Solution and solid-support synthesis of a potential leishmaniasis carbohydrate vaccine

A Ferrier glycosylation/cis-dihydroxylation strategy to synthesize Leishmania spp. lipophosphoglycan-associated βGal(1,4)Man disaccharide

The Galβ(1→4)Man disaccharide, found in the cell surface lipophosphoglycan (LPG) of Leishmania species, has been synthesized by a Ferrier glycosylation/cis-dihydroxylation strategy. This stereoselective method proved efficient for synthesizing the target saccharide in good yield. In addition, we prepared two clickable O-glycoside and phospho-glycoside versions of Galβ(1→4)Man to enable conjugation to protein carriers for further immunological and antibody-binding studies.

Temporary ether protecting groups at the anomeric center in complex carbohydrate synthesis

The synthesis of a carbohydrate building block usually starts with introduction of a temporary protecting group at the anomeric center and ends with its selective cleavage for further transformation. Thus, the choice of the anomeric temporary protecting group must be carefully considered because it should retain intact during the whole synthetic manipulation, and it should be chemoselectively removable without affecting other functional groups at a late stage in the synthesis. Etherate groups are the most widely used temporary protecting groups at the anomeric center, generally including allyl ethers, MP (p-methoxyphenyl) ethers, benzyl ethers, PMB (p-methoxybenzyl) eithers, and silyl ethers. This chapter provides a comprehensive review on their formation, cleavage, and applications in the synthesis of complex carbohydrates.

Synthetic Glycobiology: Parts, Systems, and Applications

Protein glycosylation, the attachment of sugars to amino acid side chains, can endow proteins with a wide variety of properties of great interest to the engineering biology community. However, natural glycosylation systems are limited in the diversity of glycoproteins they can synthesize, the scale at which they can be harnessed for biotechnology, and the homogeneity of glycoprotein structures they can produce. Here we provide an overview of the emerging field of synthetic glycobiology, the application of synthetic biology tools and design principles to better understand and engineer glycosylation. Specifically, we focus on how the biosynthetic and analytical tools of synthetic biology have been used to redesign glycosylation systems to obtain defined glycosylation structures on proteins for diverse applications in medicine, materials, and diagnostics. We review the key biological parts available to synthetic biologists interested in engineering glycoproteins to solve compelling problems in glycoscience, describe recent efforts to construct synthetic glycoprotein synthesis systems, and outline exemplary applications as well as new opportunities in this emerging space.

Leishmania lipophosphoglycan components: A potent target for synthetic neoglycoproteins as a vaccine candidate for leishmaniasis

Leishmania is an obligate intracellular pathogen that invades phagocytic host cells. Due to its high morbidity and mortality rates, leishmaniasis attracts significant attention. The disease, which is caused by Leishmania parasites, is distributed worldwide, particularly among developing communities, and causes fatal complications if not treated expediently. Unfortunately, the existing treatments are not preventive and do not impede Leishmania infection. Many drugs available for leishmaniasis are becoming less effective due to emerging resistance in some Leishmania species. Other drugs have drawbacks such as low cost-effectiveness, toxicity, and side effects. The World Health Organization (WHO) considers leishmaniasis to be a major public health problem and suggests that the best prevention is to develop a vaccine for this dangerous disease. In this review, we focus on the unique components of lipophosphoglycan (LPG), a component of the Leishmania cell wall, particularly [Galp(1 → 4)-β-[Manp-(1 → 2)-α-Manp-(1 → 2)-α]-Manp] in the cryptic tetrasaccharide cap, and on synthetic approaches as a potent candidate for a leishmaniasis vaccine.

