Synthetic N- and O-glycosyl derivatives of L-asparagine, L-serine, and L-threonine

A Hitchhiker's Guide to Problem Selection in Carbohydrate Synthesis

Oligosaccharides are ubiquitous in molecular biology and are used for functions ranging from governing protein folding to intercellular communication. Perhaps paradoxically, the exact role of the glycan in most of these settings is not well understood. One reason for this contradiction concerns the fact that carbohydrates often appear in heterogeneous form in nature. These mixtures complicate the isolation of pure material and characterization of structure-activity relationships. As a result, a major bottleneck in glycoscience research is the synthesis and modification of pure materials. While synthetic and chemoenzymatic methods have enabled access to homogeneous tool compounds, a central problem, particularly for newer synthetic chemists, is the matter of problem selection. This outlook aims to provide an entry level overview of fundamental principles in carbohydrate chemistry with an eye toward enabling solutions to frontier challenges.

Self-promoted and stereospecific formation of N-glycosides

A stereoselective and self-promoted glycosylation for the synthesis of various N-glycosides and glycosyl sulfonamides from trichloroacetimidates is presented.

A stereoselective and self-promoted glycosylation for the synthesis of various N-glycosides and glycosyl sulfonamides from trichloroacetimidates is presented. No additional catalysts or promoters are needed in what is essentially a two-component reaction. When α-glucosyl trichloroacetimidates are employed, the reaction resulted in the stereospecific formation of the corresponding β-N-glucosides in high yields at ambient conditions. On the other hand, when equatorial glucosyl donors were used, the stereospecificity decreased and resulted in a mixture of anomers. By NMR-studies, it was concluded that this decrease in stereospecificity was due to an, until now, unpresented anomerization of the trichloroacetimidate under the very mildly acidic conditions. The mechanism and kinetics of the glycosylations have been studied by NMR-experiments, which gave an insight into the activation of trichloroacetimidates, suggesting an S_Ni-like mechanism involving ion pairs. The scope of glycosyl donors and sulfonamides was found to be very broad including popular N-protective groups and common glycosyl donors of various reactivity. Peracetylated GlcNAc trichloroacetimidate could be used without the need for any promotors or additives and a tyrosine side chain was glycosylated as an N-glycosyl carbamate. The N-carbamates and the N-sulfonyl groups functioned as orthogonal protective groups of the N-glycoside and hence allowed further N-functionalization without risking mutarotation of the N-glycoside. The N-glycosylation was also performed on a gram scale, without a drop in stereoselectivity nor yield.

Glycosylated neuropeptides: a new vista for neuropsychopharmacology?

The application of endogenous neuropeptides (e.g., enkephalins) as analgesics has been retarded by their poor stability in vivo and by their inability to effectively penetrate the blood-brain barrier (BBB). Effective BBB transport of glycosylated enkephalins has been demonstrated in several labs now. Analgesia (antinociception) levels greater than morphine, and with reduced side effects have been observed for several glycopeptides related to enkephalin. Somewhat paradoxically, enhanced BBB transport across this lipophilic barrier is achieved by attaching water-soluble carbohydrate groups to the peptide moieties to produce biousian glycopeptides that can be either water-soluble or membrane bound. Transport is believed to rely on an endocytotic mechanism (transcytosis), and allows for systemic delivery and transport of the water-soluble glycopeptides. Much larger endorphin/dynorphin glycopeptide analogs bearing amphipathic helix address regions also have been shown to penetrate the BBB in mice. This holds forth the possibility of transporting much larger neuropeptides across the BBB, which may encompass a wide variety of receptors beyond the opioid receptors.

Polymorphism of monolayer lipid membrane structures made from unsymmetrical bolaamphiphiles

The crystal structures of synthetic unsymmetrical 1-glucosamide- and 1-galactosamide bolaamphiphiles, 13-[(beta-d-glucopyranosyl)carbamoyl]tridecanoic acid (1) and 15-[(beta-d-galactopyranosyl)carbamoyl]pentadecanoic acid (2), respectively, were elucidated by single-crystal X-ray analysis. The space group for 1 is P2(1), Z=2 with cell dimensions: a=8.6816(9), b=4.8578(5), c=26.250(3)A, beta=91.460(2) degrees ; that for 2P2(1), Z=2 with cell dimensions: a=4.90(1), b=40.139(1), 6.289(1)A, beta=106.48(1) degrees . The glucopyranosyl and galactopyranosyl rings in 1 and 2, respectively, take a (4)C(1) chair conformation. In the crystal lattice, the 1-glucosamide 1 forms a symmetrical monolayer lipid membrane (MLM) structure in which the molecules are packed in an antiparallel fashion, while 1-galactosamide 2 has an unsymmetrical MLM with parallel molecular packing. The stereochemistry of the sugar hydroxy group proved to affect their hydrogen-bonding networks and induce the polymorphism of the MLM.

