Volume 85, Issue 9

The 26th International Carbohydrate Symposium (ICS 2012) took place in Madrid, Spain from 22 to 27 July 2012. One thousand delegates and accompanying persons attended the meeting from all continents. The meeting was a tremendous success, gathering both senior scientists and several hundreds of young glycoscientists. This conference is the most important one in carbohydrate chemistry and biochemistry, and the participants presented their latest contributions in the different aspects of the field. From organic synthesis to biomedicine, passing through structural aspects, molecular recognition features, materials science applications, and mechanistic biochemistry advances, among many others. From the chemical aspects, it is today evident that the glycochemistry field is alive and continuously expanding. Synthetic chemistry methods permit access to a variety of oligosaccharides, both neutral and charged, in amounts that in turn allow the study of their biochemical and biomedical properties at a level of complexity that was elusive a few years ago. Additionally, the latest advances in purification techniques together with the improvement in analytical tools permit working with pure glycoconjugates under many different experimental conditions. This multidisciplinary issue of Pure and Applied Chemistry gathers 10 representative contributions from eminent scientists working in the field. They encompass the different aspects described above. The importance of NMR techniques to unravel the conformation, dynamics, and molecular recognition features of oligosaccharides is described, as well as the importance of natural and modified saccharides as sources of new nanomaterials or molecular transporters. Organic synthesis methods together with enzymatic approaches are presented as complementary approaches to obtain saccharides and their glycomimetics, showing different properties as ligands and/or inhibitors and strikingly diverse structures. Modifications of polysaccharides to access novel biomaterials are also considered. Fundamental mechanistic aspects are described from the chemical and biochemical perspectives. Therefore, we feel that this PAC issue serves to show to the chemical community different aspects of modern carbohydrate chemistry, which is today at the cutting edge of diverse scientific disciplines and acts as a glue to bring together scientists with different expertise to tackle key problems for science and society. Jesús Jiménez-Barbero and Sonsoles Martín-Santamaría Conference Editors

Glycan structural information is a prerequisite for elucidation of carbohydrate function in biological systems. To this end we employ a tripod approach for investigation of carbohydrate 3D structure and dynamics based on organic synthesis; different experimental spectroscopy techniques, NMR being of prime importance; and molecular simulations using, in particular, molecular dynamics (MD) simulations. The synthesis of oligosaccharides in the form of glucosyl fluorides is described, and their use as substrates for the Lam16A E115S glucosyl synthase is exemplified as well as a conformational analysis of a cyclic β-(1→3)-heptaglucan based on molecular simulations. The flexibility of the N -acetyl group of aminosugars is by MD simulations indicated to function as a gatekeeper for transitions of glycosidic torsion angles to other regions of conformational space. A novel approach to visualize glycoprotein (GP) structures is presented in which the protein is shown by, for example, ribbons, but instead of stick or space-filling models for the carbohydrate portion it is visualized by the colored geometrical figures known as CFG representation in a 3D way, which we denote 3D-CFG, thereby effectively highlighting the sugar residues of the glycan part of the GP and the position(s) on the protein.

The recent introduction of saturation transfer difference (STD) NMR has increased the tools for the study of protein–carbohydrate complexes. This is useful when it is combined with transfer nuclear Overhauser enhancement spectroscopy (NOESY) measurement, or when it is interpreted using the expected calculated values of transference, yielding additional, very valuable information for the study of this type of complex. The objective of this work is to cover the advances of the STD technique as exemplified by the investigations of DC-SIGN (dendritic cell-specific ICAM-3 grabbing non-integrin) recognition by simple carbohydrates or mimics of them, based on structures containing a terminal mannose or fucose. We also will discuss the methods for quantification of the STD values based on the initial growing rates with the saturation time.

The use of thioglycosides and other glycan derivatives with anomeric sulfur linkages is gaining increasing interest, both in synthesis and in various biological contexts. Herein, we demonstrate the occurrence and circumvention of anomerization during 1- S -glycosylation reactions, and present highly efficient and stereocontrolled syntheses of a series of photoprobe-thiosaccharide conjugates. Mutarotation of glycosyl thiols proved to be the origin of the anomeric mixtures formed, and kinetic effects could be used to circumvent anomerization. The synthesized carbohydrate conjugates were then evaluated by both solution- and solid-phase-based techniques. Both binding results showed that the S -linked glycosides interact with their cognate lectins comparably to the corresponding O -analogs in the present cases, thus demonstrating the reliability of the solid-support platform built upon our photo-initiated carbohydrate immobilization method for probing protein bindings, and showing the potential of combining these two means for studying carbohydrate–protein interactions.

