2-Bromoethyl glycosides: applications in the synthesis of spacer-arm glycosides

Characterization of the binding of alpha-L-Fuc (1-->2)-beta-D-Gal (1-->), a xyloglucan signal, in blackberry protoplasts

Previous work showed that the fucose-->galactose moiety of the xyloglucan nonasaccharide XXFG is responsable for its biological activity. We used this side chain of XXFG (alpha-L-Fuc (1-->2)-beta-D-Gal (1-->)) in ligand-binding experiments to demonstrate its role as a signal molecule in plant cells. Proteins solubilized from plasma membrane enriched fractions isolated from Rubus fruticosus protoplasts were tested for their ability to bind the side chain of XXFG, using a digoxigenin- or biotin-conjugated neoglycoprotein specific for 2'-fucosyl-lactose in blots and k-ELISA tests. A putative receptor for the signaling sugar was identified, and the ligand specificity is reported. The role of structural elements important for biological activities was investigated using compounds structurally related to xyloglucan, and a variety of phytohormones such as 2,4-D.

PapG adhesin from E. coli J96 recognizes the same saccharide epitope when present on whole bacteria and as isolated protein

Purified PapG adhesin from the genetically well-defined uropathogenic Escherichia coli strain J96, as well as whole bacteria, were bound to microtiter plates that carried covalently bound globotetraose and galabiose. The binding was inhibited by soluble saccharide derivatives corresponding to the glycolipids, including all di-, tri-, tetra-, and pentasaccharide fragments of the Forssman antigen and all monodeoxy analogues of galabiose. Analysis of the inhibition pattern showed no significant difference between purified adhesin and whole bacteria. The glucose unit at the reducing end of the natural saccharides was detrimental to PapG binding since deletion of the glucose unit increased the inhibitory power 10-20 fold. The five hydroxyl groups HO-6, -2', -3', -4', -6' of the galabiose unit were shown to be important for PapG binding, presumably via intermolecular hydrogen bonds.

Synthesis of four glycosides of a disaccharide fragment representing the terminus of the O-polysaccharide of Vibrio cholerae O:1, serotype Inaba, bearing aglycons suitable for linking to proteins

Methyl 4-azido-3-O-benzyl-4,6-dideoxy-alpha-D-mannopyranoside was converted into the crystalline 2-(trimethylsilyl)ethyl 4-azido-2-O-benzoyl-3-O-benzyl-4,6-dideoxy-alpha-D-mannopyranoside. Debenzoylation of the latter, followed by glycosylation of the resulting 2-hydroxy derivative with 2-O-acetyl-4-azido-4,6-dideoxy-alpha-D-mannopyranosyl chloride, gave the 2-(trimethylsilyl)ethyl glycoside of the corresponding disaccharide (8). Deacetylation of 8, followed by reduction of the resulting 4-azido-2-hydroxy derivative with H2S, gave the corresponding amine 10. The latter was treated with 4-O-benzyl-3-deoxy-L-glycero-tetronic acid to give, after debenzylation and acetylation, the fully protected 2-(trimethylsilyl)ethyl alpha-glycoside of the disaccharide fragment of the O-PS of Vibrio cholerae O:1, serotype Inaba (13). Compound 13 was transformed into the corresponding 1-trichloroacetimidate which was treated, separately, with methyl 6-hydroxy-hexanoate and 2-(2-methoxycarbonylethylthio)ethanol, to give two analogs of 13 possessing a differing linkage arm, namely the methyl esters 16 and 17. Each of 16 and 17 was treated with aqueous sodium hydroxide, followed by a cation-exchange resin, to give the two corresponding carboxylic acids (19 and 22). Alternately, treatment of 16 and 17 with hydrazine hydrate gave the acid hydrazides 20 and 23.