Synthesis of an isostere of an O-linked glycopeptide

[reaction: see text] A route for the synthesis of an electrophilic, carbocyclic galactose equivalent from D-galactose is described. The strategy utilizes ring-closing metathesis with Grubbs's second-generation catalyst as the key step. The galactose-derived electrophile reacted in an S(N)2 fashion with N-Boc-cysteine methyl ester to provide an alpha-galactosylserine isostere. The method was extended to the synthesis of a glycopeptide isostere.

Synthesis of S-linked glycopeptides in aqueous solution

Investigation of direct S-glycosylation of homocysteine and cysteine containing peptides with O-acetyl protected glycosyl halides led under two-phase conditions in the presence of sodium carbonate as base to excellent results. Thus, from glucosyl bromide, galactosyl bromide, lactosyl bromide, sialyl chloride, and N-Troc protected 2-amino-2-deoxyglucosyl bromide S-glycosylated dipeptides 15, 18-21, 23, 24, and 26-29, respectively, were obtained in excellent yields. Alternatively, depending on the solubility of the peptide moiety, mixtures of DMF and water could be employed for successfully carrying out this reaction. Thus, S-glycosylated tripeptides 42-45 could be obtained. Combination of this method with chemical ligation was also successfully carried out.

The synthesis of deoxy-alpha-Gal epitope derivatives for the evaluation of an anti-alpha-Gal antibody binding

Alpha-Gal epitopes (also termed as alpha-Gal) are carbohydrate structures bearing the alpha-D-Gal-(1-->3)-beta-D-Gal terminus 1 and are known to be the antigen responsible for antibody-mediated hyperacute rejection in xenotransplantation. Terminal 2-, 3-, 4-, and 6-deoxy-Gal derivatives of alpha-Gal were synthesized. Inhibition ELISA using mouse laminin was established to determine the binding affinity of the synthesized alpha-Gal derivatives. 4-Deoxy-alpha-Gal derivative 7 showed a significant reduction in antibody recognition. The IC(50) value was 15-fold poorer than the standard alpha-Gal epitopes alpha-D-Gal-(1-->3)-beta-D-Gal-(1-->4)-beta-D-Glc-NHAc (39) and alpha-D-Gal-(1-->3)-beta-D-Gal-(1-->4)-beta-D-Glc-OBn (40). A similar observation was seen with 2-deoxy-alpha-Gal derivative 5, whose IC(50) value was nearly tenfold higher than the standards. Interestingly, substitution at the terminal 3-position resulted in only a fourfold decrease in antibody recognition, suggesting a possible point of future derivation. Finally, 6-deoxy-alpha-Gal derivative 8 exhibited similar antibody recognition to both alpha-Gal epitope 39 and alpha-Gal epitope 40. This strongly suggests that derivatization at the 6-position can be accomplished without loss of antibody recognition. These findings can be utilized for the future design of other alpha-Gal derivatives.

Synthesis of C-terminal glycopeptides from resin-bound glycosyl azides via a modified Staudinger reaction

The solid-phase synthesis of glycopeptides containing the sugar at the C-terminus is reported. The method is demonstrated on a model, the endogenous antinociceptive peptide Leu-enkephalin. 2,3,4-Tri-O-acetyl-1-azido-1-deoxy-beta-D-glucopyranuronic acid was synthesized and immobilized onto a variety of derivatized resins. Conjugation of the first amino acid was accomplished by reaction of the resin-bound glycosyl azide with an activated amino acid, in one step, via a modified Staudinger reaction. Standard solid-phase peptide synthesis then resulted in the desired amide-linked glycopeptide. Reaction conditions and reagents for the glycosylation were varied to optimize the yield and purity of the product. The optimum conditions were found to be the use of a 4-fold molar excess of activated amino acid and 3-fold excess of tri-n-butylphosphine in tetrahydrofuran. This methodology is generally applicable to most peptide sequences and is compatible with both Boc- and Fmoc- synthetic strategies on a variety of resins.

