Vibrational normal modes calculation in the crystalline state of methylated monosaccharides: Anomers of the methyl-D-glucopyranoside and methyl-D-xylopyranoside molecules

A structural investigation of the organic molecules is being carried out using vibrational spectroscopy. In this study, normal co-ordinate calculations of anomers of the methyl-D-glucopyranoside and methyl-β-D-xylopyranoside in the crystalline state have been performed using the modified Urey-Bradley-Shimanouchi force field (mUBSFF) combined with an intermolecular potential energy function. The latter includes Van der Waals interactions, electrostatic terms, and explicit hydrogen bond functions. The vibrational spectra of the compounds recorded in the crystalline state, in the 4000-500 cm(-1) spectral region for the IR spectra, and in the 4000-20 cm(-1) spectral range for the Raman spectra are presented. After their careful examination, several differences in the intensities and frequency shifts have been observed. The theoretical spectra have been obtained after a tedious refinement of the force constants. Thus, on the basis of the obtained potential distribution, each observed band in IR and in Raman has been assigned to a vibrational mode. The obtained results are indeed in agreement with those observed experimentally and thus confirm the previous assignments made for the methyl-α and β-D-glucopyranoside, as well as for the methyl-β-D-xylopyranoside.

Effects of various vitamins and coenzymes Q on reactions involving α-hydroxyl-containing radicals

Effects of vitamins B, C, E, K and P, as well as coenzymes Q, on formation of final products of radiation-induced free-radical transformations of ethanol, ethylene glycol, alpha-methylglycoside and glucose in aqueous solutions were studied. Based on the obtained results, it can be concluded that there are substances among vitamins and coenzymes that effectively interact with alpha-hydroxyl-containing radicals. In the presence of these substances, recombination reactions of alpha-hydroxyalkyl radicals and fragmentation of alpha-hydroxy-beta-substituted organic radicals are suppressed. It has been established that the observed effects are due to the ability of the vitamins and coenzymes under study to either oxidize alpha-hydroxyl-containing radicals yielding the respective carbonyl compounds or reduce them into the initial molecules.

DFT and NMR studies of 2JCOH, 3JHCOH, and 3JCCOH spin-couplings in saccharides: C-O torsional bias and H-bonding in aqueous solution

Density functional theory (DFT) has been used to investigate the structural dependencies of NMR spin-coupling constants (J-couplings) involving the exchangeable hydroxyl protons of saccharides. 3JHCOH, 3JCCOH, and 2JCOH values were calculated at different positions in model aldopyranosyl rings as a function of one or more torsion angles, and results support the use of a generalized Karplus equation to treat 3JHCOH involving the non-anomeric OH groups. The presence of O5 appended to the H1-C1-O1-H coupling pathway introduces asymmetry in 3JH1,O1H Karplus curves due to internal electronegative substituent effects on the gauche couplings, thus requiring separate equations to treat this coupling. 3JCCOH values depend not only on the C-C-O-H torsion angle but also on the orientation of terminal substituents on the coupled carbon, similar to 3JCOCC studied previously (Bose et al., J. Am. Chem. Soc. 1998, 120, 11158-11173). "In-plane" oxygen increased 3JCCOH by approximately 3-4 Hz, whereas "in-plane" carbon gave more modest enhancements ( approximately 1 Hz). Three Karplus equations were derived for non-anomeric 3JCCOH based on the nature and orientation of substituents on the coupled carbon. Like 3JH1,O1H, 3JC2,O1H is also subject to internal electronegative substituent effects on the gauche couplings, thus necessitating separate equations to treat this coupling. 2JCOH values were found not to be useful probes of C-O torsions as a result of their nonsystematic dependence on these torsions. Experimental measurements of 3JHCOH and 3JCCOH in doubly 13C-labeled methyl beta-lactoside 20 and its constituent 13C-labeled methyl aldopyranosides in H2O/acetone-d6 at -20 degrees C showed that some C-O torsion angles are influenced by molecular context and do not experience complete rotational averaging in solution. A strong bias in the H3-C3-O3-H torsion angle in the Glc residue of 20 favoring a gauche conformation suggests the presence of inter-residue H-bonding between O3HGlc and O5Gal. Quantitative analysis of 3JHCOH and 3JCCOH values in 20 indicates that approximately 85% of the forms in solution have geometries consistent with H-bonding. These results suggest that H-bonding between adjacent and/or remote residues may play a role in dictating preferred glycosidic bond conformation in simple and complex oligosaccharides in aqueous solution.

Conformational analysis of methyl 5-O-methyl septanosides: effect of glycosylation on conformer populations

Methyl 5-O-methyl-alpha-d-glycero-d-idoseptanoside (3) and methyl 5-O-methyl-beta-d-glycero-d-guloseptanoside (4) were investigated as (1-->5)-linked di-/oligoseptanoside mimetics. Here we report the synthesis of 3 and 4 and describe their preferred solution conformations through a combination of ab initio/DFT calculations and (1)H (3)J(H,H) NMR coupling constant analysis. The conformations of 3 and 4 observed in this study are discussed in comparison to those of the parent (C5 hydroxy) compounds 1 and 2. The results indicate that methyl 5-O-methyl-alpha-septanoside 3 is relatively rigid and adopts the same (3,4)TC(5,6) conformation as 1. Methyl 5-O-methyl-beta-septanoside 4 is somewhat less rigid than its parent septanoside (2). In addition to the (6,O)TC(4,5) conformation adopted by 2, beta-septanoside 4 also populates the adjacent (3,4)TC(5,6) conformation. Glycosylation at C5 on beta-septanoside 4 therefore increases its overall flexibility and allows access to alternative ring conformations.

Secondary metabolites and insecticidal activity of Anemone pavonina

The insecticidal properties of the crude extracts of the leaves and flowers of Anemone pavonina were evaluated on Pheidole pallidula ants and showed significant levels of activity. Bioassay-guided fractionations led to the isolation of the butenolide ranunculin (1) as the active principle. Chemical investigations of the extracts showed them to contain as major components the sitosterol glycopyranoside lipids 2-5 and the glycerides 6-8. The structures of the metabolites were elucidated, following acetylation and hydrolysis of the natural products, by interpretation of their NMR and mass spectral data. The uncommon lipid metabolites 2-8 were isolated for the first time from the genus Anemone and this is the first report of insecticidal activity of the Anemone metabolite ranunculin against ants.

Alkylalanes and methyl furanosides: regioselective O-debenzylation or acetal cleavage

Perbenzylated methyl pentofuranosides were submitted to the action of three alkylalanes and regioselective debenzylation at O-2 of the four pentoses was observed when choosing the right match between anomeric configuration and aluminium reagent. Diisobutylalane (DIBAL-H) allowed an easy access to reduced open-chain compounds, whereas trimethylalane (TMAL) stereoselectively produced methylated open chain derivatives.

