Synthesis and biological activities of three sulfated sialyl Le(x) ganglioside analogues for clarifying the real carbohydrate ligand structure of L-selectin

An efficient synthetic route to O-(2-O-benzyl-3,4-di-O-acetyl-α/β-l-fucopyranosyl)-trichloroacetimidate

An efficient synthetic route to prepare O-(2-O-benzyl-3,4-di-O-acetyl-α/β-l-fucopyranosyl)-trichloroacetimidate from l-fucose was developed by introducing the thiophenyl group at the anomeric center and the benzylidene functional group to protect the 3 and 4 positions. Although three approaches were considered, the best result was obtained when, after the 2-hydroxyl benzylation, both protective groups were simultaneously removed by using acetic anhydride and perchloric acid supported on silica as catalyst. Selective deacetylation of the obtained tri-O-acetate followed by the reaction of the resultant hemiacetal with trichloroacetonitrile and DBU afforded the trichloroacetimidate with an overall yield of 56% from the l-fucose.

Systematic Chemoenzymatic Synthesis of O-Sulfated Sialyl Lewis x Antigens

O-Sulfated sialyl Lewis x antigens play important roles in nature. However, due to their structural complexity, they are not readily accessible by either chemical or enzymatic synthetic processes. Taking advantage of a bacterial sialyltransferase mutant that can catalyze the transfer of different sialic acid forms from the corresponding sugar nucleotide donors to Lewis x antigens which are fucosylated glycans as well as an efficient one-pot multienzyme (OPME) sialylation system, O-sulfated sialyl Lewis x antigens containing different sialic acid forms and O-sulfation at different locations were systematically synthesized by chemoenzymatic methods.

Overcoming antibiotic resistance in Pseudomonas aeruginosa biofilms using glycopeptide dendrimers

Antibiotic resistance in the opportunistic pathogen Pseudomonas aeruginosa is partly caused by biofilms forming a physical barrier to antibiotic penetration. Here we focused on modifying tetravalent glycopeptide dendrimer ligands of P. aeruginosa lectins LecB or LecA to increase their biofilm inhibition activity. First heteroglycoclusters were investigated displaying one pair each of LecB specific fucosyl groups and LecA specific galactosyl groups and binding simultaneously to both lectins, one of which gave the first fully resolved crystal structure of a peptide dendrimer as LecB complex providing a structural model for dendrimer-lectin interactions (PDB ; 5D2A). Biofilm inhibition was increased by introducing additional cationic residues in these dendrimers but resulted in bactericidal effects similar to those of non-glycosylated polycationic antimicrobial peptide dendrimers. In a second approach dendrimers displaying four copies of the natural LecB ligand Lewisa were prepared leading to slightly stronger LecB binding and biofilm inhibition. Finally synergistic application of a LecB specific non-bactericidal antibiofilm dendrimer with the antibiotic tobramycin at sub-inhibitory concentrations of both compounds allowed effective biofilm inhibition and dispersal.

Total syntheses of disulphated glycosphingolipid SB1a and the related monosulphated SM1a

Total syntheses of two natural sulphoglycolipids, disulphated glycosphingolipid SB1a and the structurally related monosulphated SM1a, are described. They have common glycan sequences and ceramide moieties and are associated with human epithelial carcinomas. The syntheses featured efficient glycan assembly and the glucosyl ceramide cassette as a versatile building block. The binding of the synthetic sulphoglycolipids by the carcinoma-specific monoclonal antibody AE3 was investigated using carbohydrate microarray technology.

Appel-reagent-mediated transformation of glycosyl hemiacetal derivatives into thioglycosides and glycosyl thiols

A series of glycosyl hemiacetal derivatives have been transformed into thioglycosides and glycosyl thiols in a one-pot two-step reaction sequence mediated by Appel reagent (carbon tetrabromide and triphenylphosphine). 1,2-trans-Thioglycosides and β-glycosyl thiol derivatives were stereoselectively formed by the reaction of the in situ generated glycosyl bromides with thiols and sodium carbonotrithioate. The reaction conditions are reasonably simple and yields were very good.