Synthesis of single- and double-chain fluorocarbon and hydrocarbon galactosyl amphiphiles and their anti-HIV-1 activity

Galactosylceramide (GalCer) is an alternative receptor allowing HIV-1 entry into CD4(-)/GalCer(+) cells. This glycosphingolipid recognizes the V3 loop of HIV gp120, which plays a key role in the fusion of the HIV envelope and cellular membrane. To inhibit HIV uptake and infection, we designed and synthesized analogs of GalCer. These amphiphiles and bolaamphiphiles consist of single and double hydrocarbon and/or fluorocarbon chain beta-linked to galactose and galactosamine. They derive from serine (GalSer), cysteine (GalCys), and ethanolamine (GalAE). The anti-HIV activity and cytotoxicity of these galactolipids were evaluated in vitro on CEM-SS (a CD4(+) cell line), HT-29, a CD4(-) cell line expressing high levels of GalCer receptor, and/or HT29 genetically modified to express CD4. GalSer and GalAE derivatives, tested in aqueous medium or as part of liposome preparation, showed moderate anti-HIV-1 activities (IC50 in the 20-220 microM range), whereas none of the GalCys derivatives was found to be active. Moreover, only some of these anti-HIV active analogs inhibited the binding of [3H]suramin (a polysulfonyl compound which displays a high affinity for the V3 loop) to SPC3, a synthetic peptide which contains the conserved GPGRAF region of the V3 loop. Our results most likely indicate that the neutralization of the virion through masking of this conserved V3 loop region is not the only mechanism involved in the HIV-1 antiviral activity of our GalCer analogs.

Molecular structures and hydrogen-bond networks in crystals of synthetic 1-d-galactosamide bolaamphiphiles

The crystal structures of synthetic 1-galactosamide bolaamphiphiles, N,N'-bis(beta-D-galactopyranosyl)decane-1,10-dicarboxamide (1) and N,N'-bis(beta-D-galactopyranosyl)dodecane-1,12-dicarboxamide (2), were determined by single-crystal X-ray analysis. The space groups are P21, Z = 2 with cell dimensions: a = 13.624(2), b = 4.832(3), c = 21.178(3) Angstroms, beta = 98.57(1) degrees for 1; a = 13.521(2), b = 4.838(1), c = 23.706(2) Angstroms, beta = 104.945(10) degrees for 2. The galactopyranosyl rings of 1 and 2 are in a 4C1 chair conformation. The bolaamphiphile molecules are arranged in a layered structure for 1 and 2, with the alkylene chains packed parallel all over the layers. The hydroxyl groups of the galactopyranosyl rings in 1 and 2 form identical three-dimensional hydrogen-bond networks. The connecting decamethylene spacer of 1 has a kink conformation, while the dodecamethylene chain of 2 an all-trans zigzag conformation.

Detailed studies on substrate structure requirements of glycoamidases A and F

Glycoamidases (peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase, EC 3.5.1.52; also known as peptide: N-glycanases (PNGases) release N-linked oligosaccharides from glycopeptides and/or glycoproteins by hydrolyzing the glycosylated beta-amide bond of the asparagine side chain. The most widely used glycoamidases are those from Flavobacterium meningosepticum (glycoamidase F or PNGase F) and almond emulsin (glycoamidase A or PNGase A). To study the substrate structure requirement of these enzymes systematically, we synthesized >30 glycopeptides containing cellobiose, lactose, GlcNAc, and di-N,N'-acetylchitobiose (CTB). The length of the peptide was varied from one to five amino acids, and glycosylamines were linked to either Asn or Gln located at different positions in the peptide, including NH2 and COOH termini. Neither enzyme could cleave cellobiose and lactose glycopeptides, indicating that the 2-acetamido group on the Asn-linked GlcNAc is important in the recognition by both glycoamidases A and F. GlcNAc peptides could be cleaved by both enzymes, albeit not as effectively as CTB glycopeptides. Neither enzyme requires the Asn-Xaa-(Ser/Thr) sequence (required for N-glycosylation) for activity. Glycoamidase A could even hydrolyze a Gln-bound CTB glycopeptide, whereas the action of glycoamidase F on this substrate is minimal. While glycoamidase A could act on CTB dipeptides, glycoamidase F preferred a tripeptide or longer. The Km and Vmax values of glycoamidase A for t-butoxycarbonyl-(CTB)-Asn-Ala-Ser-OMe were 2.1 mM and 0.66 micromol/min/mg, respectively. A natural glycodipeptide, Man9-GlcNAc2-Asn-Phe, was also completely hydrolyzed by glycoamidase A.