Methyl 4-O-beta-L-galactopyranosyl-beta-D-glucopyranoside (methyl beta-L-lactoside)

Methyl beta-L-lactoside, C13H24O11, (II), is described by glycosidic torsion angles phi (O5Gal-C1Gal-O4Glc-C4Glc) and psi (C1Gal-O1Gal-C4Glc-C5Glc) of 93.89 (13) and -127.43 (13) degrees , respectively, where the ring atom numbering conforms to the convention in which C1 is the anomeric C atom and C6 is the exocyclic hydroxymethyl (CH2OH) C atom in both residues (Gal is galactose and Glc is glucose). Substitution of L-Gal for D-Gal in the biologically relevant disaccharide, methyl beta-lactoside [Stenutz, Shang & Serianni (1999). Acta Cryst. C55, 1719-1721], (I), significantly alters the glycosidic linkage interface. In the crystal structure of (I), one inter-residue (intramolecular) hydrogen bond is observed between atoms H3OGlc and O5Gal. In contrast, in the crystal structure of (II), inter-residue hydrogen bonds are observed between atoms H6OGlc and O5Gal, H6OGlc and O6Gal, and H3OGlc and O2Gal, with H6OGlc serving as a donor with two intramolecular acceptors.

Crystal structure of methyl 3-amino-2,3-dideoxy-beta-D-arabino-hexopyranoside. Stabilization of the crystal lattice by a double network of N-H...O, O-H...N and O-H...O interactions

The structure, conformation and configuration of methyl 3-amino-2,3-dideoxy-beta-D-arabino-hexopyranoside were investigated by (1)H NMR, (13)C NMR and IR spectroscopy, as well as by optical rotation. The crystal structure was confirmed by single-crystal X-ray crystallographic analysis at 293 K and R = 0.0434 based on 910 independent reflections. The crystal belongs to the monoclinic system, space group of P2(1) with cell dimensions a = 6.050(1) Angstroms, b = 7.284(1) Angstroms, c = 10.289(2) Angstroms, beta = 104.69(3) degrees, D(c) = 1.341 Mg cm(-3) and V = 438.9(1) Angstroms(3) for Z = 2. Furthermore, the molecule has a typical (4)C(1) chair conformation. Hydrogen bonds between sugar molecules are responsible for stabilizing the crystal lattice.

Methyl 4-O-β-D-galactopyranosyl α-D-glucopyranoside (methyl α-lactoside)

Methyl alpha-lactoside, C13H24O11, (I), is described by glycosidic torsion angles varphi (O5gal-C1gal-O1gal-C4glc) and psi (C1gal-O1gal-C4glc-C5glc), which have values of -93.52 (13) and -144.83 (11) degrees, respectively, where the ring atom numbering conforms to the convention in which C1 is the anomeric C atom and C6 is the exocyclic hydroxymethyl (-CH2OH) C atom in both residues. The linkage geometry is similar to that observed in methyl beta-lactoside methanol solvate, (II), in which varphi is -88.4 (4) degrees and psi is -161.3 (4) degrees. As in (II), an intermolecular O3glc-H...O5gal hydrogen bond is observed in (I). The hydroxymethyl group conformation in both residues is gauche-trans, with torsion angles omegagal (O5gal-C5gal-C6gal-O6gal) and omega(glc) (O5glc-C5glc-C6glc-O6glc) of 69.15 (13) and 72.55 (14) degrees, respectively. The latter torsion angle differs substantially from that found for (II) [-54.6 (2) degrees; gauche-gauche]. Cocrystallization of methanol, which is hydrogen bonded to O6glc in the crystal structure of (II), presumably affects the hydroxymethyl conformation in the Glc residue in (II).

Deoxy and deoxyfluoro analogues of acetylated methyl beta-D-xylopyranoside--substrates for acetylxylan esterases

Four modified substrates for acetylxylan esterases, 2-deoxy, 3-deoxy, 2-deoxy-2-fluoro, and 3-deoxy-3-fluoro derivatives of di-O-acetylated methyl beta-D-xylopyranoside were synthesized via 2,3-anhydropentopyranoside precursors. Methyl 2,3-anhydro-4-O-benzyl-beta-D-ribopyranoside was transformed into methyl 2,3-anhydro-4-O-benzyl-beta-D-lyxopyranoside in three steps. The epoxide ring opening of 2,3-anhydropentopyranosides was accomplished either by hydride reduction or hydrofluorination. Methyl beta-D-xylopyranoside 2,3,4-tri-O-, 2,4-di-O-, and 3,4-di-O-acetates, and the prepared diacetate analogues were tested as substrates of acetylxylan esterases from Schizophyllum commune and Trichoderma reesei. Measurement of their rate of deacetylation pointed to unique structural requirements of the enzymes for the substrates. The enzymes differed particularly in the requirement for the trans vicinal hydroxy group in the deacetylation at C-2 and C-3 and in the tolerance to the presence of trans vicinal acetyl groups esterifying the OH group at C-2 and C-3.

Determination of sugar structures in solution from residual dipolar coupling constants: methodology and application to methyl beta-D-xylopyranoside

We have developed methodology for the determination of solution structures of small molecules from residual dipolar coupling constants measured in dilute liquid crystals. The power of the new technique is demonstrated by the determination of the structure of methyl beta-d-xylopyranoside (I) in solution. An oriented sample of I was prepared using a mixture of C(12)E(5) and hexanol in D(2)O. Thirty residual dipolar coupling constants, ranging from -6.44 to 4.99 Hz, were measured using intensity-based J-modulated NMR techniques. These include 15 D(HH), 4 (1)D(CH), and 11 (n)D(CH) coupling constants. The accuracy of the dipolar coupling constants is estimated to be < +/- 0.02 Hz. New constant-time HMBC NMR experiments were developed for the measurement of (n)D(CH) coupling constants, the use of which was crucial for the successful structure determination of I, as they allowed us to increase the number of fitted parameters. The structure of I was refined using a model in which the directly bonded interatom distances were fixed at their ab initio values, while 16 geometrical and 5 order parameters were optimized. These included 2 CCC and 6 CCH angles, and 2 CCCC and 6 CCCH dihedral angles. Vibrationally averaged dipolar coupling constants were used during the refinement. The refined solution structure of I is very similar to that obtained by ab initio calculations, with 11 bond and dihedral angles differing by 0.8 degrees or less and the remaining 5 parameters differing by up to 3.3 degrees . Comparison with the neutron diffraction structure showed larger differences attributable to crystal packing effects. Reducing the degree of order by using dilute liquid crystalline media in combination with precise measurement of small residual dipolar coupling constants, as shown here, is a way of overcoming the limitation of strongly orienting liquid crystals associated with the complexity of (1)H NMR spectra for molecules with more than 12 protons.

Water T2 relaxation in sugar solutions

1H spin-spin relaxation times of water were measured with the CPMG sequence in dilute aqueous solutions of glucitol, mannitol, glycerol, glycol, the methyl D-pyranosides of alpha-glucose, beta-glucose, alpha-galactose, beta-galactose, alpha-xylose, beta-xylose, beta-arabinose and sucrose, alpha,alpha-trehalose, beta-maltose, maltotriose and maltoheptaose. The relaxation-time dispersion was measured by varying the CPMG pulse spacing, tau. These data were interpreted by means of the Carver-Richards model in which exchange between water protons and labile solute hydroxyl protons provides a significant contribution to the relaxation. From the dependences on temperature and tau, parameters characteristic of the pool of hydroxyls belonging to a given solute were extracted by nonlinear regression, including: the fraction of exchangeable protons, P, the chemical-shift difference between water protons and hydroxyl protons, deltaomega, the intrinsic spin-spin relaxation time, T2, and the chemical exchange rate, k. These solute-specific parameters are related, respectively, to the concentration, identity, mobility and exchange life-time of the hydroxyl site. At 298 K, values of deltaomega, T2 and k were found to be of the order of 1 ppm, 100 ms and 1000 s(-1), respectively. Effects of molecular size, conformation and solute concentration were investigated. The exchange mechanism was characterised by Eyring activation enthalpies and entropies with values in the ranges 50-70 kJ mol(-1) and -10 to 60 J K(-1)mol(-1), respectively.

