (methyl beta-d-Glcp(p-Br-Bz)-(1-->x)-beta/alpha-d-Glcp) containing beta-glycosidic linkages (1-->2, 1-->3, 1-->4, and 1-->6) were synthesized and analyzed by means of CD and NMR spectroscopy in three different solvents. For each of these four types of disaccharides, a correlation was observed between the hydroxymethyl rotational populations around the C5-C6 bond of the glucopyranosyl residue II with the substituents and the anomeric configuration of the methoxyl group in residue I, as well as with the solvent. Nonbonded interactions, the stereoelectronic exo-anomeric effect, and hydrogen bonding were found to be responsible for the observed rotameric differences. Whereas the rotational populations of the (1-->6)-linked disaccharides are mainly dependent on the exo-anomeric effect, the (1-->2)-bonded disaccharides are strongly dependent on the anomeric configuration at C1, and the (1-->3)- and (1-->4)-linked disaccharides are mainly dependent on the substituents and the solvent. The population of the gt rotamer decreases as nonbonded interactions increase but increases as the exo-anomeric effect becomes greater, as well as in the presence of intramolecular hydrogen bonding to the endocyclic oxygen O5'. Comparison of the hydroxymethyl rotational preferences between our model disaccharides revealed a dependence on the glycosidic linkage type. Thus the population of the gg and gt rotamers decreases/increases from (1-->2)- (beta series), to (1-->6)-, to (1-->2)- (alpha series), to (1-->4)-, and to (1-->3)-bonded disaccharides respectively, while the tg rotamer population remains almost constant (around 20%), except for the (1-->3)- and (1-->4)-linked disaccharides with the intramolecular hydrogen bonding to O5', where this population decreases to 10%.

2-Deoxy-beta-D-erythro-pentofuranose: hydroxymethyl group conformation and substituent effects on molecular structure, ring geometry, and NMR spin-spin coupling constants from quantum chemical calculations.

The effect of hydroxymethyl conformation (gg, gt, and tg rotamers about the C4-C5 bond) on the conformational energies and structural parameters (bond lengths, bond angles, bond torsions) of the 10 envelope forms of the biologically relevant aldopentofuranose, 2-deoxy-beta-D-erythro-pentofuranose (2-deoxy-D-ribofuranose) 2, has been investigated by ab initio molecular orbital calculations at the HF/6-31G level of theory. C4-C5 bond rotation induces significant changes in the conformational energy profile of 2 (2gt and 2tg exhibit one global energy minimum, whereas 2gg exhibits two nearly equivalent energy minima), and structural changes, especially those in bond lengths, are consistent with predictions based on previously reported vicinal, 1,3- and 1,4-oxygen lone pair effects. HF/6-31G-optimized envelope geometries of 2gg were re-optimized using density functional theory (DFT, B3LYP/6-31G), and the resulting structures were used in DFT calculations of NMR spin-spin coupling constants involving 13C (i.e., J(CH) and J(CC) over one, two, and three bonds) in 2gg according to methods described previously. The computed J-couplings were compared to those reported previously in 2gt to assess the effect of C4-C5 bond rotation on scalar couplings within the furanose ring and hydroxymethyl side chain. The results confirm prior predictions of correlations between 2J(CH), 3J(CH), 2J(CC) and 3J(CC), and ring conformation, and verify the usefulness of a concerted application of these couplings (both their magnitudes and signs) in assigning preferred ring and C4-C5 bond conformations in aldopentofuranosyl rings. The new calculated J-couplings in 2gg have particular relevance to related J-couplings in DNA (and RNA indirectly), where the gg rotamer, rather than the gt rotamer, is observed in most native structures. The effects of two additional structural perturbations on 2 were also studied, namely, deoxygenation at C5 (yielding 2,5-dideoxy-beta-D-erythro-pentofuranose 4) and methyl glycosidation at O1 (yielding methyl 2-deoxy-beta-D-erythro-pentofuranoside 5) at the HF/6-31G level. The conformational energy profile of 4 resembles that found for 2gt, not 2gg, indicating that 4 is an inappropriate structural mimic of the furanose ring in DNA. Glycosidation failed to induce differential stabilization of ring conformations containing an axial C1-O1 bond (anomeric effect), contrary to experimental data. The latter discrepancy indicates that either the magnitude of this differential stabilization depends on ring configuration or that solvent effects, which are neglected in these calculations, play a role in promoting this stabilization.

Quantitative conformational analysis of the core region of N-glycans using residual dipolar couplings, aqueous molecular dynamics, and steric alignment.

A method is described for quantitatively investigating the dynamic conformation of small oligosaccharides containing an alpha(1 --> 6) linkage. It was applied to the oligosaccharide Man-alpha(1 --> 3) [Man-alpha(1 --> 6)] Man-alpha-O-Me, which is a core region frequently observed in N-linked glycans. The approach tests an aqueous molecular dynamics simulation, capable of predicting microscopic dynamics, against experimental residual dipolar couplings, by assuming that alignment is caused purely by steric hindrance. The experimental constraints were heteronuclear and homonuclear residual dipolar couplings, and in particular those within the alpha(1 --> 6) linkage itself. Powerful spin-state-selective pulse sequences and editing schemes were used to obtain the most relevant couplings for testing the model. Molecular dynamics simulations in water over a period of 50 ns were not able to predict the correct rotamer population at the alpha(1 --> 6) linkage to agree with the experimental data. However, this sampling problem could be corrected using a simple maximum likelihood optimisation, indicating that the simulation was modelling local dynamics correctly. The maximum likelihood prediction of the residual dipolar couplings was found to be an almost equal population of the gg and gt rotamer conformations at the alpha(1--> 6) linkage, and the tg conformation was predicted to be unstable and unpopulated in aqueous solution. In this case all twelve measured residual dipolar couplings could be satisfied. This conformer population could also be used to make predictions of scalar couplings with the use of a previously derived empirical equation, and is qualitatively in agreement with previous predictions based on NMR, X-ray crystallography and optical data.

A conformational study of the xyloglucan oligomer, XXXG, by NMR spectroscopy and molecular modeling

A structural study of the XXXG xyloglucan heptasaccharide (X = α-D-Xylp(1 → 6)-β-D-Glcp and G = β-D-Glcp) isolated from apple fruit has been undertaken with nmr and molecular mechanics methods. Quantitative 400 MHz nmr data including nuclear Overhauser effect spectroscopy (NOESY) volumes were recorded at both 6 and 20°C. In spite of severe overlapping of resonances, it was possible to estimate summed NOEs for the majority of the anomeric and glucosyl methylene protons. An ensemble-average population of preferred geometries has been established with the CICADA conformational searching algorithm associated with the MM3 force field. Comparison of the theoretical data obtained by back-calculation of the NOESY volumes from the ensemble-average distance matrix program and motional models based on the Stokes–Einstein–Debye relation satisfactorily reproduce the experimental data. Conformational averaging about the mainchain glycosidic linkages includes both the syn and anti conformers and a minor gauche–gauche population is highly probable. The theoretical data overestimate the syn preference of the Glcc → Glcb linkage as well as the Glcc GT rotamer population. Finally, both the motional models and the conformational search indicate a fairly rigid backbone and greater flexiblity for the xylose side chains. © 2000 John Wiley & Sons, Inc. Biopoly 54: 11–26, 2000