Major histocompatibility complex class I binding glycopeptides for the estimation of 'empty' class I molecules

Different forms of major histocompatibility complex (MHC) class I heavy chains are known to be expressed on the cell surface, including molecules which are functionally 'empty'. Direct peptide binding to cells is obvious during sensitization of target cells in vitro for cytotoxic T lymphocyte killing and 'empty' MHC-I molecules are comparatively abundant on TAP-1/2 peptide transporter mutant cells. In the present work we have estimated the fraction of 'empty' MHC class I molecules using glycosylated peptides and cellular staining with carbohydrate specific monoclonal antibodies. Synthetic Db and Kb binding peptides were coupled at different positions with different di- or trisaccharides, using different spacing between the carbohydrate and the peptide backbone. Binding of sugar specific mAbs was compared in ELISA and cellular assays. An optimal Db binding glycopeptide was used for comparative staining with anti-Db and anti-carbohydrate monoclonal antibodies to estimate fractions of 'empty' molecules on different T lymphoid cells. On activated normal T cells, a large fraction of Db molecules were found to be 'empty'. The functional role of such 'empty' MHC class I molecules on T cells is presently unclear. However, on antigen presenting cells they might participate in the antigen presentation process.

Carbohydrate specificity of the Escherichia coli P-pilus papG protein is mediated by its N-terminal part

The adherence of pyelonephritic Escherichia coli isolates to mammalian host cells is mediated by the P-pili structures on the bacterial surface. The protein constituting the distal part of the pili structure, papG, interacts with glycan receptors on the host cell. Variation in specificity for different glycoconjugates between the isolates, that may reflect variation in host tropism, has been correlated to three different classes of papG. Truncated variants of the class I, II and III papG adhesins were produced as fusion protein in E. coli and analysed for carbohydrate binding. The results showed that both carbohydrate binding and specificity of the papG adhesin resided in a linear part of the N-terminus of the protein.

Lipid modulation of glycolipid receptor function. Availability of Gal(alpha 1-4)Gal disaccharide for verotoxin binding in natural and synthetic glycolipids

Verotoxins bind to glycosphingolipids containing terminal Gal(alpha 1-4)Gal residues. Globotriaosylceramide is the most effective receptor for verotoxin-1 in vitro and is the functional plasma-membrane receptor which mediates cytopathology for most sensitive cells. Binding of verotoxin-1 to a series of galabiose-containing or globotriaose-containing synthetic glycolipids with monoalkylsulfides and bisalkylsulfides or sulfones as the lipid moiety, have been studied for toxin binding by TLC overlay and in solid phase in the presence of auxiliary lipids. The results demonstrate that for an identical carbohydrate, binding is dramatically altered according to the nature of the lipid moiety. The close proximity of the galabiose sequence and the hydrophobic species also compromised recognition. The lipid environment is also a major determinant of receptor function, since species that were effective, even preferred toxin receptors as monitored by TLC overlay, were not necessarily recognized in the presence of auxiliary lipids. Certain glycolipids, which were not recognized by TLC overlay, were nevertheless found to be effective receptors in an auxiliary lipid matrix. These results demonstrate the crucial role of the lipid moiety in verotoxin/glycolipid recognition and are discussed in relation to toxin pathogenesis and glycolipid receptor function.

Major histocompatibility complex class I-binding peptides are recycled to the cell surface after internalization

Cytotoxic T lymphocytes (CTL) recognize target antigens as short, processed peptides bound to major histocompatibility complex class I (MHC-I) heavy and light chains (beta 2-microglobulin; beta 2-m). The heavy chain, which comprise the actual peptide binding alpha-1 and alpha-2 domains, can exist at the cell surface in different forms, either free, bound to beta 2-m or as a ternary complex with beta 2-m and peptides. MHC-I chains are also known to internalize, and recycle to the cell surface, and this has been suggested to be important in peptide presentation. Whether MHC-I-bound peptides also can recycle is not known. We have investigated this by using both peptide transporter mutant RMA-S cells and EL4 cells loaded with Db-binding peptides, by two different approaches. First, peptides were covalently linked with galabiose (Gal alpha 4Gal) at a position which did not interfere with Db binding or immunogenicity, and peptide recycling tested with Gal2-specific monoclonal antibodies. By flow cytometry, a return of Gal2 epitopes to the cell surface was found, after cellular internalization and cell surface clearance by pronase treatment. This peptide recycling could be discriminated from free fluid-phase uptake and was inhibited by methylamine, chloroquine and low temperature (18 degrees C) but not by leupeptin. Second, specific CTL were reacted with peptide-loaded target cells after complete removal of surface Db molecules by pronase, and after different times of incubation at 37 degrees C to allow reexpression. By this procedure, reappearance of target cell susceptibility was confirmed. The results are in agreement with a model for optimizing peptide presentation by recycling through an intracellular compartment similar to early endosomes in certain antigen-presenting cells.