Septanose Carbohydrates: Synthesis and Conformational Studies of Methyl α-d-glycero-d-Idoseptanoside and Methyl β-d-glycero-d-Guloseptanoside

We report the synthesis of methyl alpha-D-glycero-D-idoseptanoside (1) and methyl beta-D-glycero-D-guloseptanoside (2) and the characterization of their preferred solution conformations by computational chemistry and (1)H NMR (3)J(H,H) coupling constant analysis. Central to the synthetic approach was the epoxidation of glucose-derived oxepine 3 using DMDO. Nucleophilic attack on the resulting 1,2-anhydroseptanose using NaOCH(3) in CH(3)OH followed by deprotection provided the 1,2-trans diastereomers 1 and 2. The computational approach for determining the preferred low energy septanose conformations began with a pseudo Monte Carlo search for each isomer using minimization with the AMBER force field. Single-point energy calculations (HF/6-31G *and B3LYP/6-31+G**) as well as full geometry optimizations in a model for aqueous solvent were then conducted using the conformers within 5 kcal/mol of the AMBER global minimum. Calculated (3)J(H,H) values, based on a Boltzmann distribution of the computed low energy conformers, were compared to experimental (3)J(H,H) values from (1)H NMR coupling constant analyses. The correlation between calculated and observed values suggest that septanose carbohydrates are not so flexible and should generally prefer one twist-chair (TC) conformation.

X-ray and conformational analysis of methyl 3-amino-2,3-dideoxy-α-d-arabino-hexopyranoside

The structure, conformation and configuration of methyl 3-amino-2,3-dideoxy-alpha-d-arabino-hexopyranoside were confirmed by (1)H NMR, (13)C NMR and IR spectroscopy, as well as by optical rotation. The structure of the compound studied was also determined by single crystal X-ray crystallography at 293 K and refined to a final R=0.0521 based on 1798 independent reflections. The title compound crystallized in the tetragonal space group P4(3) with a=6.572(1) angstrom, b=6.572(1) angstrom, c=41.161(8) angstrom, D(c)=1.324 Mgcm(-3) and V=1777.8(5) angstrom(3) for Z=8. The packing arrangement in the unit cell displayed a stratified structure. Moreover, medium-strength N-H. . .O and O-H. . .O hydrogen bonds, which stabilized the 3-D structure of compound I, were observed.

An efficient chemoenzymatic route to methyl 4-O-benzyl-2,3-anhydro-β-d-lyxopyranoside from methyl β-d-xylopyranoside

Methyl 4-O-benzyl-2,3-anhydro-beta-D-lyxopyranoside, an intermediate for the preparation of methyl beta-D-xylopyranoside derivatives modified at C-2, was obtained in five steps in 58% yield. The synthetic sequence starts from methyl beta-D-xylopyranoside through two main steps involving regioselective enzymatic acetylation and deacetylation catalyzed by lipase PS.

Transacetylations to carbohydrates catalyzed by acetylxylan esterase in the presence of organic solvent

Various conditions were applied to test the ability of acetylxylan esterase (AcXE) from Schizophyllum commune to catalyze acetyl group transfer to methyl beta-D-xylopyranoside (Me-beta-Xylp) and other carbohydrates. The best performance of the enzyme was observed in an n-hexane-vinyl acetate-sodium dioctylsulfosuccinate (DOSS)-water microemulsion at a molar water-detergent ratio (w(0)) of about 4-5. Although the enzyme was found to have a half-life of about 1 h in the system, more than 60% conversion of Me-beta-Xylp to acetylated derivatives was achieved. Under identical reaction conditions, the enzyme acetylated other carbohydrates such as methyl beta-D-cellobioside (Me-beta-Cel), cellotetraose, methyl beta-D-glucopyranoside (Me-beta-Glcp), 2-deoxy-D-glucose, D-mannose, beta-1,4-mannobiose, -mannopentaose, -mannohexaose, beta-1,4-xylobiose and -xylopentaose. This work is the first example of reverse reactions by an acetylxylan esterase and a carbohydrate esterase belonging to family 1.

Mode of action of acetylxylan esterase from Streptomyces lividans: a study with deoxy and deoxy-fluoro analogues of acetylated methyl beta-D-xylopyranoside

Streptomyces lividans acetylxylan esterase removes the 2- or 3-O-acetyl groups from methyl 2,4-di-O-acetyl- and 3,4-di-O-acetyl beta-D-xylopyranoside. When the free hydroxyl group was replaced with a hydrogen or fluorine, the rate of deacetylation was markedly reduced, but regioselectivity was not affected. The regioselectivity of deacetylation was found to be independent of the prevailing conformation of the substrates in solution as determined by 1H-NMR spectroscopy. These observations confirm the importance of the vicinal hydroxyl group and are consistent with our earlier hypothesis that the deacetylation of positions 2 and 3 may involve a common ortho-ester intermediate. Another possible role of the free vicinal hydroxyl group could be the activation of the acyl leaving group in the deacetylation mechanism. Involvement of the free hydroxyl group in the enzyme-substrate binding is not supported by the results of inhibition experiments in which methyl 2,4-di-O-acetyl beta-D-xylopyranoside was used as substrate and its analogues or methyl beta-D-xylopyranoside as inhibitors. The enzyme requires for its efficient action the trans arrangement of the free and acetylated hydroxyl groups at positions 2 and 3.

An access to various sulfation patterns in dermatan sulfate: chemical syntheses of sulfoforms of trisaccharide methyl glycosides

The syntheses are reported for the first time of alpha-L-IdopA2SO(3)-(1-->3)-beta-D-GalpNAc4SO(3)-(1-->4)-alpha-L-IdopA2SO(3)-(1-->OMe), its disulfated analogue alpha-L-IdopA2SO(3)-(1-->3)-beta-D-GalpNAc-(1-->4)-alpha-L-IdopA2SO(3)-(1-->OMe), and of beta-D-GalpNAc4SO(3)-(1-->4)-alpha-L-IdopA2SO(3)-(1-->3)-beta-D-GalpNAc4SO(3)-(1-->OMe), which represent structural fragments of dermatan sulfate, unavailable directly by chemical or enzymatic degradation of the glycosaminoglycan polymer. These molecules were readily obtained from a pair of key disaccharide intermediates, in which the relative difference of stability of the D-GalNAc 4-hydroxy protecting groups (acetate or pivalate) toward saponification conditions allowed access to various sulfoforms from a common precursor. For the preparation of these blocks, the 4-O-pivaloyl-D-galacto moiety was readily obtained through a one-pot stereospecific intramolecular nucleophilic displacement on an easily available 3-O-pivaloyl-D-gluco precursor, and the L-IdoA moiety through selective radical oxidation at C-6 of a L-ido 4,6-diol derivative with oxoammonium salts.

