Solid-phase synthesis of thioether-linked glycopeptide mimics for application to glycoprotein semisynthesis

Halomethyl-Triazoles for Rapid, Site-Selective Protein Modification

Post-translational modifications (PTMs) are used by organisms to control protein structure and function after protein translation, but their study is complicated and their roles are not often well understood as PTMs are difficult to introduce onto proteins selectively. Designing reagents that are both good mimics of PTMs, but also only modify select amino acid residues in proteins is challenging. Frequently, both a chemical warhead and linker are used, creating a product that is a misrepresentation of the natural modification. We have previously shown that biotin-chloromethyl-triazole is an effective reagent for cysteine modification to give S-Lys derivatives where the triazole is a good mimic of natural lysine acylation. Here, we demonstrate both how the reactivity of the alkylating reagents can be increased and how the range of triazole PTM mimics can be expanded. These new iodomethyl-triazole reagents are able to modify a cysteine residue on a histone protein with excellent selectivity in 30 min to give PTM mimics of acylated lysine side-chains. Studies on the more complicated, folded protein SCP-2L showed promising reactivity, but also suggested the halomethyl-triazoles are potent alkylators of methionine residues.

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.

Characterizing non-hydrolyzing Neisseria meningitidis serogroup A UDP-N-acetylglucosamine (UDP-GlcNAc) 2-epimerase using UDP-N-acetylmannosamine (UDP-ManNAc) and derivatives

Neisseria meningitidis serogroup A non-hydrolyzing uridine 5'-diphosphate-N-acetylglucosamine (UDP-GlcNAc) 2-epimerase (NmSacA) catalyzes the interconversion between UDP-GlcNAc and uridine 5'-diphosphate-N-acetylmannosamine (UDP-ManNAc). It is a key enzyme involved in the biosynthesis of the capsular polysaccharide [-6ManNAcα1-phosphate-]n of N. meningitidis serogroup A, one of the six serogroups (A, B, C, W-135, X, and Y) that account for most cases of N. meningitidis-caused bacterial septicemia and meningitis. N. meningitidis serogroup A is responsible for large epidemics in the developing world, especially in Africa. Here we report that UDP-ManNAc could be used as a substrate for C-terminal His6-tagged recombinant NmSacA (NmSacA-His6) in the absence of UDP-GlcNAc. NmSacA-His6 was activated by UDP-GlcNAc and inhibited by 2-acetamidoglucal and UDP. Substrate specificity study showed that NmSacA-His6 could tolerate several chemoenzymatically synthesized UDP-ManNAc derivatives as substrates although its activity was much lower than non-modified UDP-ManNAc. Homology modeling and molecular docking revealed likely structural determinants of NmSacA substrate specificity. This is the first detailed study of N. meningitidis serogroup A UDP-GlcNAc 2-epimerase.

Preparation of aminoethyl glycosides for glycoconjugation

The synthesis of a number of aminoethyl glycosides of cell-surface carbohydrates, which are important intermediates for glycoarray synthesis, is described. A set of protocols was developed which provide these intermediates, in a short number of steps, from commercially available starting materials.

Synthesis and cytotoxic activity of the N-acetylglucosamine-bearing triterpenoid saponins

Fourteen ursolic acid and oleanolic acid saponins with N-acetyl-beta-D-glucosamine, and (1-->4)-linked and (1-->6)-linked N-acetyl-beta-D-glucosamine oligosaccharide residues were synthesized in a convergent manner. The structures of all compounds were confirmed by (1)H NMR and (13)C NMR spectroscopy and by mass spectrometry, and their cytotoxic activities were assayed in three cancer cell lines. Only oleanolic acid-3-yl beta-D-GluNAc showed significant cytotoxicity against HL-60 and BGC-823.

Carbohydrate-pi interactions: what are they worth?