Synthesis of omega-(methoxycarbonyl)alkyl and 9-(methoxycarbonyl)-3,6-dioxanonyl glycopyranosides for the preparation of carbohydrate-protein conjugates

omega-(Methoxycarbonyl)alkyl glycopyranosides of D-mannose having C4, C7, C9, C12, and C15 carbon chains, L-fucose and 2-acetamido-2-deoxy-D-mannose having C7 and C9 carbon chains, D-xylose and 2-acetamido-2-deoxy-L-fucose having a C9 carbon chain, and 9-(methoxycarbonyl)-3,6-dioxanonyl glycopyranosides of D-mannose, 2-acetamido-2-deoxy-D-mannose, and L-fucose were synthesized as intermediates for coupling to human serum albumin in order to examine the effect of chain length and hydrophobicity of the spacer arm on the binding specificity of lectins. 8-(Methoxycarbonyl)octyl glycosides of beta-D-Man-(1----2)-alpha-D-Man, alpha-D-Man-(1----2)-alpha-D-Man, alpha-D-ManNAc-(1----2)-alpha-D-Man, beta-D-GlcNAc-(1----2)-alpha-D-Man, and their 6-O-positional isomers, beta-D-Man-(1----6)-alpha-D-Man, alpha-D-Man-(1----6)-alpha-D-Man, alpha-D-ManNAc-(1----6)-alpha-D-Man, and beta-D-GlcNAc-(1----6)-alpha-D-Man, were also synthesized.

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

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

Interaction of viruses, bacteria and bacterial toxins with host cell surface glycolipids. Aspects on receptor identification and dissection of binding epitopes

An overview and perspective is presented on animal cell surface carbohydrate (primarily lipid-linked oligosaccharides) as specific receptors for viruses, bacteria and bacterial toxins. Although carbohydrate has been known for many years to be specific attachment sites for these ligands, it is only in very recent time that carbohydrate technology and receptor assays in combination afford a rational approach. One generalization from present experience is the property of microbiological ligands to recognize sequences placed internally in an oligosaccharide chain which differs from antibody recognition of short sequences which most often involves terminally placed determinants. This is of both biological and technical importance. Biologically it may assure attachment by avoiding differences between host individuals often residing in terminal parts (e.g. blood group determinants), and may also make a shift of target cells by mutations more efficient. Technically this property is an important help when dissecting narrow binding epitopes, and for disclosing receptor-binding variants with only slight differences in binding epitopes (e.g. different epitopes on the same disaccharide). Such variants representing a kind of "epitope drift" are probably a consequence of point mutations in the binding site of the lectin-like proteins to select a proper host environment. Current technology allows an efficient screening for carbohydrate receptors with interesting consequences for applications within medicine (diagnosis and therapy) and biotechnology.

Streptococcus pneumoniae type XIV polysaccharide: synthesis of a repeating branched tetrasaccharide with dioxa-type spacer-arms

beta-Glycosides of 2-acetamido-2-deoxy-D-glucopyranose were synthesised, using either 7-methoxycarbonyl-3,6-dioxa-1-heptanol or 8-azido-3,6-dioxa-1-octanol. Selective beta-lactosylation of 7-methoxycarbonyl-3,6-dioxaheptyl 2-acetamido-3-O-benzyl-2-deoxy-beta-D-glucopyranoside with hepta-O-acetyl-lactosyl-trichloroacetimidate, followed by beta-galactosylation of the secondary hydroxyl group with O-(2,3,4,6-tetra-O-acetyl-alpha-D-galactopyranosyl)trichloroacetimida te, catalytic hydrogenolysis, and O-deacetylation, gave 7-methoxycarbonyl-3,6-dioxaheptyl 2-acetamido-2-deoxy-4-O-beta-D-galactopyranosyl-6-O-(4-O-beta-D- galactopyranosyl-beta-D-glucopyranosyl)-beta-D-glucopyranoside. Selective beta-lactosylation of 8-azido-3,6-dioxaoctyl 2-acetamido-3-O-benzyl-2-deoxy-beta-D-glucopyranoside with hepta-O-acetyl-lactosyl bromide in the presence of silver triflate, followed by condensation with 2,3,4,6-tetra-O-acetyl-alpha-D-galactopyranosyl bromide in the presence of silver triflate, catalytic hydrogenolysis, and O-deacetylation, gave 8-azido-3,6-dioxaoctyl 2-acetamido-2-deoxy-4-O-beta-D-galactopyranosyl-6-O-(4-O-beta-D- galactopyranosyl-beta-D-glucopyranosyl)-beta-D-glucopyranoside.