Further evidence for the gelation ability-structure correlation in sugar-based gelators

Eight methyl glycosides of 4,6-O-benzylidene derivatives of the monosaccharides D-glucose, D-mannose, D-allose and D-altrose were synthesized to systematically study the effect of small configurational changes on the ability to gelate organic solvents. Among the beta anomers, only the D-mannose glycoside exhibits a strong gelation ability, whereas in the alpha-series the D-glucose and D-mannose derivatives act as versatile gelators. Also, as a general rule we found that the beta anomers possess a higher ability to gelate solvents than the alpha anomers. The gelation properties are discussed on the basis of SAXS, FTIR, differential scanning calorimetric (DSC) measurements and scanning electron microscopy (SEM) observations. The temperature-dependent SAXS measurements were carried out to elucidate the sol-gel transition temperature. The present study emphasizes that the saccharide family provides, not only valuable information of the structural requirements for the design of new gelators, but also for molecular assembly systems in general.

The 2.2 A resolution structure of the O(H) blood-group-specific lectin I from Ulex europaeus

The tertiary and quaternary structure of the lectin I from Ulex europaeus (UE-I) has been determined to 2.2 A resolution. UE-I is a dimeric metalloglycoprotein that binds the H-type 2 human blood group determinant [alpha-L-Fucalpha(1-->2)-beta-D-Galbeta(1-->4)-beta-D-Glc NAcalpha-]. Nine changes from the published amino acid sequence were necessary to account for the electron density. The quaternary structural organization of UE-I is that of the most commonly occurring legume lectin dimer. The tertiary structure of the monomeric subunits is similar to that in the conventional lectin subunit; however, some structural differences are noted. These differences include a four-stranded anti-parallel "S" sheet in UE-I versus the five-stranded S sheet in other lectin monomers. The Ala residue of the Ala-Asp cis-peptide bond present in the carbohydrate-binding site of the conventional lectin monomer is replaced with a Thr in the UE-I structure. Also, a novel disulfide bridge linking Cys115 and Cys150 is present. There are two metallic ions, one calcium and the other manganese, per subunit. N-linked oligosaccharides are at residues 23 and 111 of each subunit. One molecule of R-2-methyl-2, 4-pentanediol (R-MPD) is present in a shallow depression on the surface of each subunit. In order to examine the binding of the H-type 2 blood group determinant by UE-I, its beta-methyl glycoside (H-type 2-OMe) was docked into the binding site of R-MPD. The epitope previously identified for H-type 2-OMe by chemical mapping proved, with only minor adjustment of amino acid residues, to be complementary to the shallow cavity occupied by R-MPD in the structure. Several key interactions have been proposed between the H-type 2-OMe and UE-I.

The mild cleavage of 2-amino-2-deoxy-D-glucoside methoxycarbonyl derivatives

The conversion of methyl carbamate to the corresponding free amine is described for a series of 2-amino-2-deoxy-D-glucosamine derivatives. Cleavage of methoxycarbonyl moiety with MeSiCl(3) and triethylamine in dry THF at 60 degrees C and subsequent aqueous hydrolysis yields the free amine in 54 to 93% yields. The selective cleavage of methyl carbamates with MeSiCl(3) in the presence of a 2,2,2-trichloroethoxycarbonyl group or 2-azido glycosides affords selectively, orthogonal N-deprotected carbohydrates.

Structural characterization of the O-antigenic polysaccharide of the lipopolysaccharide from Rhizobium etli strain CE3. A unique O-acetylated glycan of discrete size, containing 3-O-methyl-6-deoxy-L-talose and 2,3,4-tri-O-,methyl-l fucose

The O-antigenic polysaccharide of the Rhizobium etli CE3 lipopolysaccharide (LPS) was structurally characterized using chemical degradations (Smith degradation and beta-elimination of uronosyl residues) in combination with alkylation analysis, electrospray, and matrix-assisted laser desorption ionization-time of flight mass spectrometry, tandem mass spectrometry, and (1)H COSY and TOCSY nuclear magnetic resonance spectroscopy analyses of the native polysaccharide and the derived oligosaccharides. The polysaccharide was found to be a unique, relatively low molecular weight glycan having a fairly discrete size, with surprisingly little variation in the number of repeating units (degree of polymerization = 5). The polysaccharide is O-acetylated and contains a variety of O-methylated glycosyl residues, rendering the native glycan somewhat hydrophobic. The molecular mass of the major de-O-acetylated species, including the reducing end 3-deoxy-d-manno-2-octulosonic acid (Kdo) residue, is 3330 Da. The polysaccharide is comprised of a trisaccharide repeating unit having the structure -->4)-alpha-d-GlcpA-(1-->4)-[alpha-3-O-Me-6-deoxy-Talp-(1--> 3)]-alpha -l-Fucp-(1-->. The nonreducing end of the glycan is terminated with the capping sequence alpha-2,3, 4-tri-O-Me-Fucp-(1-->4)-alpha-d-GlcpA-(1-->, and the reducing end of the molecule consists of the non-repeating sequence -->3)-alpha-l-Fucp-(1-->3)-beta-d-Manp-(1-->3)-beta-QuiNA cp-(1-->4)-a lpha-Kdop-(2-->, where QuiNAc is N-acetylquinovosamine (2-N-acetamido-2,6-dideoxyglucose). The reducing end Kdo residue links the O-chain polysaccharide to the core region oligosaccharide, resulting in a unique location for a Kdo residue in LPS, removed four residues distally from the lipid A moiety. Structural heterogeneity in the O-chain arises mainly from the O-acetyl and O-methyl substitution. Methylation analysis using trideuteriomethyl iodide indicates that a portion of the 2,3,4-tri-O-methylfucosyl capping residues, typically 15%, are replaced with 2-O-methyl- and/or 2,3-di-O-methylfucosyl residues. In addition, approximately 25% of the 3,4-linked branching fucosyl residues and 10% of the 3-linked fucosyl residues are 2-O-methylated. A majority of the glucuronosyl residues are methyl-esterified at C-6. These unique structural features may be significant in the infection process.

Determination of constituents of sulphated proteoglycans using a methanolysis procedure and gas chromatography/mass spectrometry of heptafluorobutyrate derivatives

A major impediment in the analysis of glycosaminoglycans is the difficulty to cleave quantitatively the glycosidic bonds because of the stabilisation of glycosidic bonds and of the relative instability of the liberated constituents. This manuscript describes a modified procedure of methanolysis in the presence of barium acetate, reducing the destruction of uronic acids and increasing the cleavage yield. The reaction products could be identified and analysed quantitatively by GC and GC/MS of the heptafluorobutyrate derivatives of O-methyl glycosides of monosaccharides (for keratan sulphate and chondroitin sulphate B), or as a mixture of O-methyl glycosides of monosaccharides and of disaccharides (for the other sulphated glycosaminoglycans). Quantitative molar ratio between the different monosaccharide constituents (including the linkage region constituents) could be obtained, even when proteoglycans also contain classical N-glycans or O-glycans.