Design and synthesis of multivalent α-1,2-trimannose-linked bioerodible microparticles for applications in immune response studies of Leishmania major infection

Leishmaniasis, a neglected tropical disease, currently infects approximately 12 million people worldwide with 1 to 2 million new cases each year in predominately underdeveloped countries. The treatment of the disease is severely underdeveloped due to the ability of the Leishmania pathogen to evade and abate immune responses. In an effort to develop anti-leishmaniasis vaccines and adjuvants, novel carbohydrate-based probes were made to study the mechanisms of immune modulation. In this study, a new bioerodible polyanhydride microparticle was designed and conjugated with a glycodendrimer molecular probe. This molecular probe incorporates a pathogen-like multivalent display of α-1,2-trimannose, for which a more efficient synthesis was designed, with a tethered fluorophore. Further attachment of the glycodendrimer to a biocompatible, surface eroding microparticle allows for targeted uptake and internalization of the pathogen-associated oligosaccharide by phagocytic immune cells. The α-1,2-trimannose-linked bioerodible microparticles were found to be safe after administration into the footpad of mice and demonstrated a similar response to α-1,2-trimannose-coated latex beads during L. major footpad infection. Furthermore, the bioerodible microparticles allowed for investigation of the role of pathogen-associated oligosaccharides for recognition by pathogen-recognition receptors during L. major-induced leishmaniasis.

Recent Advances in Toll Like Receptor-Targeting Glycoconjugate Vaccines

Many malignant cell surface carbohydrates resulting from abnormal glycosylation patterns of certain diseases can serve as antigens for the development of vaccines against these diseases. However, carbohydrate antigens are usually poorly immunogenic by themselves, thus they need to be covalently coupled with immunologically active carrier molecules to be functional. The most well established and commonly used carriers are proteins. In recent years, the use of toll-like receptor (TLR) ligands to formulate glycoconjugate vaccines has gained significant attention because TLR ligands can serve not only as carrier molecules but also as built-in adjuvants to form fully synthetic and self-adjuvanting conjugate vaccines, which have several advantages over carbohydrate-protein conjugates and formulated mixtures with external adjuvants. This article reviews recent progresses in the development of conjugate vaccines based on TLR ligands. Two major classes of TLR ligands, lipopeptides and lipid A derivatives will be covered with more focus on monophosohoryl lipid A (MPLA) and related analogs, which are TLR4 ligands demonstrated to be able to provoke T cell-dependent, adaptive immune responses. Corresponding conjugate vaccines have shown promising application potentials to multiple diseases including cancer.

Gold-catalyzed glycosylation in the synthesis of complex carbohydrate-containing natural products

Gold(i)- and gold(iii)-catalyzed glycosylation reactions and their application in the synthesis of natural glycoconjugates are reviewed.

Acyclic Sesquiterpenes from the Fruit Pericarp of Sapindus saponaria Induce Ultrastructural Alterations and Cell Death in Leishmania amazonensis

Previous studies reported antiprotozoal activities of Sapindus saponaria L. The aim of this work was the evaluation of antileishmanial activity and mechanism of action of extract and fractions of S. saponaria L. Hydroethanolic extract (EHA) obtained from fruit pericarps was fractionated using solid-phase extraction in a reversed phase, resulting in fractions enriched with saponins (SAP fraction) and acyclic sesquiterpene oligoglycosides (OGSA fraction). The activities of EHA, SAP, and OGSA were evaluated by antiproliferative assays with promastigote and intracellular amastigote forms. Cytotoxicity on macrophages and hemolytic activity were also analyzed. Morphological and ultrastructural changes in Leishmania amazonensis promastigotes were evaluated by electron microscopy. Flow cytometry was used to investigate mitochondrial dysfunction and phosphatidylserine exposure. OGSA was more selective for parasites than mammalian J774A1 macrophage cells, with selectivity indices of 3.79 and 7.35, respectively. Our results showed that only the OGSA fraction did not present hemolytic activity at its IC₅₀ for promastigote growth. Electron microscopy revealed changes in parasite flagellum, cell body shape, and organelle size, mainly mitochondria. Flow cytometry analysis indicated mitochondrial membrane and cell membrane dysfunction. OGSA showed antileishmanial activity, resulting in several changes to protozoa cells, including mitochondrial depolarization and early phosphatidylserine exposure, suggesting a possible apoptotic induction.