Synthesis of spacer-arm, lipid, and ethyl glycosides of the terminal trisaccharide [alpha-D-Gal-(1----4)-beta-D-Gal-(1----4)-beta-D-GlcNAc] portion of the blood-group P1 antigen: preparation of neoglycoproteins

The title compounds were prepared via the acetylated 2-bromoethyl beta-glycoside (5) of alpha-D-Gal-(1----4)-beta-D-Gal-(1----4)-beta-D-GlcNAc by displacement of bromide ion with methyl 3-mercaptopropionate, octadecanethiol, and hydrogen, respectively. Silver triflate-promoted glycosylation of 2-bromoethyl 3,6-di-O-benzyl-2-deoxy-2-phthalimido-beta-D-glucopyranoside with 2,3,6-tri-O-acetyl-4-O-(2,3,4,6-tetra-O-acetyl-alpha-D-galactopyranosyl) -alpha-D-galactopyranosyl bromide gave 5. The spacer-arm glycoside derived from methyl 3-mercaptopropionate was coupled to bovine serum albumin and key-hole limpet haemocyanin to give neoglycoproteins.

Synthesis from pullulan of spacer-arm, lipid, and ethyl glycosides of a tetrasaccharide [alpha-D-Glc-(1----6)-alpha-D-Glc-(1----4)-alpha-D-Glc-(1----4)-D-Glc] found in human urine; preparation of neoglycoproteins

Enzymic hydrolysis of pullulan, followed by acetylation and chromatography, gave acetylated alpha-D-Glcp-(1----6)-alpha-D-Glcp-(1----4)-alpha-D-Glcp-(1----4)-D-Glcp which, with 2-bromoethanol and boron trifluoride etherate in dichloromethane, gave the 2-bromoethyl glycoside. The reactions of the glycoside with methyl 3- mercaptopropionate , methyl 11- mercaptoundecanoate , and octadecanethiol are described, and also its hydrogenolysis to give an ethyl glycoside. The mercaptopropionate -derived, spacer-arm glycoside has been coupled to bovine serum albumin and keyhole limpet haemocyanin.

Synthesis of spacer-arm, lipid, and ethyl glycosides of the trisaccharide portion [alpha-D-Gal-(1----4)-beta-D-Gal-(1----4)-beta-D-Glc] of the blood-group Pk antigen: preparation of neoglycoproteins

The title compounds were prepared via the acetylated 2-bromoethyl glycoside 11 of alpha-D-Gal-(1----4)-beta-D-Gal-(1----4)-beta-D-Glc by displacement of bromide ion with methyl 3- mercaptopropionate , octadecanethiol , and hydrogen, respectively. Silver triflate -promoted glycosylation of 2-bromoethyl 2,3,6-tri-O-benzyl-beta-D-glucopyranoside with 2,3,6-tri-O-acetyl-4-O-(2,3,4,6-tetra-O-acetyl-alpha-D-galactopyranosyl) -alpha -D-galactopyranosyl bromide gave 11. A tetradeuterated analogue of 11 was prepared by essentially the same route. The spacer-arm glycoside formed from methyl 3- mercaptopropionate was coupled to bovine serum albumin and keyhole limpet haemocyanin.

2-Bromoethyl glycosides in glycoside synthesis: preparation of glycoproteins containing alpha-L-Fuc-(1----2)-D-Gal and beta-D-Gal-(1----4)-D-GlcNAc

The applicability of 2-bromoethyl glycosides in carbohydrate synthesis is demonstrated by the synthesis of glycosides of alpha-L-Fuc-(1----2)-D-Gal and beta-D-Gal-(1----4)-D-GlcNAc. The bromoethyl aglycon was transformed into the methoxycarbonylethylthioethyl spacer, which allowed coupling of the sugars to proteins (BSA and KLH).