Isothiocyanates and cyclic thiocarbamates of alpha,alpha'-trehalose, sucrose, and cyclomaltooligosaccharides

6,6'-Dideoxy-6,6'-diisothiocyanato-alpha,alpha'-trehalose (4), 6-deoxy-6-isothiocyanato-alpha-D-fructo-furanose beta-D-fructopyranose 1,2':2,1'-dianhydride (11), 6,6'-dideoxy-6,6'-diisothiocyanatosucrose (16), and per(6-deoxy-6-isothiocyanato)-cyclomaltohexaose (23), -cyclomaltoheptaose (27), and -cyclomaltooctaose (31) have been prepared in high yield by reaction of the corresponding amino sugars with thiophosgene. In the absence of base, all isothiocyanates were stable and could be stored and acetylated without decomposition. In the presence of triethylamine, 6,6'-dideoxy-6,6'-diisothiocyanato-alpha,alpha'-trehalose underwent intramolecular cyclisation involving HO-4 to give the corresponding bis(cyclic thiocarbamate). The product of cyclisation at a single glucopyranosyl unit was obtained in the treatment of the above diisothiocyanate with mixed (H+, HO-) ion-exchange resin. Under identical reaction conditions, 6,6'-dideoxy-6,6'-diisothiocyanatosucrose yielded exclusively the product of intramolecular cyclisation at the D-glucopyranosyl moiety, while derivatives of alpha-D-fructofuranose beta-D-fructopyranose 1,2':2,1'-dianhydride and cyclomaltooligosaccharides remained unchanged.

Synthesis and structural studies of asparagine-modified 2-deoxy-alpha-N-glycopeptides associated with the renin-angiotensin system

Following addition of N-iodosuccinimide to glycals, reductive hydrogenolysis and ring opening gave 2-deoxy-alpha-N-glycopeptides carrying a deaminated asparagine unit. This reaction could be performed employing glucal, galactal, L-rhamnal, L-fucal and lactal to give the corresponding glycoconjugate building blocks 11, 12, 17, 22, 27 and 32. Further NIS-mediated glycosylation of the rhamno derivative 21 led to simple trisaccharide peptide adducts 45. Peptide synthesis of the gluco building unit with different preassembled oligopeptides afforded glycoconjugates 36, 39, 41 and 42 assumed to be of interest as potential inhibitors of the renin-angiotensin system.

Chemical and enzymatic synthesis of multivalent sialoglycopeptides

Linear and branched glycopeptides containing multiple sialyl-N-acetyllactosamine side chains have been synthesized using a combined chemical and enzymatic approach. Peptide backbones in which beta-GlcNAc-Asn residues were incorporated were obtained in good yields by optimized solid-phase synthesis following the Boc strategy. The resulting multivalent glycopeptides were galactosylated in near-quantitative yields using bovine galactosyltransferase, UDP-galactose, and calf alkaline phosphatase that destroys the inhibiting side product UDP. Subsequent enzymatic sialylation yielded the desired glycopeptides containing asparagine-linked sialyl-N-acetyllactosamine side chains. The compounds were characterized by 1H NMR and FABMS. Recombinant sialyltransferase and CMP-sialate synthetase were used for the enzymatic synthesis of sialosides on a preparative scale. The synthetic glycopeptides were tested as inhibitors of influenza virus to cells, revealing that most of the multivalent sialoglycopeptides exhibit increased binding that depends on the spacing when compared to monovalent compounds. A possible mechanism for increased binding is proposed.

Enzymatic synthesis of some O-beta-D-digalactosyl glycopeptides, using beta-D-galactosidase

Disaccharide-peptide conjugates were obtained in yields of 30-50% from o-nitrophenyl beta-D-galactopyranoside by employing beta-D-galactosidase from E. coli as catalyst. Two series of beta-D-galactosyldipeptides were examined as galactosyl acceptors. They both contain an L-serine residue beta-linked to the anomeric carbon of galactose. In the first series, serine is in the N-terminal position of the dipeptide; in the second series, serine is in the C-terminal position. The second amino acid is L-alanine or glycine. Some of our substrates gave a high yield of beta-(1-->3)-digalactosyldipeptide derivatives and all gave very little of the beta-(1-->6) regioisomer. The conditions and the limitations of the transgalactosylation reaction are discussed.