Temporary Protection and Activation in the Regioselective Synthesis of Saccharide Sulfates

Studies of carbohydrate-carbohydrate-interactions by atomic force microscopy employing functionalized 4-acetylthio-butyl glucopyranosides

By Fischer glycosylation both anomers of 4-chlorobutyl gluco-as well as galactopyranosides were obtained and transformed into the corresponding 4-acetylthio-butyl glycopyranosides. Dependent on the precursors two straightforward routes were followed to obtain the appropriate 3-O-sulfated derivatives. Unsubstituted and sulfated glucopyranosides were attached to gold surfaces a gold tips. Their interactions were studied using atomic force microscopy for simulations of intercellular glycoside-based interactions and discussed in-depth.

Diarylborinic Acid-Catalyzed, Site-Selective Sulfation of Carbohydrate Derivatives

Sulfated carbohydrates have been implicated in diverse biological processes, with the position and extent of sulfation of a glycoside often playing important roles in determining the affinity and specificity of its binding to a biomolecular partner. Methods for the site-selective introduction of sulfate groups to carbohydrates are thus of interest. Here, we describe the development of a diarylborinic acid-catalyzed protocol for selective sulfation of pyranoside derivatives at the equatorial position of a cis-1,2-diol group. This method, which employs the sulfur trioxide-trimethylamine complex as the electrophile, has been employed for installation of a sulfate group at the 3-position of a range of galacto- and mannopyranosides, including substrates having a free primary OH group. By using a full equivalent of the diarylborinic acid, selective syntheses of more complex monosulfated glycosides, namely, a 3'-sulfolactose derivative and 3'-sulfo-β-galactosylceramide, have been accomplished. Preliminary kinetics experiments suggested that the catalyst resting state is a tetracoordinate diarylborinic ester that reacts with the SO₃ complex in the turnover-limiting step. Catalyst inhibition by the pyranoside sulfate product and trialkylamine byproduct of the reaction was demonstrated.

"One-Pot" Protection, Glycosylation, and Protection-Glycosylation Strategies of Carbohydrates

Carbohydrates, which are ubiquitously distributed throughout the three domains of life, play significant roles in a variety of vital biological processes. Access to unique and homogeneous carbohydrate materials is important to understand their physical properties, biological functions, and disease-related features. It is difficult to isolate carbohydrates in acceptable purity and amounts from natural sources. Therefore, complex saccharides with well-defined structures are often most conviently accessed through chemical syntheses. Two major hurdles, regioselective protection and stereoselective glycosylation, are faced by carbohydrate chemists in synthesizing these highly complicated molecules. Over the past few years, there has been a radical change in tackling these problems and speeding up the synthesis of oligosaccharides. This is largely due to the development of one-pot protection, one-pot glycosylation, and one-pot protection-glycosylation protocols and streamlined approaches to orthogonally protected building blocks, including those from rare sugars, that can be used in glycan coupling. In addition, new automated strategies for oligosaccharide syntheses have been reported not only for program-controlled assembly on solid support but also by the stepwise glycosylation in solution phase. As a result, various sugar molecules with highly complex, large structures could be successfully synthesized. To summarize these recent advances, this review describes the methodologies for one-pot protection and their one-pot glycosylation into the complex glycans and the chronological developments associated with automated syntheses of oligosaccharides.

Boronic esters as protective groups in carbohydrate chemistry: processes for acylation, silylation and alkylation of glycoside-derived boronates

Boronic esters are employed in streamlined protocols for protection, functionalization and deprotection of glycosides, avoiding isolation and purification of intermediates.