Sialic acids are involved in a plethora of important biological events; among these the most known certainly is the binding of N -acetylneuraminic acid (Neu5Ac) with influenza virus sialidase. Considering Neu5Ac as the template that led to the structure-based development of the two potent antiviral agents zanamivir and oseltamivir, we developed the synthesis of the sialyl exo enitol, a new class of sialyl derivative that was used as precursor in powerful hetero-Diels–Alder reactions to form the corresponding spiroketals. Docking calculations employing the crystallographic structure of influenza virus sialidase indicate that these scaffolds could probably interact with most of the active site residues that stabilize Neu5Ac. In addition, their reduced polar nature with respect to Neu5Ac derivatives might provide inhibitors with increased bioavailability.

Aβ-peptide ligands based on a cis -glycofused benzopyran structure have been fluorescently labeled using coumarine derivatives. Among the synthesized compounds, two conserved their binding ability to β-amyloid peptides, as shown by NMR experiments. Moreover, exploiting its fluorescent property, it was demonstrated that one of such compounds was able to cross an in vitro model of blood–brain barrier (BBB) and to stain Aβ‑deposits.

Conventional drugs consist of a formulation of a bioactive species and a carrier, the former accounting for most of the sophistication of the design. In the case of biomolecular drugs, however, the role of the carrier becomes decisive in enabling the load to reach its target to carry out its designed therapeutic function. Thus, the clinical success of gene therapy, where the active principles are nucleic acids, critically depends on the use of efficient and safe delivery systems. Carbohydrates have proven particularly useful in this regard. Glycocoating, similarly to poly(ethylene)glycol (PEG)-coating ( pegylation ), can stabilize colloidal aggregates by improving solvation and preventing nonspecific interactions, for example, with serum proteins. Moreover, glycoconjugates can drive specific recognition and receptor-mediated internalization in target cells. Actually, the inherent flexibility of carbohydrate and glycoconjugate chemistry has greatly contributed to enlarging the range of functional materials that can be rationally conceived for gene delivery. Herein, this is illustrated with selected examples that focus on controlling the architectural parameters of the vectors to make them suitable for structure–activity relationship (SAR) and optimization studies. The members of the cyclomaltooligosaccharide (cyclodextrin, CD) family will be the central actors of the story.

The synthetic application of endohexosaminidase enzymes (e.g., Endo A, Endo M, Endo D) promises to allow ready access to a wide variety of defined homogeneous glycoproteins and glycopeptides. The use of N -glycan oligosaccharides that are activated at the reducing terminus as oxazolines allows their high-yielding attachment to almost any amino acid, peptide, or protein that contains a GlcNAc residue as an acceptor. A wide variety of oxazoline donors are readily available, either by total synthesis or by isolation of the corresponding oligosaccharide from natural sources and then conversion to the oxazoline in water. The synthetic potential of the enzymes is particularly augmented by the production of mutant glycosynthases, the use of which allows the synthesis of a wide variety of glycopeptides and glycoproteins bearing defined homogeneous N -glycan structures.

C -Glycosylation presents a rare mode of sugar attachment to the core structure of natural products and is catalyzed by a special type of Leloir C -glycosyltransferases (C-GTs). Elucidation of mechanistic principles for these glycosyltransferases (GTs) is of fundamental interest, and it could also contribute to the development of new biocatalysts for the synthesis of valuable C -glycosides, potentially serving as analogues of the highly hydrolysis-sensitive O ‑glycosides. Enzymatic glucosylation of the natural dihydrochalcone phloretin from UDP‑D-glucose was applied as a model reaction in the study of a structurally and functionally homologous pair of plant glucosyltransferases, where the enzyme from rice ( Oryza sativa ) was specific for C -glycosylation and the enzyme from pear ( Pyrus communis ) was specific for O -glycosylation. We show that distinct active-site motifs are used by the two enzymes to differentiate between C - and O -glucosylation of the phloretin acceptor. An enzyme design concept is therefore developed where exchange of active-site motifs results in a reversible switch between C / O -glycosyltransferase (C/O-GT) activity. Mechanistic proposal for enzymatic C -glycosylation involves a single nucleophilic displacement at the glucosyl anomeric carbon, proceeding through an oxocarbenium ion-like transition state. Alternatively, the reaction could be described as Friedel–Crafts-like direct alkylation of the phenolic acceptor.