Synthetic Glycan Microarrays

Structurally diverse glycans are expressed by all animate beings and exert diverse biological functions through specific interactions with glycan binding proteins (GBPs). In humans, glycan-GBP interactions are implicated in many disease-relevant processes in development, infection and immune response to bacterial and viral pathogens. Recent progress in chemical synthesis, including automated glycan assembly, has facilitated access to complex glycans that cannot be isolated from biological material. Glycan immobilization on microarrays allows rapid, multiplexed glycan-GBP interaction studies to reveal biological functions. Synthetic glycan microarrays have enabled, for instance, the identification of glycan ligands for lectins, the definition of vaccine antigens, revealed viral glycan receptors and can serve as diagnostic tools for human disease. Here, we describe the methods to fabricate custom glycan microarrays that are used to examine glycan-GBP binding specificities. Conjugation-ready synthetic glycans are covalently attached to microarray surfaces through nucleophilic linker moieties. Microarrays are incubated with GBPs, and binding events are quantitatively detected by fluorescent signals. These methods are readily adaptable to a multitude of purposes from basic research to biomedical applications.

Multivalent display of minimal Clostridium difficile glycan epitopes mimics antigenic properties of larger glycans

Synthetic cell-surface glycans are promising vaccine candidates against Clostridium difficile. The complexity of large, highly antigenic and immunogenic glycans is a synthetic challenge. Less complex antigens providing similar immune responses are desirable for vaccine development. Based on molecular-level glycan–antibody interaction analyses, we here demonstrate that the C. difficile surface polysaccharide-I (PS-I) can be resembled by multivalent display of minimal disaccharide epitopes on a synthetic scaffold that does not participate in binding. We show that antibody avidity as a measure of antigenicity increases by about five orders of magnitude when disaccharides are compared with constructs containing five disaccharides. The synthetic, pentavalent vaccine candidate containing a peptide T-cell epitope elicits weak but highly specific antibody responses to larger PS-I glycans in mice. This study highlights the potential of multivalently displaying small oligosaccharides to achieve antigenicity characteristic of larger glycans. The approach may result in more cost-efficient carbohydrate vaccines with reduced synthetic effort.

Immunologically-active glycans are promising vaccine candidates but can be difficult to synthesize. Here, the authors show that pentavalent display of a minimal disaccharde epitope on a chemical scaffold can mimic a native C. difficile glycan antigen, representing a simple approach to synthetic vaccine production.

Expeditious synthesis of the tetrasaccharide cap domain of the Leishmania donovani lipophosphoglycan using one-pot glycosylation reactions

Expeditious syntheses of the tetrasaccharide cap related to the lipophosphoglycan ofLeishmania donovaniwere achieved by sequential one-pot glycosylation reactions.

Leishmania lipophosphoglycan: how to establish structure-activity relationships for this highly complex and multifunctional glycoconjugate?

A key feature of many pathogenic microorganisms is the presence of a dense glycocalyx at their surface, composed of lipid-anchored glycoproteins and non-protein-bound polysaccharides. These surface glycolipids are important virulence factors for bacterial, fungal and protozoan pathogens. The highly complex glycoconjugate lipophosphoglycan (LPG) is one of the dominant surface macromolecules of the promastigote stage of all Leishmania parasitic species. LPG plays critical pleiotropic roles in parasite survival and infectivity in both the sandfly vector and the mammalian host. Here, we review the composition of the Leishmania glycocalyx, the chemical structure of LPG and what is currently known about its effects in the mammalian host, specifically. We will then discuss the current approaches employed to elucidate LPG functions. Finally, we will provide a viewpoint on future directions that this area of investigation could take to unravel in detail the biological activity of the specific molecular elements composing the structurally complex LPG.

