Studies Directed to the Synthesis of Oligochitosans – Preparation of Building Blocks and Their Evaluation in Glycosylation Studies

Controllable Enzymatic Synthesis of Natural Asymmetric Human Milk Oligosaccharides

Among human milk oligosaccharides (HMOs), linear HMOs are synthesized through mature but varied routes. Although branched HMOs can be synthesized by chemical, enzymatic, or chemoenzymatic methods, these methods cannot be easily applied to the synthesis of asymmetric multiantennary oligosaccharides. Herein, we developed a controllable method to synthesize asymmetric biantennary HMOs. In our synthetic route, GlcNAcβ1,3(GlcN3β1,6)Glaβ1,4Glc was first chemically synthesized as the core tetrasaccharide, which contains β1,6GlcN3 as the "stop" sugar in transferase-catalyzed glycosylation. The desired sugars at the GlcNAcβ1-3Gal arm can be assembled using galactosyltransferase, N-acetylglucosaminyltransferase, and fucosyltransferase. Then, the Staudinger reduction and acetylation were used to transform GlcN3 to GlcNAc and assemble sugars by initiating the "go" process. By manipulating transferase-catalyzed glycosylations, 22 natural asymmetric biantennary oligosaccharides were synthesized.

Chemical Synthesis of Acetobacter pasteurianus Lipid A with a Unique Tetrasaccharide Backbone and Evaluation of Its Immunological Functions

Lipopolysaccharide (LPS), a cell surface component of Gram-negative bacteria, activates innate immunity. Its active principle is the terminal glycolipid lipid A. Acetobacter pasteurianus is a Gram-negative bacterium used in the fermentation of traditional Japanese black rice vinegar (kurozu). In this study, we focused on A. pasteurianus lipid A, which is a potential immunostimulatory component of kurozu. The active principle structure of A. pasteurianus lipid A has not yet been identified. Herein, we first systematically synthesized three types of A. pasteurianus lipid As containing a common and unique tetrasaccharide backbone. We developed an efficient method for constructing the 2-trehalosamine skeleton utilizing borinic acid-catalyzed glycosylation to afford 1,1'-α,α-glycoside in high yield and stereoselectivity. A common tetrasaccharide intermediate with an orthogonal protecting group pattern was constructed via [2+2] glycosylation. After introducing various fatty acids, all protecting groups were removed to achieve the first chemical synthesis of three distinct types of A. pasteurianus lipid As. After evaluating their immunological function using both human and murine cell lines, we identified the active principles of A. pasteurianus LPS. We also found the unique anomeric structure of A. pasteurianus lipid A contributes to its high chemical stability.

Optimizations of lipid II synthesis: an essential glycolipid precursor in bacterial cell wall synthesis and a validated antibiotic target

Lipid II is an essential glycolipid found in bacteria. Accessing this valuable cell wall precursor is important both for studying cell wall synthesis and for studying/identifying novel antimicrobial compounds. Herein, we describe optimizations to the modular chemical synthesis of lipid II and unnatural analogues. In particular, the glycosylation step, a critical step in the formation of the central disaccharide unit (GlcNAc-MurNAc), was optimized. This was achieved by employing the use of glycosyl donors with diverse leaving groups. The key advantage of this approach lies in its adaptability, allowing for the generation of a wide array of analogues through the incorporation of alternative building blocks at different stages of synthesis.