Synthetic studies on glycosphingolipids from protostomia phyla: synthesis of glycosphingolipids and related carbohydrate moieties from the parasite Schistosoma mansoni

Stereocontrolled syntheses of three neutral glycosphingolipids and six oligosaccharide derivatives found from the parasite Schistosoma mansoni have been accomplished. A pentasaccharide glycosphingolipid β-D-Galp-(1→4)-[α-L-Fucp-(1→3)]-β-D-GlcpNAc-(1→3)-β-D-GalpNAc-(1→4)-β-D-Glcp-(1↔1)-Cer (1), two hexasaccharide glycosphingolipids α-L-Fucp-(1→3)-β-D-Galp-(1→4)-[α-L-Fucp-(1→3)]-β-D-GlcpNAc-(1→3)-β-D-GalpNAc-(1→4)-β-D-Glcp-(1↔1)-Cer (2) and β-D-Galp-(1→4)-[α-L-Fucp-(1→3)]-β-D-GlcpNAc-(1→3)-β-D-GlcpNAc-(1→3)-β-D-GalpNAc-(1→4)-β-D-Glcp-(1↔1)-Cer (3), together with their non-reducing end tri- and tetrasaccharides, β-D-Galp-(1→4)-[α-L-Fucp-(1→3)]-β-D-GlcpNAcOR (4) and α-L-Fucp-(1→3)-β-D-Galp-(1→4)-[α-L-Fucp-(1→3)]-β-D-GlcpNAcOR (5), were synthesized by block synthesis. Moreover, non-reducing end oligosaccharides of schistosomal glycosphingolipids, β-D-GalpNAc-(1→4)-[α-L-Fucp-(1→3)]-β-D-GlcpNAcOR (6), α-L-Fucp-(1→3)-β-D-GalpNAc-(1→4)-[α-L-Fucp-(1→3)]-β-D-GlcpNAcOR (7), α-L-Fucp-(1→3)-β-D-GalpNAc-(1→4)-[α-L-Fucp-(1→2)-α-L-Fucp-(1→3)]-β-D-GlcpNAcOR (8) and α-L-Fucp-(1→3)-β-D-GalpNAc-(1→4)-[α-L-Fucp-(1→2)-α-L-Fucp-(1→2)-α-L-Fucp-(1→3)]-β-D-GlcpNAcOR (9) [R=2-(trimethylsilyl)ethyl], were synthesized as probes to explore their diagnostic potential to detect schistosomiasis from patients' sera.

Synthesis and biological activity of alpha-L-fucosyl ceramides, analogues of the potent agonist, alpha-D-galactosyl ceramide KRN7000

Several L-fucoglycolipids are associated with diseases such as cancer, cystic fibrosis and rheumatoid arthritis. Activation of iNKT cells is known to lead to the production of cytokines that can help alleviate or exacerbate these conditions. alpha-Galactosyl ceramide (alpha-GalCer) is a known agonist of iNKT cells and it is believed that its fucosyl counterpart might have similar immunogenic properties. We herein report the synthesis of alpha-L-fucosyl ceramide derivatives and describe their biological evaluation. The key challenge in the synthesis of the target molecules involved the stereoselective synthesis of the alpha-glycosidic linkage. Of the methods examined, the per-TMS-protected glycosyl iodide donor was completely alpha-selective, and could be scaled up to provide gram quantities of the azide precursor 11, from which a range of N-acylated alpha-L-fucosyl ceramides were readily obtained and evaluated for ex vivo expansion of human iNKT cells.

Synthesis and biological activities of glycosphingolipid analogues from marine sponge Aplysinella rhax

A novel glycosphingolipid, beta-D-GalNAcp(1-->4)[alpha-D- Fucp(1-->3)]-beta-D-GlcNAcp(1-->)Cer (1), isolated from the marine sponge Aplysinella rhax, has a unique structure, with D-fucose and N-acetyl-D-galactosamine attached to a reducing-end N-acetyl-D-glucosamine through an alpha1-->3 and beta1-->4 linkage, respectively. We synthesized glycolipid analogues carrying a 2-branched fatty alkyl residue or a 2-trimethylsilyl ethyl residue in place of ceramide (2 and 3), non-natural type trisaccharide analogue containing an L-fucose residue (4), and other analogues (5 and 6). Among these prepared compounds, 2 showed the most potent nitric oxide (NO) production inhibitory activity against LPS-activated J774.1 cells. In addition, their structure-activity relationships were established.