Carbohydrate-Based Vaccines: An Overview

Vaccination is one of the key developments in the fight against infectious diseases. It is based on the principle that immunization with pathogen-derived antigens provides protection from the respective infection by inducing an antigen-specific immune response. The discovery by Avery and Heidelberger in the 1920s that capsular polysaccharides (CPS) from Streptococcus pneumoniae are immunoreactive was the starting point of the development of carbohydrate-based vaccines. CPS-specific neutralizing antibodies were found to mediate protection against S. pneumoniae infection. Since the majority of bacterial pathogens carry a dense array of polysaccharides on their surface, the carbohydrate-based vaccine approach was applied to a variety of bacterial strains. The first CPS-based vaccines against S. pneumoniae were licensed in the 1940s. The increasing emergence of antibiotic-resistant bacterial strains since the 1960s boosted the development of carbohydrate-based vaccines and led to the approval of CPS-based vaccines against Neisseria meningitidis, Haemophilus influenzae type b (Hib), and Salmonella typhi. Meanwhile, it was observed that CPS generally do not elicit protective antibody responses in children below the age of 2 years who are at the greatest risk of infection. As a consequence, studies refocused on the conjugation of oligosaccharides to proteins in order to increase vaccine immunogenicity which led to the introduction of the first glycoconjugate vaccine against Hib in 1987. Due to the success of the first glycoconjugate vaccines, higher valent formulations were developed against numerous bacterial infections to achieve broad serotype coverage. Current research also focuses on the development of carbohydrate-based vaccines against other pathogens such as viruses, fungi, protozoan parasites, or helminths.

Epitope recognition of antibodies against a Yersinia pestis lipopolysaccharide trisaccharide component

Today, the process of selecting carbohydrate antigens as a basis for active vaccination and the generation of antibodies for therapeutic and diagnostic purposes is based on intuition combined with trial and error experiments. In efforts to establish a rational process for glycan epitope selection, we employed glycan array screening, surface plasmon resonance, and saturation transfer difference (STD)-NMR to elucidate the interactions between antibodies and glycans representing the Yersinia pestis lipopolysaccharide (LPS). A trisaccharide epitope of the LPS inner core glycan and different LPS-derived oligosaccharides from various Gram-negative bacteria were analyzed using this combination of techniques. The antibody-glycan interaction with a heptose substructure was determined at atomic-level detail. Antibodies specifically recognize the Y. pestis trisaccharide and some substructures with high affinity and specificity. No significant binding to LPS glycans from other bacteria was observed, which suggests that the epitopes for just one particular bacterial species can be identified. On the basis of these results we are beginning to understand the rules for structure-based design and selection of carbohydrate antigens.

Immunological evaluation of a synthetic Clostridium difficile oligosaccharide conjugate vaccine candidate and identification of a minimal epitope

Clostridium difficile is the cause of emerging nosocomial infections that result in abundant morbidity and mortality worldwide. Thus, the development of a vaccine to kill the bacteria to prevent this disease is highly desirable. Several recently identified bacterial surface glycans, such as PS-I and PS-II, are promising vaccine candidates to preclude C. difficile infection. To circumvent difficulties with the generation of natural PS-I due to its low expression levels in bacterial cultures, improved chemical synthesis protocols for the pentasaccharide repeating unit of PS-I and oligosaccharide substructures were utilized to produce large quantities of well-defined PS-I related glycans. The analysis of stool and serum samples obtained from C. difficile patients using glycan microarrays of synthetic oligosaccharide epitopes revealed humoral immune responses to the PS-I related glycan epitopes. Two different vaccine candidates were evaluated in the mouse model. A synthetic PS-I repeating unit CRM197 conjugate was immunogenic in mice and induced immunoglobulin class switching as well as affinity maturation. Microarray screening employing PS-I repeating unit substructures revealed the disaccharide Rha-(1→3)-Glc as a minimal epitope. A CRM197-Rha-(1→3)-Glc disaccharide conjugate was able to elicit antibodies recognizing the C. difficile PS-I pentasaccharide. We herein demonstrate that glycan microarrays exposing defined oligosaccharide epitopes help to determine the minimal immunogenic epitopes of complex oligosaccharide antigens. The synthetic PS-I pentasaccharide repeating unit as well as the Rha-(1→3)-Glc disaccharide are promising novel vaccine candidates against C. difficile that are currently in preclinical evaluation.