Stereoselective Synthesis of Sialyl Lewisa Antigen and the Effective Anticancer Activity of Its Bacteriophage Qβ Conjugate as an Anticancer Vaccine

Sialyl Lewisa (sLea ), also known as cancer antigen 19-9 (CA19-9), is a tumor-associated carbohydrate antigen. The overexpression of sLea on the surface of a variety of cancer cells makes it an attractive target for anticancer immunotherapy. However, sLea -based anticancer vaccines have been under-explored. To develop a new vaccine, efficient stereoselective synthesis of sLea with an amine-bearing linker was achieved, which was subsequently conjugated with a powerful carrier bacteriophage, Qβ. Mouse immunization with the Qβ-sLea conjugate generated strong and long-lasting anti-sLea IgG antibody responses, which were superior to those induced by the corresponding conjugate of sLea with the benchmark carrier keyhole limpet hemocyanin. Antibodies elicited by Qβ-sLea were highly selective toward the sLea structure, could bind strongly with sLea -expressing cancer cells and human pancreatic cancer tissues, and kill tumor cells through complement-mediated cytotoxicity. Furthermore, vaccination with Qβ-sLea significantly reduced tumor development in a metastatic cancer model in mice, demonstrating tumor protection for the first time by a sLea -based vaccine, thus highlighting the significant potential of sLea as a promising cancer antigen.

Efficient and versatile formation of glycosidic bonds via catalytic strain-release glycosylation with glycosyl ortho-2,2-dimethoxycarbonylcyclopropylbenzoate donors

Catalytic glycosylation is a vital transformation in synthetic carbohydrate chemistry due to its ability to expediate the large-scale oligosaccharide synthesis for glycobiology studies with the consumption of minimal amounts of promoters. Herein we introduce a facile and efficient catalytic glycosylation employing glycosyl ortho-2,2-dimethoxycarbonylcyclopropylbenzoates (CCBz) promoted by a readily accessible and non-toxic Sc(III) catalyst system. The glycosylation reaction involves a novel activation mode of glycosyl esters driven by the ring-strain release of an intramolecularly incorporated donor-acceptor cyclopropane (DAC). The versatile glycosyl CCBz donor enables highly efficient construction of O-, S-, and N-glycosidic bonds under mild conditions, as exemplified by the convenient preparation of the synthetically challenging chitooligosaccharide derivatives. Of note, a gram-scale synthesis of tetrasaccharide corresponding to Lipid IV with modifiable handles is achieved using the catalytic strain-release glycosylation. These attractive features promise this donor to be the prototype for developing next generation of catalytic glycosylation.

Differential Recognition of Deacetylated PNAG Oligosaccharides by a Biofilm Degrading Glycosidase

Exopolysaccharides consisting of partially de-N-acetylated poly-β-d-(1→6)-N-acetyl-glucosamine (dPNAG) are key structural components of the biofilm extracellular polymeric substance of both Gram-positive and Gram-negative human pathogens. De-N-acetylation is required for the proper assembly and function of dPNAG in biofilm development suggesting that different patterns of deacetylation may be preferentially recognized by proteins that interact with dPNAG, such as Dispersin B (DspB). The enzymatic degradation of dPNAG by the Aggregatibacter actinomycetemcomitans native β-hexosaminidase enzyme DspB plays a role in biofilm dispersal. To test the role of substrate de-N-acetylation on substrate recognition by DspB, we applied an efficient preactivation-based one-pot glycosylation approach to prepare a panel of dPNAG trisaccharide analogs with defined acetylation patterns. These analogs served as effective DspB substrates, and the rate of hydrolysis was dependent on the specific substrate de-N-acetylation pattern, with glucosamine (GlcN) located +2 from the site of cleavage being preferentially hydrolyzed. The product distributions support a primarily exoglycosidic cleavage activity following a substrate assisted cleavage mechanism, with the exception of substrates containing a nonreducing GlcN that were cleaved endo leading to the exclusive formation of a nonreducing disaccharide product. These observations provide critical insight into the substrate specificity of dPNAG specific glycosidase that can help guide their design as biocatalysts.