Poly-β-(1→6)- N -acetyl-D-glucosamine is an exopolysaccharide secreted by numerous pathogenic bacteria, including Staphylococcus aureus , Escherichia coli , Yersinia pestis , Bordetella pertussis , Acinetobacter baumannii , Burkholderia spp . , and others. A convergent approach was developed for the synthesis of oligosaccharide fragments consisting of 5, 7, 9, and 11 glucosamine or N -acetylglucosamine units and for the preparation of five nona-β-(1→6)-D-glucosamines with various N -acetylation patterns. Penta- and nona-β‑(1→6)-D-glucosamines conjugated to protein carriers through a specially developed sulfhydryl linker proved to be highly immunogenic in mice and rabbits and elicited antibodies that mediated opsonic killing of multiple strains of S. aureus (including methicillin-resistant S. aureus , MRSA) and E. coli , and protected against S. aureus skin abscesses and lethal E. coli and B. cenocepacia peritonitis. These findings provide a basis for the construction of a unique semisynthetic vaccine against multiple bacterial targets. Conformational studies by means of special NMR experiments and computer modeling revealed that the oligo-β-(1→6)-D-glucosamine chain exists mostly in a helix-like conformation, where the terminal monosaccharides are arranged close to each other. Owing to this feature, oligoglucosamines consisting of 2 to 7 residues easily form products of cycloglycosylation. Cyclooligo-β-(1→6)-D-glucosamines represent a new family of functionalized cyclic oligosaccharides. Owing to their molecular architectonics, these compounds are convenient scaffolds for the design of conjugates with defined valency, symmetry, flexibility, and function.

Partial periodate oxidation of hyaluronan (HA) leads to ring opening of the 4-linked D-glucuronate (GlcA) residues, providing a particularly flexible element within otherwise semi-flexible chains. This leads to compaction of the chains as demonstrated by a pronounced decrease in the intrinsic persistence length, which was determined on the basis of the molecular weight dependence of radius of gyration ( R G ) and the intrinsic viscosity. These parameters were readily obtained using size-exclusion chromatography (SEC) with an online multi-angle laser light scattering (MALLS) detector, a viscosity detector, and a concentration-sensitive detector. The electrostatic contribution to the total persistence length increased with increasing degree of oxidation. Compared to alginates and chitosans, HA becomes less degraded during the oxidation, which is attributed to a protective effect of periodate-resistant N -acetyl-D-glucosamine (GlcNAc) residues adjacent to periodate-sensitive GlcA residues in HA.

Global climate change is a major concern as it leads to an increase in the average temperature of the earth’s atmosphere. The existence and persistence of some gaseous species in the atmosphere contribute to global warming. Experimental techniques are used to study the kinetics and degradation of global warming gases. However, quantum mechanical methods are also useful for the kinetic and radiative forcing study of global warming species and can precede experimental investigations. Research has also been targeted to develop more adapted procedures using ab initio and density functional theory (DFT) methods. This report provides a global perspective, in simplified manner, of the theoretical studies of the degradation of gas species in the atmosphere with an emphasis on the hydrogen abstraction kinetics of global warming gas species during their degradation and byproduct formation. En route, the results obtained from these studies are analysed and compared with experimental data where available. Our analyses indicate that the theoretical predictions are in agreement with experimental findings but the predicted parameters are dependent on the method being used. Theoretical methods are used to predict the thermodynamic parameters of reactions, and, with relevance to this report, the global warming potential (GWP) index can also be calculated. This report can be useful for future investigations involving global warming gaseous species while providing suggestions on how computations can fill in data gaps when experimental data are unavailable.