Enhancement in therapeutic efficacy of miltefosine in combination with synthetic bacterial lipopeptide, Pam3Cys against experimental Visceral Leishmaniasis

Existing drugs for visceral leishmaniasis (VL) are partially effective, toxic, having high cost and long term treatment. Their efficacies are also compromised due to suppression of immune function associated during the course of infection. Combination therapy including a potential and safe immunostimulant with lower doses of effective drug has proven as a significant approach which is more effective than immunotherapy or drug therapy alone. In the present study, we have used the combination of Pam3Cys (an in-built immunoadjuvant and TLR2 ligand) and miltefosine. Initially dose optimization of both the agents was carried out and after that, antileishmanial effect of their combination was evaluated. All experiments were done in BALB/c mouse model. The immunomodulatory role of Pam3Cys on the immune functions of the host receiving combination treatment was also determined using immunological and biochemical parameters viz. phagocytosis, Th1/Th2 cytokines and production of ROS, RNS and H(2)O(2). Combination group showed significant enhancement in parasitic inhibition as compared to groups receiving miltefosine and Pam3Cys separately. Enhanced production of Th1 cytokines as well as ROS, RNS and H(2)O(2) was witnessed during the study of immunological alterations. Remarkable increase in phagocytosis index was also observed. Thus, the risk of development of drug resistance against miltefosine can be resolved through using low doses of it and Pam3Cys (single-dose) in combination and also provide a promising alternative for cure of leishmaniasis, with a pronounced transformation of the host immune response.

Synthetic neoglycoconjugates of cell-surface phosphoglycans of Leishmania as potential anti-parasite carbohydrate vaccines

Leishmania are a genus of sandfly-transmitted protozoan parasites that cause a spectrum of debilitating and often fatal diseases in humans throughout the tropics and subtropics. During the parasite life cycle, Leishmania survive and proliferate in highly hostile environments. Their survival strategies involve the formation of an elaborate and dense cell-surface glycocalyx composed of diverse stage-specific glycoconjugates that form a protective barrier. Phosphoglycans constitute the variable structural and functional domain of major cell-surface lipophosphoglycan and secreted proteophosphoglycans. In this paper, we discuss structural aspects of various phosphoglycans from Leishmania with the major emphasis on the chemical preparation of neoglycoconjugates (neoglycoproteins and neoglycolipids) based on Leishmania lipophosphoglycan structures as well as the immunological evaluation for some of them as potential anti-leishmaniasis vaccines.

A fluorous phosphate protecting group with applications to carbohydrate synthesis

The first fluorous protecting group for phosphate is reported. This group can be used as a facile tag for purification and be removed under mild reducing conditions using zinc and ammonium formate. Synthesis of a disaccharide from Leishmania using this fluorous protecting group demonstrated the group's stability to the acidic conditions necessary for glycosylation as well as its orthogonality to several other common protecting groups.

Synthesis of multivalent tuberculosis and Leishmania-associated capping carbohydrates reveals structure-dependent responses allowing immune evasion

Mycobacterium tuberculosis and the protozoan parasites of the genus Leishmania are intracellular pathogens that can survive in macrophages--the very white blood cells of the immune system responsible for engulfing and ultimately clearing foreign invaders. The ability of these pathogens to hide within immune cells has made the design of effective therapies, including vaccines, to control tuberculosis and leishmaniasis particularly challenging. Herein we present the synthesis and development of carbohydrate-based probes to demonstrate that changes in pathogen-associated surface oligosaccharides are sufficient to alter cellular immune responses and thereby let a pathogen hide from immune surveillance.