Design, synthesis and cytotoxic activity of N-Modified oleanolic saponins bearing A glucosamine

A series of N-acyl, N-alkoxycarbonyl, and N-alkylcarbamoyl derivatives of 2'-deoxy-glucosyl bearing oleanolic saponins were synthesized and evaluated against HL-60, PC-3, and HT29 tumor cancer cells. The SAR studies revealed that the activity increased in order of conjugation of 2' -amino group with carbamate > amide > urea derivatives. Lengthening the alkyl chain increased the cytotoxicity, the peak activity was found to around heptyl to nonyl substitutions. 2'-N-heptoxycarbonyl derivative 56 was found to be the most cytotoxic (IC₅₀ = 0.76 μM) against HL-60 cells. Due to the interesting SARs of alkyl substitutions, we hypothesized that their location in the cell was different, and pursued a location study using 2'-(4″-pentynoylamino) 2'-deoxy-glucosyl OA, which suggested that these compounds distributed mainly in the cytosol.

Synthesis of a poly-hydroxypyrolidine-based inhibitor of Mycobacterium tuberculosis GlgE

Long treatment times, poor drug compliance, and natural selection during treatment of Mycobacterium tuberculosis (Mtb) have given rise to extensively drug-resistant tuberculosis (XDR-TB). As a result, there is a need to identify new antituberculosis drug targets. Mtb GlgE is a maltosyl transferase involved in α-glucan biosynthesis. Mutation of GlgE in Mtb increases the concentration of maltose-1-phosphate (M1P), one substrate for GlgE, causing rapid cell death. We have designed 2,5-dideoxy-3-O-α-d-glucopyranosyl-2,5-imino-d-mannitol (9) to act as an inhibitor of GlgE. Compound 9 was synthesized using a convergent synthesis by coupling thioglycosyl donor 14 and 5-azido-3-O-benzyl-5-deoxy-1,2-O-isopropylidene-β-d-fructopyranose (23) to form disaccharide 24. A reduction and intramolecular reductive amination transformed the intermediate disaccharide 24 to the desired pyrolidine 9. Compound 9 inhibited both Mtb GlgE and a variant of Streptomyces coelicolor (Sco) GlgEI with K_i = 237 ± 27 μM and K_i = 102 ± 7.52 μM, respectively. The results confirm that a Sco GlgE-V279S variant can be used as a model for Mtb GlgE. In conclusion, we designed a lead transition state inhibitor of GlgE, which will be instrumental in further elucidation of the enzymatic mechanism of Mtb GlgE.

Biotechnological approaches for field applications of chitooligosaccharides (COS) to induce innate immunity in plants

Plants have evolved mechanisms to recognize a wide range of pathogen-derived molecules and to express induced resistance against pathogen attack. Exploitation of induced resistance, by application of novel bioactive elicitors, is an attractive alternative for crop protection. Chitooligosaccharide (COS) elicitors, released during plant fungal interactions, induce plant defenses upon recognition. Detailed analyses of structure/function relationships of bioactive chitosans as well as recent progress towards understanding the mechanism of COS sensing in plants through the identification and characterization of their cognate receptors have generated fresh impetus for approaches that would induce innate immunity in plants. These progresses combined with the application of chitin/chitosan/COS in disease management are reviewed here. In considering the field application of COS, however, efficient and large-scale production of desired COS is a challenging task. The available methods, including chemical or enzymatic hydrolysis and chemical or biotechnological synthesis to produce COS, are also reviewed.

Synthesis of β-Glycosyl Amides from N-Glycosyl Dinitrobenzenesulfonamides

The N-glycosyl-2,4-dinitrobenzenesulfonamides were accessed via benzoyl-protected β-glycosyl azides. The azides were reduced with Adams' catalyst to the corresponding amines. The glycosylamines were sulfonated with 2,4-dinitrobenzenesulfonyl chloride to form N-glycosyl-2,4-dinitrobenzenesulfonamides in moderate yields. β-Glycosyl amides were then prepared in 67 - 81 % yields by treatment of the sulfonamides with thioacetic acid and cesium carbonate. The conversion of the glycosylsulfonamide to the glycosyl amide proceeded with high stereoselectivity.