Gas-liquid chromatography of the heptafluorobutyrate derivatives of the O-methyl-glycosides on capillary columns: a method for the quantitative determination of the monosaccharide composition of glycoproteins and glycolipids

We have developed a method involving the formation of hepta-fluorobutyrate derivatives of O-methyl-glycosides liberated from glycoproteins and glycolipids following methanolysis. The stable derivatives of the most common monosaccharides of these glycoconjugates (Ara, Rha, Xyl, Fuc, Gal, Man, Glc, GlcNAc, GalNAc, Neu5Ac, KDN) can be separated and quantitatively and reproducibly determined with a high degree of sensitivity level (down to 25 pmol) in the presence of lysine as an internal standard. The GlcNAc residue bound to Asn in N-glycans is quantitatively recovered as two peaks. The latter were easily distinguished from the other GlcNAc residues of N-glycans, thus allowing a considerable improvement of the data on structure of N-glycans obtained from a single carbohydrate analysis. The most common contaminants present in buffers commonly used for the isolation of soluble or membrane-bound glycoproteins (SDS, Triton X-100, DOC, TRIS, glycine, and polyacrylamide or salts, as well as monosaccharide constituents of proteoglycans or degradation products of nucleic acids) do not interfere with these determinations. A carbohydrate analysis of glycoproteins isolated from a SDS/PAGE gel or from PDVF membranes can be performed on microgram amounts without significant interferences. Since fatty acid methyl esters and sphingosine derivatives are separated from the monosaccharide peaks, the complete composition of gangliosides can be achieved in a single step starting from less than 1 microg of the initial compound purified by preparative Silicagel TLC. Using electron impact ionization mass spectrometry, reporter ions for the different classes of O-methyl-glycosides (pentoses, deoxy-hexoses, hexoses, hexosamines, uronic acids, sialic acid, and KDN) allow the identification of these compounds in very complex mixtures. The mass of each compound can be determined in the chemical ionization mode and detection of positive or negative ions. This method presents a considerable improvement compared to those using TMS derivatives. Indeed the heptafluorobutyrate derivatives are stable, and acylation of amino groups is complete. Moreover, there is no interference with contaminants and the separation between fatty acid methyl-esters and O-methyl glycosides is achieved.

Reductive cleavage of permethylated polysaccharides with borane-methyl sulfide complex and butyltin trichloride

Several per-O-methylated monosaccharides and polysaccharides were used as models in an attempt to identify more convenient reagents for accomplishing reductive cleavage of glycosidic linkages. Included in the model studies were methyl alpha- and beta-D-glucopyranoside, methyl alpha- and beta-D-mannopyranoside, and methyl alpha- and beta-D-ribofuranoside. These studies led to the identification of a new promoter, butyltin trichloride, for carrying out reductive cleavage when borane-methyl sulfide complex was used as the reducing agent. These reagents were found to accomplish the reductive cleavage of per-O-methylated amylose, cellulose, and pullulan to give only the expected derivatives of 1,5-anhydro-D-glucitol. These reagents also accomplished reductive cleavage of per-O-methylated insulin to give only the expected derivatives of 2,5-anhydro-D-mannitol and 2,5-anhydro-D-glucitol. Reductive cleavage using these reagents is easy to perform, and subsequent acetylation of the products is readily accomplished in situ.

Synthesis of the methyl alpha-glycoside of a trisaccharide mimicking the terminus of the O antigen of Vibrio cholerae O:1, serotype Inaba

Coupling of methyl 4-amino-4,6-dideoxy-2-O-4-methoxybenzyl-alpha-D-mannopyranoside, obtained from the corresponding 4-azido derivative by treatment with H2S, with 3-deoxy-L-glycero-tetronolactone gave the crystalline methyl 4-(3-deoxy-L-glycero-tetronamido)-4,6-dideoxy-2-O-4-methoxybenzyl- alpha-D- mannopyranoside (7). Subsequent acetylation of 7, followed by O-demethoxybenzylation of the 8 formed gave the crystalline methyl 3-O-acetyl-4,6-dideoxy-4-(2,4-di-O-acetyl-3-deoxy-L-glycero-tetronami do)-alpha- D-mannopyranoside (9), which was used as the key intermediate in the construction of the title trisaccharide. To make a glycosyl donor allowing the extension of the oligosaccharide chain at O-2, compound 9 was converted, via conventional transformations, into 3-O-acetyl-2-O-bromoacetyl-4,6-dideoxy-4-(2,4-di-O-acetyl-3-deoxy-L-glyc ero- tetronamido)-alpha-D-mannopyranosyl chloride (12). Condensation of 12 with 9 afforded the disaccharide 20 having a selectively removable protecting group at O-2(2). The latter was O-debromoacetylated, and the disaccharide nucleophile thus obtained was treated with 2,3-di-O-acetyl-4,6-dideoxy-4-(2,4-di-O-acetyl-3-deoxy-L-glycero-tetr onamido)- alpha-D-mannopyranosyl chloride to give, after O-deacetylation, the target, title trisaccharide. The constituent monosaccharide of the O-specific polysaccharide antigen of Vibrio cholerae serotype Inaba, 4-(3-deoxy-L-glycero-tetronamido)-4,6-dideoxy-D-mannopyranose (18), was obtained from the peracetate of its methyl alpha-glycoside by acetolysis, followed by O-deacetylation. The amorphous compound 18 was characterized by 1H and 13C NMR spectroscopy and through its crystalline alpha-per-O-acetyl derivative.

Synthesis of the methyl alpha-glycosides of some isomalto-oligosaccharides specifically deoxygenated at position C-4

Methyl alpha-isomaltoside and methyl alpha-isomaltotrioside specifically deoxygenated at position C-4 of various glucopyranosyl units were synthesized by condensation of either 1,6-di-O-acetyl-2,3-di-O-benzyl-4-deoxy-alpha,beta-D-xylo-hexopyranos e (7) or 1,6-di-O-acetyl-2,3,4-tri-O-benzyl-alpha,beta-D-glucopyranose (10) [mediated by silver perchlorate and tin(IV) chloride] with suitably blocked derivatives of methyl alpha-D-glucopyranoside, its 4-deoxy analog 6, or methyl 4'-deoxy alpha-isomaltoside (13), respectively.

Studies of the bound conformations of methyl alpha-lactoside and methyl beta-allolactoside to ricin B chain using transferred NOE experiments in the laboratory and rotating frames, assisted by molecular mechanics and dynamics calculations

The conformation in solution of methyl beta-galactopyranosyl-(1-->4)-alpha-glucopyranoside (methyl alpha-lactoside) and methyl beta-galactopyranosyl-(1-->6)-beta-glucopyranoside (methyl beta-allolactoside) has been studied through NMR spectroscopy and molecular mechanics calculations. NOE measurements both in the laboratory and rotating frames, have been interpreted in terms of an ensemble average distribution of conformers. Molecular mechanics calculations have been performed to estimate the probability distribution of conformers from the steric energy maps. The experimental results indicate that methyl alpha-lactoside spends about 90% of its time in a broad low-energy region close to the global minimum, while methyl beta-allolactoside presents much higher flexibility. The conformational changes that occur when both disaccharides are bound to the ricin B chain in aqueous solution have been studied using transferred NOE experiments at several protein/ligand ratios. The observed data indicate that the protein causes a conformational variation in the torsion angles of methyl alpha-lactoside changing towards smaller angle values (phi/psi approximately -20/-20), although the recognized conformer is still within the lowest energy region. In particular, the torsional changes separate Gal H1 from Glc H3 and Glc H6 protons, with a noticeable decrease in the intensities of the corresponding NOE cross-peaks, which were clearly observed for the free disaccharide. On the other hand, different conformations around the phi, psi, and omega glycosidic bonds of methyl beta-allolactoside are recognized by the lectin. In fact, for the methyl-beta-allolactoside-ricin-B complex, only the NOESY cross-peaks corresponding to the protons of the galactose residue are negative, as expected for a molecule in the slow motion regime. In contrast, the corresponding cross peaks for the glucose residue were about zero, as expected for a molecule whose motion is practically independent of the protein. However, for the methyl-alpha-lactoside-ricin-B complex, all the NOESY cross-peaks for both the galactose and glucose moieties were clearly negative. From the NMR experimental point of view, it is demonstrated that the comparison of longitudinal and transversal transferred NOEs allows one to clearly differentiate direct enhancements from spin diffusion effects, which are of major concern when analysing NOE spectra of macromolecules.(ABSTRACT TRUNCATED AT 250 WORDS)