Protein-carbohydrate interactions play an important role in many biologically important processes. The recognition is mediated by a number of noncovalent interactions, including an interaction between the alpha-face of the carbohydrate and the aromatic side chain of the protein. To elucidate this interaction, it has been studied in the context of a beta-hairpin in aqueous solution, in which the interaction can be investigated in the absence of other cooperative noncovalent interactions. In this beta-hairpin system, both the aromatic side chain and the carbohydrate were varied in an effort to gain greater insight into the driving force and magnitude of the carbohydrate-pi interaction. The magnitude of the interaction was found to vary from -0.5 to -0.8 kcal/mol, depending on the nature of the aromatic ring and the carbohydrate. Replacement of the aromatic ring with an aliphatic group resulted in a decrease in interaction energy to -0.1 kcal/mol, providing evidence for the contribution of CH-pi interactions to the driving force. These findings demonstrate the significance of carbohydrate-pi interactions within biological systems and also their utility as a molecular recognition element in designed systems.

Recent trends in the synthesis of O-glycosides of 2-amino-2-deoxysugars

The discovery of new methods for stereoselective glycoside synthesis and convergent oligosaccharide assembly has been critical for the area of glycosciences. At the heart of this account is the discussion of the approaches for stereoselective synthesis of glycosides of 2-amino-2-deoxysugars that have emerged during the past two decades. The introductory part provides general background information and describes the key features and challenges for the synthesis of this class of compounds. Subsequently, major approaches to the synthesis of 2-amino-2-deoxyglycosides are categorized and discussed. Each subsection elaborates on the introduction (or protection) of the amino functionality, synthesis of glycosyl donors by introduction of a suitable leaving group, and glycosidation. Wherever applicable, the deprotection of a temporary amino group substituent and the conversion onto the natural acetamido functionality is described. The conclusions part evaluates the current standing in the field and provides a perspective for future developments.

A novel neoglycopeptide linkage compatible with native chemical ligation

The straightforward synthesis of a novel class of neoglycopeptide and its fusion with a larger peptide thioester using sequential chemoselective ligations is described.

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.

N-Acyl-N-alkyl-sulfonamide anchors derived from Kenner’s safety-catch linker: powerful tools in bioorganic and medicinal chemistry

In 1971 Kenner et al. introduced the safety-catch principle into solid phase peptide synthesis. Thus two contradicting needs were addressed. On the one hand, sufficient stability of the linker substrate bond to impede hydrolysis or similar side reactions, on the other hand mild chemical conditions allowing for unscathed liberation of the precious products. Over the years this linker type emerged in several different chemical disciplines and nowadays it presents a useful and broadly applicable tool. Recent advancements and applications based on Kenner's safety-catch linker are reviewed.

Tight binding ligand approach to oligosaccharide-grafted protein

A novel type of artificial glycoprotein was developed, by using dihydrofolate reductase (DHFR) and methotrexate (MTX) as a protein-ligand pair. Various oligosaccharides linked to MTX were shown to bind tightly with DHFR and afforded oligosaccharide-grafted protein, which could be isolated easily by lectin beads.

An Efficient Synthetic Route to Glycoamino Acid Building Blocks for Glycopeptide Synthesis

[reaction: see text] Chemical glycopeptide synthesis requires access to gram quantities of glycosylated amino acid building blocks. Hence, the efficiency of synthesis of such building blocks is of great importance. Here, we report a fast and highly efficient synthetic route to Fmoc-protected asparaginyl glycosides from unprotected sugars in three steps with high yields. The glycosylated amino acids were successfully incorporated into target glycopeptides 7 and 8 by standard Fmoc solid-phase peptide synthesis.

Cyanogen Bromide Cleavage Generates Fragments Suitable for Expressed Protein and Glycoprotein Ligation

Herein cyanogen bromide is employed for the efficient production of N-terminal cysteine containing protein fragments for expressed protein ligation (EPL) from polyhistidine-tagged precursors. We provide three examples of efficient CNBr cleavage of fragments of the glycoprotein erythropoietin that can be ligated with peptides or glycopeptide mimetics potentially giving rise to semisynthetic glycoprotein therapeutics.

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.