Carbohydrate vaccines: developing sweet solutions to sticky situations?

Recent technological advances in glycobiology and glycochemistry are paving the way for a new era in carbohydrate vaccine design. This is enabling greater efficiency in the identification, synthesis and evaluation of unique glycan epitopes found on a plethora of pathogens and malignant cells. Here, we review the progress being made in addressing challenges posed by targeting the surface carbohydrates of bacteria, protozoa, helminths, viruses, fungi and cancer cells for vaccine purposes.

Application of carbohydrate array technology to antigen discovery and vaccine development

Carbohydrate arrays are a new technology developed for high-throughput evaluation of interactions between carbohydrates and proteins, cells or viruses. Carbohydrate arrays contain many different carbohydrate structures on a solid support. The format allows one to probe hundreds or thousands of potential receptor-ligand interactions while using only tiny amounts of material. Recently, carbohydrate arrays have been applied to vaccine development in several ways. First, carbohydrate arrays have been utilized for the discovery and characterization of carbohydrate antigens. Second, they have been used to evaluate immune responses to vaccine candidates. Third, carbohydrate arrays have been used to identify and characterize reagents necessary for vaccine development. Although still at an early stage, carbohydrate array technology has tremendous potential for accelerating vaccine development.

Enhancement of the immunogenicity of synthetic carbohydrates by conjugation to virosomes: a leishmaniasis vaccine candidate

Novel virosomal formulations of a synthetic oligosaccharide were prepared and evaluated as vaccine candidates against leishmaniasis. A lipophosphoglycan-related synthetic tetrasaccharide antigen was conjugated to a phospholipid and to the influenza virus coat protein hemagglutinin. These glycan conjugates were embedded into the lipid membrane of reconstituted influenza virus virosomes. The virosomal formulations elicited both IgM and IgG anti-glycan antibodies in mice, indicating an antibody isotype class switch to IgG. The antisera cross-reacted in vitro with the corresponding natural carbohydrate antigens expressed by leishmania cells. These findings support the concept of using virosomes as universal antigen delivery platform for synthetic carbohydrate vaccines.

Carbohydrate based vaccines

In the past decades, a gradual increase in the resistance to antibiotics has been observed, leading to a serious thread for successful treatment of bacterial infections. This feature in addition to difficulties in developing adequate drugs against (tropical) diseases caused by parasites has stimulated the interest in vaccines to prevent infections. In principle, various types of cell surface epitopes, characteristic for the invading organism or related to aberrant growth of cells, can be applied to develop vaccines. The progress in establishing the structure of carbohydrate immuno-determinants in conjunction with improvements in carbohydrate synthesis has rendered it feasible to develop new generations of carbohydrate-based vaccines.

Syntheses of oligomannosides in solution and on a soluble polymer support: a comparison

The alpha-(1-->6)-linked and the alpha-(1-->2)-linked linear mannotetraose glycosides and, respectively, and the branched mannopentaoside [R=CH2(CH2)2CH2Cl] were synthesised by conventional methods in solution, using trichloroacetimidate donors, and the products were obtained in 39%, 42% and 40% overall yield, respectively. For comparative purposes, the same two linear tetrasaccharides were prepared by use of MPEG as a soluble polymer support, the yields being 34% and 14%, respectively. An attempted MPEG-supported synthesis of the branched pentasaccharide was unsuccessful. The merits and shortcomings of oligosaccharide syntheses on MPEG are discussed.

Automated synthesis of oligosaccharides as a basis for drug discovery

Carbohydrates present both potential and problems - their biological relevance has been recognized, but problems in procuring sugars rendered them a difficult class of compounds to handle in drug discovery efforts. The development of the first automated solid-phase oligosaccharide synthesizer and other methods to assemble defined oligosaccharides rapidly has fundamentally altered this situation. This review describes how quick access to oligosaccharides has not only contributed to biological, biochemical and biophysical investigations, but also to drug discovery. Particular focus will be placed on the development of carbohydrate-based vaccines, defined heparin oligosaccharides and aminoglycosides that have recently begun to affect drug discovery.

