Urea-catalyzed N-Glycosylation of Amides/Azacycles with Glycosyl Halides.

The efficient synthesis of N-glycosides via direct N-glycosylation of amides/azacycles has been reported. The glycosylation of amides/azacycles with glycosyl halides in the presence of a catalytic amount of urea proceeded smoothly to provide the corresponding N-glycosylated amides or nucleosides in good to excellent yields with 1,2-trans-stereoselectivity. Moreover, by the addition of terpyridine, the 1,2-cis-stereoselectivity was achieved.

Protecting-group-free S-glycosylation towards thioglycosides and thioglycopeptides in water.

A facile and green S-glycosylation method has been developed featuring protecting-group-free and proceeding-in-water like enzymatic synthesis. Glycosylation of fluoride donors with thiol sugar acceptors using Ca(OH)2 as a promoter afforded various thioglycosides in good yields with exclusive stereoselectivity. This method also enabled the successful production of S-linked oligosaccharides and S-linked glycopeptides.

Ganglioside Extraction, Purification and Profiling.

Gangliosides are glycosphingolipids that contain one or more sialic acid residues. They are found on all vertebrate cells and tissues but are especially abundant in the brain. Expressed primarily on the outer leaflet of the plasma membranes of cells, they modulate the activities of cell surface proteins via lateral association, act as receptors in cell-cell interactions and are targets for pathogens and toxins. Genetic dysregulation of ganglioside biosynthesis in humans results in severe congenital nervous system disorders. Because of their amphipathic nature, extraction, purification, and analysis of gangliosides require techniques that have been optimized by many investigators in the 80 years since their discovery. Here, we describe bench-level methods for the extraction, purification, and preliminary qualitative and quantitative analyses of major gangliosides from tissues and cells that can be completed in a few hours. We also describe methods for larger scale isolation and purification of major ganglioside species from brain. Together, these methods provide analytical and preparative scale access to this class of bioactive molecules.

Clostridioides difficile cd2775 Encodes a Unique Mannosyl-1-Phosphotransferase for Polysaccharide II Biosynthesis.

Clostridioides difficile (C. difficile) is the leading cause of antibiotic-induced bacterial colitis and life-threatening diarrhea worldwide. The commonly existing anionic polysaccharide II (PSII) is responsible for protein anchoring involved in colonization, and the gene cd2775 located in its biosynthesis gene cluster is essential for bacterial growth. Herein, we demonstrated that cd2775 encodes a novel mannosyl-1-phosphotransferase (ManPT) responsible for the phosphorylation of PSII. Unlike typical mannosyltransferases, CD2775 transfers mannose-α1-phosphate instead of mannose from guanosine 5'-diphospho-d-mannose to disaccharide acceptors, forming a unique mannose-α1-phosphate-6-glucose linkage. The enzyme was overexpressed in E. coli and purified for biochemical characterization and substrate specificity study. It is found that CD2775 possesses a strict acceptor specificity toward Glc-ß1,3-GalNAc-diphospho-lipids but extreme promiscuity toward various sugar donors. This is the first report of a ManPT in all living systems. Given its essentiality in C. difficile growth, CD2775 can be a promising target for therapeutics development.

ADP-heptose: A new innate immune modulator.

Lipopolysaccharide (LPS) is a well-known pathogen-associated molecular pattern (PAMP) produced by gram-negative bacteria. Previous studies showed that a key metabolic intermediate in LPS biosynthesis, d-glycero-ß-d-manno-heptose 1,7-bisphosphate (HBP), could activate the NF-κB pathway and trigger the innate immune responses. However, it was unclear whether HBP could be a novel PAMP and its pattern recognition receptor (PRR) is not fully understood. Very recently, the Shao group reported that another key metabolic intermediate in LPS biosynthesis, ADP-heptose, could be transported into mammalian cells and bind with ALPK1 (alpha-kinase 1), which leads to a series of strong immune responses. These findings broaden our understanding on bacterial metabolites as a new type of PAMP and these small molecules hold great potential to be applied in the development of novel immune modulators. This minireview focuses on the roles of ADP-heptose related metabolites in innate immunity.

Synthetic Glycans and Glycomimetics: A Promising Alternative to Natural Polysaccharides.

A large quantity of polysaccharide-derived conjugate vaccines have been developed to combat various pathogenic infections. Another prominent polysaccharide, heparin, is listed as an essential drug by the World Health Organization (WHO) to treat thrombus. One of their common problems is that they all derive from natural polysaccharides. Specifically, capsular polysaccharides are mainly obtained from bacterial fermentation and unfractionated heparin is extracted from animal tissues such as porcine mucosa. The quality of natural polysaccharides is inconsistent and traces of contamination would cause a disaster. By contrast, the use of chemical or chemoenzymatic methods could provide structurally homogeneous and quality-controlled glycans. To date, large numbers of polysaccharide fragments and their analogues have been synthesized and evaluated. Some of them even showed comparable activities to their corresponding natural polysaccharides. Here, the latest advances in these synthetic glycan analogues ranging from carbohydrate-based vaccines, heparin-related therapeutics and glycomimetics of polysaccharides are summarized.

Synthesis and Antigenic Evaluation of Oligosaccharide Mimics of Vi Antigen from Salmonella typhi.

Salmonella typhi is responsible for typhoid fever, which is a serious health threat in developing countries. As a virulent factor of Salmonella typhi, the purified Vi polysaccharide (Vi PS) has become an effective vaccine to combat typhoid fever. The chemical synthesis can provide homogeneous and well-defined molecules for the development of Vi-based vaccines. However, the synthesis of Vi oligosaccharides in high yields and with exclusive α-stereoselectivities remains very challenging. In this paper, a series of Vi pseudooligosaccharides, including pseudo tetra-, hexa-, and octa-saccharides were efficiently synthesized. These oligosaccharide analogues were conjugated by carbon chain tether through olefin cross metathesis or by the 1,2,3-triazole moiety through copper (I)-catalyzed alkyne-azide cycloaddition reaction (CuAAC). The binding affinities of these oligosaccharide mimics to anti-Vi antibodies were investigated. These results will be beneficial to the further development of Vi-based oligosaccharide vaccines.

Design, synthesis and cytotoxic activities of novel hybrid compounds between dihydrobenzofuran and imidazole.

A series of novel hybrid compounds between dihydrobenzofuran and imidazole has been prepared and evaluated in vitro against a panel of human tumor cell lines. The results suggest that substitution of the imidazolyl-1-position with an electron-donating dihydrobenzofuran, and the imidazolyl-3-position with a naphthylacyl or electron-rich phenacyl group, were vital for modulating cytotoxic activity.