Substrate specificity of small-intestinal lactase: study of the steric effects and hydrogen bonds involved in enzyme-substrate interaction

Milk lactose is hydrolysed to D-galactose and D-glucose in the small intestine of mammals by the lactase-phlorizin hydrolase complex (LPH, EC 3.2.1.23-62). Lactase activity has broad substrate selectivity and several glycosides are substrates. Recently, using the monodeoxy derivatives of methyl beta-lactoside (1), we have shown the importance of each hydroxyl group in the substrate molecule concerning the interaction with the enzyme. Now we have studied the corresponding O-methyl derivatives, as well as some of the halo derivatives of 1. We have found that the enzyme presents steric restrictions to the recognition of substrates modified in the galactose moiety. In contrast, the binding site for the aglycon part of the substrate is looser. On the other hand, we have previously shown that HO-3' and HO-6 were important for the recognition of the substrate by the enzyme. Now we have found that the corresponding fluorine derivatives are not, or very poorly, recognized. This suggests that the HO-3' and HO-6 participate, as donors, in hydrogen bonds in the interaction with the enzyme.

The use of CVFF and CFF91 force fields in conformational analysis of carbohydrate molecules. Comparison with AMBER molecular mechanics and dynamics calculations for methyl alpha-lactoside

The solution conformation of methyl alpha-lactoside has been studied through molecular mechanics calculations using the AMBER/Homans, CVFF and CFF91 force fields, and compared to NMR nuclear Overhauser data. Steady-state and transient nuclear Overhauser effects (NOEs) have been interpreted in terms of the ensemble average distribution of conformers. The NOEs have been analysed using the complete relaxation matrix approach for a rigid and isotropic motion model. The molecular mechanics calculations have been performed at two dielectric constants (i.e. epsilon = 1 and 80 debyes, or epsilon = r and 80 debyes) in an exhaustive way, and, in some cases, have been complemented by specific calculations at intermediate epsilon values. Relaxed energy maps and adiabatic surfaces have been generated for the different dielectric constants. The probability distribution of conformers has been estimated from these steric energy maps. Molecular dynamics simulations in vacuo have also been performed. Our results indicate that the beta-(1-->4) glycosidic linkage shows some fluctuations between three low-energy regions, although it spends about 90% of its time in the region close to the global minimum. The observed conformation of methyl alpha-lactoside seems to be closer to that predicted by CVFF, although the AMBER/Homans results are also in qualitative agreement with the experimental data.

Synthesis of methyl glycosides of some alpha-isomalto oligosaccharides specifically deoxygenated at position C-2

Methyl alpha-isomaltoside and methyl alpha-isomaltotrioside analogues specifically deoxygenated at position C-2 of various glucopyranosyl units were synthesized by condensation of either 6-O-acetyl-3-O-benzoyl-4-O-benzyl-1-O-tert-butyldimethylsilyl-2-deoxy- beta-D-arabino-hexopyranose (trimethylsilyl triflate mediated) or 6-O-acetyl-2,3,4-tri-O-benzyl-alpha-D-glucopyranosyl chloride (mediated by silver carbonate and silver triflate) with suitably blocked derivatives of methyl alpha-D-glucopyranoside, its 2-deoxy analogue, or methyl 2'-deoxy-alpha-isomaltoside.

Synthesis of specifically deoxygenated analogues of the methyl alpha-glycoside of the intracatenary monosaccharide repeating unit of the O-polysaccharide of Vibrio cholerae O:1

Treatment of methyl alpha-D-perosaminide (1) with gamma-butyrolactone gave the 2'-deoxy analogue of methyl 4,6-dideoxy-4-(3-deoxy-L-glycero-tetronamido)-alpha-D-mannopyranos ide (13), the methyl alpha-glycoside of the intracatenary monosaccharide repeating unit of the O-polysaccharide of Vibrio cholerae O:1. The analogous 4'-deoxy derivative was obtained by hydrogenolysis of a 4'-chlorodeoxy precursor, obtained by chlorination of methyl 2,3-di-O-benzyl-4-(2-O-benzyl-3-deoxy-L-glycero-tetronamido)-4,6-d ideoxy-alpha- D-mannopyranoside with methanesulfonyl chloride in DMF. To obtain the 3-deoxy analogue of 13, methyl 4-amino-2-O-benzyl-4,6-dideoxy-3-O-p-methoxy-benzyl-alpha-D-mannopyranos ide was converted into methyl 2-O-benzyl-4,6-dideoxy-4-(2,4-di-O-benzyl-3-deoxy-L-glycero-tetronami do)- alpha-D-mannopyranoside, which was deoxygenated via the corresponding 3-O-(imidazol-1-ylthiocarbonyl) derivative. Subsequent catalytic debenzylation gave the deoxy compound (24). In an alternative synthesis, which is also generally useful for the preparation of 4-N-acyl-3-deoxy derivatives of 1, methyl 4-azido-4,6-dideoxy-alpha-D-mannopyranoside was converted through a series of transformations into methyl 4-amino-2-O-benzyl-3,4,6-tri-deoxy-alpha-D-mannopyranoside. Subsequent reaction with 2-O-benzyl-3-deoxy-L-glycero-tetronolactone, followed by hydrogenolysis of the formed tetronamido derivative, gave 24.

The use of the AMBER force field in conformational analysis of carbohydrate molecules: determination of the solution conformation of methyl alpha-lactoside by NMR spectroscopy, assisted by molecular mechanics and dynamics calculations

The solution conformation of methyl alpha-lactoside has been studied through nmr spectroscopy and molecular mechanics calculations using the assisted model building with energy refinement (AMBER) force field. The nmr data have included nuclear Overhauser effect (NOE) measurements both in the laboratory and rotating frames, longitudinal relaxation times, and homonuclear and heteronuclear coupling constants. The steady-state and transient NOEs have been interpreted in terms of an ensemble average distribution of conformers, making use of the complete relaxation matrix approach. The molecular mechanics calculations have been performed at two dielectric constants [epsilon = 1*r and 80 Debyes (D)] in an exhaustive way, and have been complemented with specific calculations at intermediate epsilon values. Relaxed energy maps and adiabatic surfaces have been generated for the different dielectric constants. The probability distribution of conformers has been estimated from these steric energy maps. Molecular dynamics simulations in vacuo have also been performed. The experimental results indicate that the beta (1-->4)-glycosidic linkage shows some fluctuations among three low energy regions, although spends ca. 85% of its time in the region close to the global minimum. It is shown that the overestimation of the electrostatic contributions in AMBER is responsible for the failure of this force field to explain the experimental results when used at low dielectric constant (epsilon < 20 D). The matching between the expected and observed facts increases for epsilon > 40 D. Different conditions have been tested to perform temperature constant molecular dynamics simulations in vacuo, which have indicated that, when used without explicit solvent, this force field should only be employed in a qualitatively way when analyzing dynamical properties of oligosaccharides.