Elucidation and chemical modulation of sulfolipid-1 biosynthesis in Mycobacterium tuberculosis

Mycobacterium tuberculosis possesses unique cell-surface lipids that have been implicated in virulence. One of the most abundant is sulfolipid-1 (SL-1), a tetraacyl-sulfotrehalose glycolipid. Although the early steps in SL-1 biosynthesis are known, the machinery underlying the final acylation reactions is not understood. We provide genetic and biochemical evidence for the activities of two proteins, Chp1 and Sap (corresponding to gene loci rv3822 and rv3821), that complete this pathway. The membrane-associated acyltransferase Chp1 accepts a synthetic diacyl sulfolipid and transfers an acyl group regioselectively from one donor substrate molecule to a second acceptor molecule in two successive reactions to yield a tetraacylated product. Chp1 is fully active in vitro, but in M. tuberculosis, its function is potentiated by the previously identified sulfolipid transporter MmpL8. We also show that the integral membrane protein Sap and MmpL8 are both essential for sulfolipid transport. Finally, the lipase inhibitor tetrahydrolipstatin disrupts Chp1 activity in M. tuberculosis, suggesting an avenue for perturbing SL-1 biosynthesis in vivo. These data complete the SL-1 biosynthetic pathway and corroborate a model in which lipid biosynthesis and transmembrane transport are coupled at the membrane-cytosol interface through the activity of multiple proteins, possibly as a macromolecular complex.

PapA3 is an acyltransferase required for polyacyltrehalose biosynthesis in Mycobacterium tuberculosis

Mycobacterium tuberculosis possesses an unusual cell wall that is replete with virulence-enhancing lipids. One cell wall molecule unique to pathogenic M. tuberculosis is polyacyltrehalose (PAT), a pentaacylated, trehalose-based glycolipid. Little is known about the biosynthesis of PAT, although its biosynthetic gene cluster has been identified and found to resemble that of the better studied M. tuberculosis cell wall component sulfolipid-1. In this study, we sought to elucidate the function of papA3, a gene from the PAT locus encoding a putative acyltransferase. To determine whether PapA3 participates in PAT assembly, we expressed the protein heterologously and evaluated its acyltransferase activity in vitro. The purified enzyme catalyzed the sequential esterification of trehalose with two palmitoyl groups, generating a diacylated product similar to the 2,3-diacyltrehalose glycolipids of M. tuberculosis. Notably, PapA3 was selective for trehalose; no activity was observed with other structurally related disaccharides. Disruption of the papA3 gene from M. tuberculosis resulted in the loss of PAT from bacterial lipid extracts. Complementation of the mutant strain restored PAT production, demonstrating that PapA3 is essential for the biosynthesis of this glycolipid in vivo. Furthermore, we determined that the PAT biosynthetic machinery has no cross-talk with that for sulfolipid-1 despite their related structures.

PapA1 and PapA2 are acyltransferases essential for the biosynthesis of the Mycobacterium tuberculosis virulence factor sulfolipid-1

Mycobacterium tuberculosis produces numerous exotic lipids that have been implicated as virulence determinants. One such glycolipid, Sulfolipid-1 (SL-1), consists of a trehalose-2-sulfate (T2S) core acylated with four lipid moieties. A diacylated intermediate in SL-1 biosynthesis, SL(1278), has been shown to activate the adaptive immune response in human patients. Although several proteins involved in SL-1 biosynthesis have been identified, the enzymes that acylate the T2S core to form SL(1278) and SL-1, and the biosynthetic order of these acylation reactions, are unknown. Here we demonstrate that PapA2 and PapA1 are responsible for the sequential acylation of T2S to form SL(1278) and are essential for SL-1 biosynthesis. In vitro, recombinant PapA2 converts T2S to 2'-palmitoyl T2S, and PapA1 further elaborates this newly identified SL-1 intermediate to an analog of SL(1278). Disruption of papA2 and papA1 in M. tuberculosis confirmed their essential role in SL-1 biosynthesis and their order of action. Finally, the Delta papA2 and Delta papA1 mutants were screened for virulence defects in a mouse model of infection. The loss of SL-1 (and SL(1278)) did not appear to affect bacterial replication or trafficking, suggesting that the functions of SL-1 are specific to human infection.