X-ray Structures and Thermodynamics of the Interaction of PA-IIL fromPseudomonas aeruginosa with Disaccharide Derivatives

Pseudomonas aeruginosa is an opportunistic bacterium showing increasing resistance to antibiotics and consequently represents elevated threatening problems in hospital environments, particularly for cystic fibrosis patients. The use of glycomimetics as an anti-adhesive strategy against microorganisms may complement the use of antibiotics. PA-IIL lectin (LecB) from P. aeruginosa constitutes an appealing target for antibacterial agents, as it has been proposed to play a key role in binding to airway epithelia and/or to be involved in biofilm formation. The lectin has an unusually high affinity for L-fucose and related oligosaccharides. In the work presented herein, the disaccharide alphaFuc1-4GlcNAc is used as a scaffold toward the synthesis of a series of glycomimetic derivatives. Microcalorimetry and structural studies indicate that several of the derivatives are potent inhibitors of the lectin, with affinity in the same range as the best known natural ligand, Lewis a, and could represent interesting leads for the development of future antibacterial compounds.

A chemical reporter strategy to probe glycoprotein fucosylation

Fucosylated glycoproteins are involved in many cell-cell recognition events and are markers of embryonic and malignant tissue. Here we report a method for rapid profiling of fucosylated glycoproteins from human cells using 6-azido fucose as a metabolic label.

Carbohydrate post-glycosylational modifications

Carbohydrate modification is a common phenomenon in nature. Many carbohydrate modifications such as some epimerization, O-acetylation, O-sulfation, O-methylation, N-deacetylation, and N-sulfation, take place after the formation of oligosaccharide or polysaccharide backbones. These modifications can be categorized as carbohydrate post-glycosylational modifications (PGMs). Carbohydrate PGMs further extend the complexity of the structures and the synthesis of carbohydrates and glycoconjugates. They also increase the capacity of the biological regulation that is achieved by finely tuning the structures of carbohydrates. Developing efficient methods to obtain structurally defined naturally occurring oligosaccharides, polysaccharides, and glycoconjugates with carbohydrate PGMs is essential for understanding the biological significance of carbohydrate PGMs. Combined with high-throughput screening methods, synthetic carbohydrates with PGMs are invaluable probes in structure-activity relationship studies. We illustrate here several classes of carbohydrates with PGMs and their applications. Recent progress in chemical, enzymatic, and chemoenzymatic syntheses of these carbohydrates and their derivatives are also presented.

Synthesis and binding properties of divalent and trivalent clusters of the Lewis a disaccharide moiety to Pseudomonas aeruginosa lectin PA-IIL

The synthesis of oligomeric glycocomimetics has been performed for targeting the Pseudomonas aeruginosa PA-IIL lectin, which is of therapeutical interest for anti-adhesive treatment. The disaccharide alpha-L-Fucp-(1-->4)-beta-D-GlcNAc, which is a high-affinity ligand of the lectin, has been coupled to dimeric and trimeric linkers with various lengths and geometries. A series of linear dimers displayed an efficient clustering effect and a very strong affinity, with a lower dissociation constant of 90 nM. The trimeric compound was less efficient in inhibition assays but displayed high affinity in solution. Titration microcalorimetry and molecular modeling allowed in-depth analysis and rationalization of the binding data. These glycoclusters could act by crosslinking the lectins present on the surface of bacteria and therefore interfere with host recognition or biofilm formation.

Synthesis of a novel glycosphingolipid from the millipede, Parafontaria laminata armigera, and the assembly of its carbohydrate moiety into multivalent structures

A novel glycosphingolipid, beta-D-Manp-(1-->4)-[alpha-L-Fucp-(1-->3)]-beta-D-Glcp-(1-->1)-Cer, found in the millipede, Parafontaria laminata armigera, and multivalent derivatives of its carbohydrate moiety were synthesized. As the key step, the target glycolipid (1) was obtained through an inversion reaction at the 2-position of a beta-glucopyranoside residue yielding a beta-mannopyranoside. In addition, the synthesis of fluorescently labeled trimer and tetramer glycoconjugates (2, 3) was achieved by iterative amide bond formation using a monomer unit (24).