Probing hydrogen-bonding interactions of bovine heart galectin-1 and methyl beta-lactoside by use of engineered ligands

The binding of different synthetic monodeoxy, O-methyl and fluorodeoxy derivatives of methyl beta-lactoside to galectin-1 from bovine heart has been studied to probe the role of hydrogen bonding in the recognition and binding. The energetic contributions of the hydroxyl groups of methyl beta-lactoside directly involved in the interaction have been estimated and the nature of the protein residues involved has been predicted on the basis of the free energy data. Interpretations of the results have been sustained by molecular modeling of the three-dimensional structure of the sugars in solution. One side of the disaccharide molecule is not involved (HO-6 and HO-2') or only marginally involved (HO-3') in hydrogen bonding. Moreover, O-methylation at these positions causes an enhancement of the binding, suggesting favourable interactions of the methyl groups which may come into contact with hydrophobic residues at the periphery of the combining site. Hydrogen-bonding interactions are almost exclusively restricted to the other side of the molecule: the C-4' and C-6' hydroxyl groups act as donors of the strongest hydrogen bonds to charged groups of the lectin, while the C-3 hydroxyl group participates in a strong hydrogen bond with a neutral group. The results also suggest that the N-acetyl NH group in N-acetyllactosamine, as well as the hydroxyl group at position C-2 in methyl beta-lactoside, are involved in a polar interaction with neutral groups of the combining site. This hydrogen-bonding pattern contrasts markedly with that previously reported for the two galactose-specific Ricinus communis lectins. The recognition of different epitopes of the same basic structure underlies the differences in the oligosaccharide-binding specificities of galectin-1 and the R. communis lectins.

Involvement of the glucose moiety in the molecular recognition of methyl beta-lactoside by ricin: synthesis, conformational analysis, and binding studies of different derivatives at the C-3 region

Syntheses of the 3-aminodeoxy (4), 3-deoxy-3-methyl (5), and 3-epi (6) derivatives of methyl beta-lactoside (1) have been achieved from 1 in a straightforward way, and their solution conformations in water and dimethyl sulfoxide analysed through molecular mechanics and dynamics calculations and nuclear magnetic resonance data. The overall shape of all the compounds studied is fairly similar and may be described by conformers included in a low energy region with phi = 15 +/- 45 degrees and psi = -25 +/- 30 degrees, that is ca. 5% of the total potential energy surface for the glycosidic linkages of the disaccharides. The binding of the different compounds to ricin, the galactose-specific toxin from Ricinus communis, has been investigated. The results confirm the involvement of the C-3 region in a nonpolar interaction with the protein at the periphery of the combining site.

The conformation of the monomethyl ethers of methyl beta-lactoside in D2O and Me2SO-d6 solutions

The solution conformations of all the possible monomethyl ethers of methyl beta-lactoside have been analysed using molecular mechanics and dynamics calculations and nuclear magnetic resonance data (variable temperature and NOE experiments). The overall shape of all the compounds studied is fairly similar and may be described by conformers included in a low-energy region with phi = -100 +/- 40 degrees and psi = -135 +/- 35 degrees, which is ca. 5% of the total potential energy surface for the glycosidic linkages of the disaccharides.

Hydrogen-bonding pattern of methyl beta-lactoside binding to the Ricinus communis lectins

The binding of O-methyl and fluorodeoxy derivatives of methyl beta-lactoside to the Ricinus communis toxin (RCA60) and agglutinin (RCA120) was studied in order to determine the donor/acceptor relationships of the hydrogen bonds between the hydroxyl groups of methyl beta-lactoside and the binding sites of the lectins. Free energy contributions of the hydrogen bonds at each position have been estimated from these data and from those previously reported for the monodeoxy derivatives [Rivera-Sagredo, A., Solís, D., Díaz-Mauriño, T., Jiménez-Barbero, J. & Martín-Lomas, M. (1991) Eur. J. Biochem. 197, 217-228; Rivera-Sagredo, A., Jiménez-Barbero, J., Martín-Lomas, M., Solís, D. & Díaz-Mauriño, T. (1992) Carbohydr. Res. 232, 207-226]. The nature of the groups of the lectins involved in hydrogen bonding has been predicted on the basis of the free energy data. Analysis of the results indicates that both the C-3' and C-4' hydroxyl groups act as hydrogen-bond donors to charged groups of both RCA60 and RCA120. The C-6' and probably also the C-2' hydroxyl groups participate both as donors and as acceptors of two hydrogen bonds with neutral groups of the lectins. And finally, the C-6 hydroxyl group possibly acts as a donor of a weak hydrogen bond to a neutral group in RCA60, but not in RCA120. The results provide a molecular basis to explain some features of the binding specificity of the lectins. Comparison of RCA60 binding data with the recently refined X-ray crystal structure of the RCA60-lactose complex shows similarities but also some discrepancies that can be attributed to the marked influence of the pH on the carbohydrate-lectin interaction.

Synthesis and 13C NMR spectra of 2,3-di-O-glycosyl derivatives of methyl alpha-L-rhamnopyranoside and methyl alpha-D-mannopyranoside

The syntheses are described of 2,3-di-O-glycosyl derivatives of methyl alpha-L-rhamnopyranoside (1-5) and alpha-D-mannopyranoside (6-9). [formula: see text] [table: see text] The deviation from additivity in 13C NMR spectra calculated for 1-9 were similar for stereochemically related trisaccharides.

Studies of the molecular recognition of synthetic methyl beta-lactoside analogues by Ricinus communis agglutinin

The 2-, 3-, 6-, 2'-, 3'-, 4'-, and 6'-deoxy derivatives and the 3-O-methyl derivative of methyl beta-lactoside have been synthesised and their binding to the galactose-specific agglutinin from Ricinus communis (RCA-120) has been investigated. The results indicate that HO-3,4,6 of the beta-D-galactopyranose moiety are the key polar groups. The main difference from the closely related ricin lectin RCA-60 involves HO-6 of the D-glucopyranose moiety, which seems to contribute to the binding of the carbohydrate to RCA-60 but not to RCA-120.

The interaction of elderberry (Sambucus sieboldiana) bark lectin and sialyloligosaccharides as detected by 1H-NMR

The interaction of Japanese elderberry bark lectin (Sambucus sieboldiana agglutinin, SSA) with carbohydrate was investigated by 1H-NMR. When a low affinity ligand, methyl beta-D-galactoside (beta MeGal), was mixed with SSA, each proton signal of beta MeGal was broadened. The signal of H-4 was markedly broad, while those of H-1, OCH3, and H-2 of beta MeGal were rather sharp. The specific broadening of Gal H-4 was more evident when SSA was mixed with methyl-beta-D-lactoside (beta MeLac). Position-dependent signal broadening suggests that beta MeGal binds to SSA such that H-4 is closely involved in the contact region, but H-1, OCH3, and H-2 are far from this region. In the case of a high affinity ligand, Neu5Ac(alpha 2-6)Gal(beta 1-4)Glc(= N6L), ligand signals of the SSA-N6L mixture did not change at all. But when a small amount of N6L was added to the SSA-beta MeGal mixture, the broad signals of bound beta MeGal became dramatically sharp. This indicates that the added N6L molecules liberated the bound beta MeGal from SSA. On the other hand, the sialyllactose with the alpha(2-3)-linkage(= N3L) could not substitute for bound beta MeGal because of its lower affinity. This demonstrates that the competitive binding experiment between two ligands is a useful technique to detect the interaction of lectins with high affinity ligands which could not be observed directly by NMR signal broadening and/or chemical shift change.