Selective manipulation of steroid hydroxyl groups with boronate esters: efficient access to antigenic C-3 linked steroid–protein conjugates and steroid sulfate standards for drug detection

The temporary protection of 17alpha-alkyl-5alpha-androstane-3beta,16beta,17beta triols as boronate esters is an efficient method for their regioselective functionalisation. This has been applied to the synthesis of protein-steroid conjugates 7-10 suitable for the development of immunoassays targeting classes of steroids banned from competition in Australian horse racing and other sports. The synthesis of steroids sulfate conjugates 42 and 44 for use as reference standards is also reported.

Regioselective synthesis of long-chain ethers and their sulfates derived from methyl beta-D-galactopyranoside and derivatives via dibutylstannylene acetal intermediates

A number of different conditions were investigated for the alkylation of the dibutylstannylene acetals of methyl beta-d-galactopyranoside with long-chain primary alkyl bromides, decyl, dodecyl, and tetradecyl bromide. The best yields of the major products, the 3-O-alkyl ethers, were obtained by reaction of the alkyl bromide with the monodibutylstannylene acetal in DMF in the presence of cesium fluoride for extended periods of time at moderate temperatures (65 degrees C). These products were always accompanied by minor amounts of the 3,6-di-O-alkyl derivative. Performing the reaction with excess alkyl halide on the bis(dibutylstannylene) acetal resulted in more of the 3,6-di-O-alkyl derivative, particularly for the shorter alkyl bromides, but this product was never predominant. Sulfation of the dibutylstannylene acetal of methyl 3-O-tetradecyl-beta-D-galactopyranoside resulted in the 6-sulfate in 96% yield.

Identification, function and structure of the mycobacterial sulfotransferase that initiates sulfolipid-1 biosynthesis

Sulfolipid-1 (SL-1) is an abundant sulfated glycolipid and potential virulence factor found in Mycobacterium tuberculosis. SL-1 consists of a trehalose-2-sulfate (T2S) disaccharide elaborated with four lipids. We identified and characterized a conserved mycobacterial sulfotransferase, Stf0, which generates the T2S moiety of SL-1. Biochemical studies demonstrated that the enzyme requires unmodified trehalose as substrate and is sensitive to small structural perturbations of the disaccharide. Disruption of stf0 in Mycobacterium smegmatis and M. tuberculosis resulted in the loss of T2S and SL-1 formation, respectively. The structure of Stf0 at a resolution of 2.6 A reveals the molecular basis of trehalose recognition and a unique dimer configuration that encloses the substrate into a bipartite active site. These data provide strong evidence that Stf0 carries out the first committed step in the biosynthesis of SL-1 and establish a system for probing the role of SL-1 in M. tuberculosis infection.

Synthesis of various sulfoforms of the trisaccharide β-d-GlcpA-(1→3)-β-d-Galp-(1→3)-β-d-Galp-(1→OMP) as probes for the study of the biosynthesis and sorting of proteoglycans

A straightforward preparation of various sulfoforms of the trisaccharide 4-methoxyphenyl O-(sodium beta-d-glucopyranosyluronate)-(1-->3)-( beta-d-galactopyranosyl)-(1-->3)-beta-d-galactopyranoside (1), namely its 6a- and 4a-monosulfate, 6b- and 4b-monosulfate and 6a,6b-disulfate derivatives, is reported for the first time. These compounds, which are partial structures of the linkage region of proteoglycans, will serve as probes for the study of the biosynthesis and sorting of these macromolecules. A key trisaccharide derivative, in which the two similar d-Gal units were differentiated at C-4,6 with 4,6-benzylidene and 4,6-di-tert-butylsilylene acetals, respectively, was used as a common intermediate. Both acetal groups showed excellent orthogonality, and allowed the preparation of all target compounds in high yield. Noteworthy is the possibility to prepare the 6a- and 6b-monosulfated and the 6a,6b-disulfated species through a one-pot regioselective procedure starting from a tetrol precursor.