Carbohydrate microarrays reveal sulphation as a modulator of siglec binding

Siglecs are receptors on cells of the immune, haemopoietic, and nervous systems that recognize sialyl-glycans with differing preferences for sialic acid linkage and oligosaccharide backbone sequence. We investigate here siglec binding using microarrays of Lewis(x) (Le(x))- and 3'-sialyl-Le(x)-related probes with different sulphation patterns. These include sulphation at position 3 of the terminal galactose of Le(x), position 6 of the galactose of Le(x) and sialyl-Le(x), position 6 of N-acetylglucosamine of Le(x) and sialyl-Le(x), or both positions of sialyl-Le(x). Recombinant soluble forms of five siglecs have been investigated: human Siglec-7, -8, -9, and murine Siglec-F and CD22 (Siglec-2). Each siglec has a different binding pattern. Unlike two C-type lectins of leukocytes, L-selectin and Langerin, which also bind to sulphated analogues of sialyl-Le(x), the siglecs do not give detectable binding signals with sulphated analogues that are lacking sialic acid. The sulphate groups modulate, however, positively or negatively the siglec binding intensities to the sialyl-Le(x) sequence.

Acylation of carbohydrates over Al2O3: preparation of partially and fully acylated carbohydrate derivatives and acetylated glycosyl chlorides

Selective and per-O-acylation of carbohydrate derivatives using acyl chlorides and Al2O3, a solid support reagent, is reported. This protocol does not require the addition of any base or activator. This methodology has been further extended to the selective acylation of carbohydrate diols and the one-pot preparation of acetylated glycosyl chlorides direct from free reducing sugars. The yields obtained in most of the cases are excellent.

One-pot synthesis of per-O-acetylated thioglycosides from unprotected reducing sugars

A sequential per-O-acetylation and thioglycosidation of unprotected reducing sugars using a stoichiometric quantity of acetic anhydride and alkyl- or arylthiols is reported. These reactions, which are catalyzed by BF3.OEt2, together constitute an efficient one-pot method for the synthesis of acetylated thioglycosides.

Synthesis of a 2,3,4-Triglycosylated Rhamnoside Fragment of Rhamnogalacturonan-II Side Chain A Using a Late Stage Oxidation Approach

Pectic polysaccharide RG-II, a key component of plant primary cell walls, is known to exist as a dimer formed by means of borate diester cross-links between apiosyl residues of one of its constituent side-chain oligosaccharides. Described herein is the strategy for the synthesis of the branched tetrasaccharide alpha-d-GalA-(1-->2)-[beta-D-GalA-(1-->3)]-[alpha-L-Fuc-(1-->4)]-alpha-L-Rha-OMe, an RG-II fragment that is linked to the apiosyl residue that is thought to be responsible for the borate complexation in RG-II dimer. Iterative glycosylation of the rhamnoside acceptors derived from the key 2,3-orthoacetate of methyl 4-O-methoxybenzyl-alpha-d-rhamnopyranoside afforded the protected tetrasaccharide. The target dicarboxylic acid saccharide was subsequently prepared by removal of protecting groups followed by TEMPO-mediated oxidation of galactopyranosyl residues to galactopyranosyluronic acids.

Synthetic studies on glycosphingolipids from Protostomia phyla: total syntheses of glycosphingolipids from the parasite, Echinococcus multilocularis

Efficient and systematic syntheses of four neutral glycosphingolipids that have been isolated from the metacestodes of Echinococcus multilocularis have been achieved. A key step is the direct glycosylation of galactosyl donors using thioglycosides with benzoyl ceramide in the presence of N-iodosuccinimide (NIS)/TfOH, which gave the desired oligosaccharide derivatives. The fully protected glycosides 13, 20, 22 and 25 were deprotected to give four target glycosphingolipids (1-4).