A 1H-NMR and MD study of intramolecular hydrogen bonds in methyl beta-cellobioside

The existence of an HO-3...O-5' intramolecular hydrogen bond in methyl beta-cellobioside in solution in Me2SO-d6 and H2O-CD3OD (4:1 w/w) was studied by 500-MHz 1H-NMR spectroscopy and MD simulations. Temperature coefficients for the chemical shift of the hydroxyl resonances in these solvents were determined and the rates of proton exchange in the latter solvent were obtained from NOE data. With H2O-CD3OD as the solvent, the HO-3...O-5' hydrogen bond was insignificant, but its presence in Me2SO-d6 was confirmed.

Binding studies on internal immunodeterminants: synthesis of beta-(1----6)-linked oligosaccharide methyl glycosides having one to four internal D-galactopyranosyl residues flanked by gentiobiose residues

The oligosaccharide glycosides beta-D-Glcp-(1----6)-beta-D-Glcp-(1----6)-[beta-D-Galp-(1----6)]n-beta-D - Glcp-(1----6)-beta-D-Glcp-1----OMe (n = 1-4) were prepared by a convergent block synthesis. Haloacetyl, tert-butyldiphenylsilyl, and dimethylthexylsilyl groups were used as temporary protective groups for the preparation of the intermediate glycosyl donors and acceptors. The deoxygenated trisaccharide glycosides beta-D-Glcp-(1----6)-beta-D-Galp-(1----6)-4-deoxy-beta-D-xylo-Hexp -1----OMe and beta-D-Glcp-(1----6)-4-deoxy-beta-D-xylo-Hexp-(1----6)-beta-D-Galp -1----OMe were also synthesized. The binding of each glycoside to the monoclonal antigalactan antibody IgA J539 was studied and the results support the previous finding that J539 can bind to internal antigenic epitopes. The data are consistent with the interpretation that subsite C of that antibody binds glucose with a Ka of approximately 6 (cf. 10.9 for galactose).

N.m.r. studies of the conformation of analogues of methyl beta-lactoside in methyl sulfoxide-d6

The 1H- and 13C-n.m.r. spectra of solutions of methyl beta-lactoside (1), all of its monodeoxy derivatives (2, 3, 6-10), the 3-O-methyl derivative (4), and methyl 4-O-beta-D-galactopyranosyl-D-xylopyranoside (5) in methyl sulfoxide-d6 have been analysed. The n.O.e.'s and specific desheildings indicate similar distributions of low-energy conformers, comparable to those in aqueous solution. The major conformer has torsion angles phi H and psi H of 49 degrees and 5 degrees, respectively, with contributions of conformers with phi/psi 24 degrees/-59 degrees, 22 degrees/32 degrees, and 6 degrees/44 degrees.

Thio and epidithio derivatives of methyl beta-lactoside

Treatment of methyl beta-lactoside with triphenylphosphine-carbon tetrabromide in pyridine gave the 3',6'-anhydro-6-bromo-6-deoxy derivative, from which 6-thio derivatives were prepared, and methyl 3',4'-O-isopropylidene-beta-lactoside gave the 6,6'-dibromo-6,6'-dideoxy derivative. A dibromide was prepared also from methyl 4',6'-O-benzylidene-beta-lactoside by bromination with Ph3P-CBr4, acetylation, and then treatment with N-bromosuccinimide. Various 6,6'-dithio derivatives were prepared from the 6,6'-dibromide by nucleophilic substitution reactions. Reaction of the 6,6'-dibromide with thiourea led to the 6,6'-epidithio derivative and, with potassium tricarbonate, the bridged 6,6'-trithiocarbonate was formed. The 6,6'-dibromo derivative underwent selective nucleophilic substitution to give a variety of 6'-bromo-6-thio derivatives. Likewise, with azide, the 6-azide was formed first, followed by the 6,6'-diazide and the product of elimination, the 6-azido-5%-ene. Raney nickel-mediated desulphuration of the various 6,6'-dithio derivatives afforded methyl 6,6'-dideoxy-beta-lactoside and desulphuration of the 6'-bromo-6-thio derivatives could be accomplished without reductive dehalogenation to give methyl 6'-bromo-6,6'-dideoxylactoside.

Regioselectivity in the sulfation of dermatan sulfate and methyl 4,6-O-benzylidene-alpha-D-idopyranoside

The sulfation of dermatan sulfate by SO3-trimethylamine in N,N-dimethylformamide led to substitution initially at HO-6 of residues of 2-acetamido-2-deoxy-beta-D-galactopyranosyl 4-sulfate (1), to produce the 4,6-disulfate (6). When this step reached a level of greater than 50%, sulfation occurred with equal facility at HO-2 and HO-3 of residues of alpha-L-idopyranosyluronic acid (2), giving rise to a mixture of 2-,3-, and 2,3-disulfates. An analogous substitution pattern was observed for HO-2 and -3 of a simpler idopyranose unit, in the sulfation of methyl 4,6-O-benzylidene-alpha-D-idopyranoside (12). This lack of regioselectivity in the reaction of 2 (and 12) contrasts markedly with the high affinity of the reagent for HO-3 of residues of alpha-L-idopyranosyluronic acid present in a modified form of heparin. It is attributed to a difference between the two polymers in the relative orientation of their neighboring amino sugar residues, whereby there is an unobstructed access of the reagent in one instance, and hindrance of HO-2 selectively in the other. Enzymolysis by chondroitinase ABC was found to yield unsaturated disaccharide containing residues of 4,6-disulfate, as well as larger fragments containing unsaturated glycosyl groups derived from L-idopyranosyluronic acid 2-sulfate, evidence of a relatively broad enzyme specificity. The presence of extra sulfate groups in dermatan sulfate did not enhance its weak antithrombotic activity, as measured by anti Xa assay, in disagreement with earlier reports.

Studies on the molecular recognition of synthetic methyl beta-lactoside analogs by ricin, a cytotoxic plant lectin

The binding of methyl beta-lactoside and of all possible monodeoxy derivatives of methyl beta-lactoside to the galactose-specific highly cytotoxin lectin ricin, has been investigated. The distribution of low-energy conformers of the disaccharide structures has been first determined using molecular-mechanics calculations and high-resolution NMR spectroscopy. The nuclear Overhauser enhancements and specific deshieldings observed are in agreement with a similar distribution of low-energy conformers for all studied compounds which may be described by a major conformer defined by phi (H1'-C1'-O1'-C4) and psi (C1'-O1'-C4-H4) torsion angles of 49 degrees and 5 degrees, respectively, with contribution of conformers with angles phi/psi 24 degrees/-59 degrees, 22 degrees/-32 degrees and 6 degrees/-44 degrees. Assuming that the disaccharides bind to the lectin in these preferred conformations, the apparent dissociation constants for the ricin-disaccharide complexes have been interpreted in terms of specific polar and nonpolar interactions. In agreement with X-ray data, the hydroxyl groups at positions 3, 4 and 6 of the beta-D-galactopyranose moiety appear as key polar groups in the interaction with ricin. These results are in contrast to previous results which have established that position 6 is not involved in lectin binding. An important nonpolar interaction involving position 3 of the beta-D-glucopyranose moiety, seems to be operative. The distribution of low-energy conformers of these disaccharide structures permits this interaction to take place with the hydroxyl group at this position intramolecularly bonded, thus rendering this region of the molecule more lipophylic in character for acceptance into nonpolar regions of the combining site.