Antigenic glycans in parasitic infections: implications for vaccines and diagnostics

Infections by parasitic protozoans and helminths are a major world-wide health concern, but no vaccines exist to the major human parasitic diseases, such as malaria, African trypanosomiasis, amebiasis, leishmaniasis, schistosomiasis, and lymphatic filariasis. Recent studies on a number of parasites indicate that immune responses to parasites in infected animals and humans are directed to glycan determinants within cell surface and secreted glycoconjugates and that glycoconjugates are important in host-parasite interactions. Because of the tremendous success achieved recently in generating carbohydrate-protein conjugate vaccines toward microbial infections, such as Haemophilus influenzae type b, there is renewed interest in defining parasite-derived glycans in the prospect of developing conjugate vaccines and new diagnostics for parasitic infections. Parasite-derived glycans are compelling vaccine targets because they have structural features that distinguish them from mammalian glycans. There have been exciting new developments in techniques for glycan analysis and the methods for synthesizing oligosaccharides by chemical or combined chemo-enzymatic approaches that now make it feasible to generate parasite glycans to test as vaccine candidates. Here, we highlight recent progress made in elucidating the immunogenicity of glycans from some of the major human and animal parasites, the potential for developing conjugate vaccines for parasitic infections, and the possible utilization of these novel glycans in diagnostics.

Iterative synthesis of Leishmania phosphoglycans by solution, solid-phase, and polycondensation approaches without involving any glycosylation

A general strategy (solution, solid-phase, and polycondensation) for the synthesis of antigenic phosphoglycans (PG) of the protozoan parasite Leishmania is presented. Phosphoglycans constitute the variable structural and functional domain of major cell-surface lipophosphoglycan (LPG) and secreted proteophosphoglycan (PPG), the molecules involved in infectivity and survival of the Leishmania parasite inside human macrophages. We have shown that the chemically labile, anomerically phosphodiester-linked phosphoglycan repeats can be assembled in an iterative and efficient manner from a single key intermediate, without involving any glycosylation steps. Furthermore, the phosphoglycan chain can be extended toward either the nonreducing (6'-OH) or the reducing (1-OH) end. We also describe a new and efficient solid-phase methodology to construct phosphoglycans based on design and application of a novel cis-allylphosphoryl solid-phase linker that enabled the selective cleavage of the first anomeric-phosphodiester linkage without affecting any of the other internal anomeric-phosphodiester groups of the growing PG chain on the solid support. The strategy to construct larger phosphoglycans in a one-pot synthesis by polycondensation of a single key intermediate is also described, enabling CD spectrometric measurements to show the helical nature of phosphoglycans. Our versatile synthetic approach provides easy access to Leishmania phosphoglycans and the opportunity to address key immunological, biochemical, and biophysical questions pertaining to the phosphoglycan family (LPG and PPG) unique to the parasite.

Site-specific discrimination by cyanovirin-N for alpha-linked trisaccharides comprising the three arms of Man(8) and Man(9)