Studies on the endogenous L-selectin ligands: systematic and highly efficient total synthetic routes to lactamized-sialyl 6-O-sulfo Lewis X and other novel gangliosides containing lactamized neuraminic acid

Systematic syntheses of lactamized neuraminic acid-containing gangliosides GM4, sulfated sialylparagloboside, and sulfated/nonsulfated sialyl Lewis X are described. The highly efficient, one-step lactamization of neuraminic acid was accomplished by treatment of the N-deacetylated sialic acid (neuraminic acid)-containing gangliosides with HBTU and HOBt in DMF at 65 degrees C. Both the lactamized neuraminic acid residue and the sulfate group at O-6 of the GlcNAc residue were found to be involved in the antigenic determinant defined by G159 monoclonal antibody, while the fucose residue may not be critical for the recognition by G159 mAb.

A mild and convenient indium(III) chloride-catalyzed synthesis of thioglycosides

The efficiency of glycosidation reactions generally involves a high chemical yield, as well as high/complete stereo- and regioselectivity. All these depend on the compatibility of the reactivity of glycosyl donors and acceptors. Among glycosyl donors, thioglycosides are widely used because of their high degree of stability in many organic reactions. Although there are number of methods available for the preparation of thioglycosides, all of them have one or more disadvantages, especially concerning the time factor and cumbersome workup procedures. Here we report a convenient and high-yielding method for the preparation of thioglycosides.

Control of disaccharide conformation by π-stacking

The conformations of a series of derivatives of the disaccharide α-L-fucopyranosyl-(1[Formula: see text]3)-2-acetamido-2-deoxy-D-glucopyranoside, part of the Lex determinant, were studied by molecular modelling using the MM3* forcefield and by 1H NMR spectroscopy. Unusually shielded O-benzyl protons were observed in the 1H NMR spectrum of phenyl 2,3,4-tri-O-benzyl-α-L-fucopyranosyl-(1[Formula: see text]3)-2-deoxy-2-phthalimido-1-thio-α-D-glucopyranoside and assigned to the 2-O-benzyl group. This observation was explained by a shift in the population of the conformational mixture present about the glycosidic linkage from the positive Ψ region in the unsubstituted disaccharide to the negative Ψ region induced by π-stacking between the phthalimide and the 2-O-benzyl phenyl ring. The experimental nuclear Overhauser enhancements confirm the accuracy of the calculations.Key words: disaccharide, conformation, π-stacking, Lex determinant, NOE measurements, MM3 calculations.

First total synthesis of alpha-(2-->3)/alpha-(2-->6)-disialyl lactotetraosyl ceramide and disialyl Lewis A ganglioside as cancer-associated carbohydrate antigens

The first total synthesis of alpha-(2-->3)/alpha-(2-->6)-disialyl lactotetraosyl (DSLc4) ceramide and alpha-(2-->3)/alpha-(2-->6)-disialyl Lewis A (DSLe(a)) ganglioside as cancer-associated antigens is described. The suitably protected lactotriose (Lc3) derivatives were successively glycosylated with sialic acid, sialyl-alpha-(2-->3)-D-galactose and/or L-fucose donors in a regio- and stereo-selective manner, to give the protected type I hexa- and hepta-saccharides, respectively, which were then converted to the target gangliosides by the introduction of ceramide and subsequent complete deprotection.

Synthesis and selectin-binding activity of N-deacetylsialyl Lewis X ganglioside

A novel analogue of sialyl Lewis X ganglioside, N-deacetylsialyl Lewis X ganglioside, was synthesized. Methyl 4,7,8,9-tetra-O-acetyl-3,5-dideoxy-5-trifluoroacetamido-D-glycero-alpha-D-galacto-2-nonulopyranosylonate-(2 --> 3)-2,4,6-tri-O-benzoyl-D-galactopyranosyl trichloroacetimidate was coupled with 2-(trimethylsilyl)ethyl [2-acetamido-6-O-benzyl-2-deoxy-3-O-(4-methoxybenzyl)-beta-D-glucopyranosyl]-(1 --> 3)-[2,4,6-tri-O-benzyl-beta-D-galactopyranosyl]-(1 --> 4)-2,3,6-tri-O-benzyl-beta-D-galactopyranoside to give the desired pentasaccharide in high yield. The glycosylation of the pentasaccharide acceptor, which was derived from its precursor by removal of the 3-methoxybenzyl group, with the phenyl 1-thioglycoside derivative of L-fucose using N-iodosuccinimide-trifluoromethanesulfonic acid as promoter, produced the hexasaccharide. Proper manipulation of the protecting groups of the hexasaccharide afforded the corresponding glycosyl imidate, which was coupled with (2S,3R,4E)-2-azido-3-O-benzoyl-4-octadecene-1,3-diol. Selective reduction of the azido group, N-acylation with octadecanoic acid, and the complete removal of the protecting groups gave the desired N-deacetylsialyl Lewis X ganglioside. L-Selectin bound more strongly to N-deacetylsialyl Lewis X ganglioside than to the sialyl Lewis X ganglioside, whereas E- and P-selectins bound equally well to the two gangliosides.