Discovery of sulfated metabolites in mycobacteria with a genetic and mass spectrometric approach

The study of the metabolome presents numerous challenges, first among them being the cataloging of its constituents. A step in this direction will be the development of tools to identify metabolites that share common structural features. The importance of sulfated molecules in cell-cell communication motivated us to develop a rapid two-step method for identifying these metabolites in microorganisms, particularly in pathogenic mycobacteria. Sulfurcontaining molecules were initially identified by mass spectral analysis of cell extracts from bacteria labeled metabolically with a stable sulfur isotope (34SO 4 2-). To differentiate sulfated from reduced-sulfur-containing molecules, we employed a mutant lacking the reductive branch of the sulfate assimilation pathway. In these sulfur auxotrophs, heavy sulfate is channeled exclusively into sulfated metabolites. The method was applied to the discovery of several new sulfated molecules in Mycobacterium tuberculosis and Mycobacterium smegmatis. Because a sulfur auxotrophic strain is the only requirement of the approach, many microorganisms can be studied in this manner. Such genetic engineering in combination with stable isotopic labeling can be applied to various metabolic pathways and their products.

Synthesis and utility of sulfated chromogenic carbohydrate model substrates for measuring activities of mucin-desulfating enzymes

A chromogenic substrate, 4-nitrophenyl 2-acetamido-2-deoxy-beta-D-glucopyranoside 6-sodium sulfate was synthesized and used in combination with beta-N-acetylhexosaminidase for detection of the sulfatase, MdsA, by release of 4-nitrophenol. MdsA was originally isolated from the bacterium Prevotella strain RS2 and is believed to be involved in desulfation of sulfomucins, major components of the mucus barrier protecting the human colon surface. The exo nature of the MdsA sulfatase was indicated by its inability to de-esterify the disaccharide 4-nitrophenyl beta-D-galactopyranosyl-(1-->4)-2-acetamido-2-deoxy-beta-D-glucopyranoside 6-sodium sulfate. This latter compound was prepared from monosaccharide precursors by two different methods, the shorter requiring just six steps from 4-nitrophenyl 2-acetamido-2-deoxy-beta-D-glucopyranoside and giving an overall yield of 26.4%. The syntheses of 4-nitrophenyl beta-D-galactopyranoside 3-triethylammonium sulfate and 6-triethylammonium sulfate and their use in combination with beta-galactosidase as chromogenic substrates for detecting Bacteroides fragilis sulfatases with different specificities was also demonstrated.

Multigram syntheses of the disaccharide repeating units of chondroitin 4- and 6-sulfates

The multigram syntheses of beta-D-glucopyranosyluronic acid-(1-->3)-2-acetamido-2-deoxy-4- and 6-O-sulfo-D-galactopyranose disodium salt, the disaccharide repeating units of chondroitin 4- and 6-sulfates, are described. The disaccharide benzyl methyl 2,3,4-tri-O-benzoyl-beta-D-glucopyranosyluronate- (1-->3)-2-acetamido-2-deoxy-alpha-D-galactopyranoside was used as a common intermediate. Selective benzoylation at O-6 followed by O-sulfonation at C-4 of the aminosugar moiety, saponification and catalytic hydrogenation afforded the 4-O-sulfo derivative, whereas selective O-sulfonation at C-6 followed by similar deprotection steps provided the 6-O-sulfo derivative in high yield.

Synthesis and 1H NMR characterization of the six isomeric mono-O-sulfates of 8-methoxycarbonyloct-1-yl O-beta-D-galactopyranosyl-(1-->4)-2-acetamido-2-deoxy-beta-D-glucopyran oside

All six isomeric mono-O-sulfates of beta-D-Galp-(1-->4)-beta-D-GlcpNAc-O-(CH2)8COOMe (LacNAc-MC) have been chemically synthesized and characterized by high resolution 1H NMR spectroscopy. Sulfation causes characteristic substitution-site-specific downfield shifts of 1H NMR signals. The 4C1 chair conformation of both pyranose residues of LacNAc are unaffected by mono-O-sulfation, and, with the exception of the 3-O-sulfate derivative, glycosidic torsion angles are also unaffected.