Cyanovirin-N (CVN) is a novel cyanobacterial protein that selectively binds with nanomolar affinities the mammalian oligosaccharides Man(8) and Man(9). Consequently, CVN potently blocks HIV entry through highly avid carbohydrate-mediated interactions with the HIV-envelope glycoprotein gp120, and is under preclinical investigation as an anti-HIV microbicide. CVN contains two non-overlapping carbohydrate-binding sites that bind the disaccharide Manalpha(1-2)Manalpha (which represents the terminal disaccharide of all three arms of Man(9)) with low to sub-micromolar affinities. The solution structure of a 1:2 CVN:Manalpha(1-2)Manalpha complex revealed that CVN recognizes the stacked conformation of Manalpha(1-2)Manalpha through a deep hydrophilic-binding pocket on one side of the protein (site 2) and a semi-circular cleft on the other (site 1). With the prominent exception of the C1 hydroxyl group of the reducing mannopyranose ring, the bound disaccharide is positioned so that each hydroxyl group is involved in a direct or water-mediated hydrogen bond to the polar or charged side-chains comprising the binding pocket. Thus, to determine whether the next-most reducing mannopyranose ring will augment CVN affinity and selectivity, we have characterized by NMR and ITC the binding of CVN to three synthetic trisaccharides representing the full-length D1, D2 and D3 arms of mammalian oligomannosides. Our findings demonstrate that site 1 is able to discriminate between the three related trisaccharides methyl Manalpha(1-2)Manalpha(1-2)Man, methyl Manalpha(1-2)Manalpha(1-3)Man and methyl Manalpha(1-2)Manalpha(1-6)Man with remarkable selectivity, and binds these trisaccharides with K(A) values ranging from 8.1x10(3)M(-1) to 6.6x10(6)M(-1). Site 2 is less selective in that it binds all three trisaccharides with similar K(A) values ranging from 1.7 to 3.7(+/-0.3)x10(5)M(-1), but overall binds these trimannosides with higher affinities than site 1. The diversity of pathogenic organisms that display alpha(1-2)-linked mannosides on their cell surfaces suggests a broad defensive role for CVN in its cyanobacterial source.

Solution- and solid-phase oligosaccharide synthesis using glucosyl iodides: a comparative study

Glycosyl iodide donors have been used in both solid- and solution-phase syntheses yielding alpha-(1 --> 6)-linked glucosyl oligomers in highly efficient protocols. While the solid-phase strategy offers advantages in terms of ease of purification, it requires a total of 7.5 equiv of donor and approximately 12 h to complete the incorporation of one monosaccharide unit. In contrast, solution-phase methods require only 2.5 equiv of donor and 2-3 h reaction time per glycosylation. Moreover, since the reactions are virtually quantitative (> 90%) column chromatography of the material is facile. The overall advantages of solution-phase oligosaccharide synthesis were further illustrated in the convergent synthesis of a hexamer (methoxycarbonylmethyl 6-O-acetyl-2,3,4-tri-O-benzyl-alpha-D-glucopyranosyl-(1 --> 6)-tetrakis-(2,3,4-tri-O-benzyl-alpha-D-glucopyranosyl-(1 --> 6))-2,3,4-tri-O-benzyl-1-thio-alpha-D-glucopyranoside) that was constructed from dimer donor iodides in a two-plus-two and a two-plus-four fashion.

Solution-phase hexasaccharide synthesis using glucosyl iodides

[reaction: see text] Oligosaccharides composed of 1,6-glucosyl residues have been prepared from glucosyl iodides. The reactions are highly stereoselective, giving the alpha-glycosides as the only isolated products in yields ranging from 84% to 94%. Oligomer synthesis can take place in an iterative 1 + 1 + 1 fashion or in a convergent manner where dimer iodides serve as donors for higher order acceptors.

Solid-phase synthesis of a branched hexasaccharide using a highly efficient synthetic strategy

The solid-phase synthesis of branched lacto-N-neohexaose derivative 1 occurring in human milk is described. The new building block of lactose 3 bearing the orthogonal temporary hydroxy protecting groups 9-fluorenylmethyloxycarbonyl (Fmoc) and levulinoyl (Lev) has been prepared. Its use, together with that of lactosamine donor 4, glucosamine donor 5, and O-galactosyl trichloroacetimidate 6, has enabled the preparation of hexasaccharide 22 following two different approaches in excellent overall yield (43%, 90% per step over eight steps). An additional key feature of this work is the successful use of newly prepared ester-type linker 2, having a benzylic spacer connected to the anomeric oxygen. This linker presents the advantage of producing a benzylic anomeric moiety after cleavage from the polymer support, which could be easily removed to obtain the unprotected oligosaccharide 1.