Oligosaccharide microarrays for high-throughput detection and specificity assignments of carbohydrate-protein interactions

We describe microarrays of oligosaccharides as neoglycolipids and their robust display on nitrocellulose. The arrays are obtained from glycoproteins, glycolipids, proteoglycans, polysaccharides, whole organs, or from chemically synthesized oligosaccharides. We show that carbohydrate-recognizing proteins single out their ligands not only in arrays of homogeneous oligosaccharides but also in arrays of heterogeneous oligosaccharides. Initial applications have revealed new findings, including: (i) among O-glycans in brain, a relative abundance of the Lewis(x) sequence based on N-acetyllactosamine recognized by anti-L5, and a paucity of the Lewis(x) sequence based on poly-N-acetyllactosamine recognized by anti-SSEA-1; (ii) insights into chondroitin sulfate oligosaccharides recognized by an antiserum and an antibody (CS-56) to chondroitin sulfates; and (iii) binding of the cytokine interferon-gamma (IFN-gamma) and the chemokine RANTES to sulfated sequences such as HNK-1, sulfo-Lewis(x), and sulfo-Lewis(a), in addition to glycosaminoglycans. The approach opens the way for discovering new carbohydrate-recognizing proteins in the proteome and for mapping the repertoire of carbohydrate recognition structures in the glycome.

Synthesis and biological evaluation of a new sialyl Lewis X mimetic derived from lactose

A sialyl Lewis X (sLe(x)) mimetic compound, 2-(trimethylsilyl)ethyl 3-O-carboxymethyl-beta-D-galactopyranosyl-(1-->4)-[alpha-L-fucosyl-(1-->6)]-beta-D-glucopyranoside (2a), has been synthesized in 14 steps from D-lactose. This synthesis features the use of the activated glycosylating donor, lactosyl iodide, in a Koenigs-Knorr sequence, the regioselective derivatization at the C-3 position of the galactose moiety, and the stereoselective construction of a fucose-alpha(1-->6)-lactose linkage. The mimetic was tested for its ability to inhibit human polymorphonuclear leukocyte (hPMNL) adhesion to immobilized recombinant human E-selectin under shear stress conditions.

Chemically synthesized solid phase oligosaccharide probes for carbohydrate-binding receptors. Interactions of the E-, L- and P-selectins with sialyl-Le(x) and O-sulphated forms linked to biotin or to polyacrylamide

There is a growing interest in chemically defined oligosaccharide reagents for identifying proteins that bind carbohydrates and determining the specificities of carbohydrate-binding proteins. Here, we compare three sets of chemically synthesized commercially available oligosaccharide conjugates as immobilized probes, for the binding signals that they elicit with known carbohydrate-binding receptors of the immune system, the E-, P- and L-selectins. The first set of conjugates is of oligosaccharides linked to biotin via a nine-carbon spacer. The second and third sets are multivalent derivatives in which the oligosaccharides are linked, via a three-carbon spacer to poly[N-(2-hydroxyethyl)acrylamide] (PAA) or to biotinylated PAA with an average of 20% substitution of the hydroxyethyl-amide groups by carbohydrate. The conjugates were immobilized on streptavidin-coated microwells if biotinylated, otherwise by drying in uncoated wells. The most robust binding curves, overall, were with the biotinylated PAA derivatives of the ligands immobilized on streptavidin wells. These reagents have permitted a reevaluation of selectin binding signals elicited by sialyl-Lewis(x) (SLe(x)) analogues having sulphate at position 6 of the galactose (6'SuSLe(x)) or of the N-acetylglucosamine (6SuSLe(x)). The results clarify the role of 6SuSLe(x), rather then 6'SuSLe(x), as a ligand for the selectins.