Sulfur and Azobenzenes, a Profitable Liaison: Straightforward Synthesis of Photoswitchable Thioglycosides with Tunable Properties.

Azobenzene photoswitches are valuable tools for controlling properties of molecular systems with light. We have been investigating azobenzene glycoconjugates to probe carbohydrate-protein interactions and to design glycoazobenzene macrocycles with chiroptical and physicochemical properties modulated by light irradiation. To date, direct conjugation of glycosides to azobenzenes was performed by reactions providing target compounds in limited yields. We therefore sought a more effective and reliable coupling method. In this paper, we report on a straightforward thioarylation of azobenzene derivatives with glycosyl thiols as well as other thiols, thereby increasing the scope of azobenzene conjugation. Even challenging unsymmetrical conjugates can be achieved in good yields via sequential or one-pot procedures. Importantly, red-shifted azoswitches, which are addressed with visible light, were easily functionalized. Additionally, by oxidation of the sulfide bridge to the respective sulfones, both the photochromic and the thermal relaxation properties of the core azobenzene can be tuned. Utilizing this option, we realized orthogonal three-state photoswitching in mixtures containing two distinct azobenzene thioglycosides.

A Photoswitchable Trivalent Cluster Mannoside to Probe the Effects of Ligand Orientation in Bacterial Adhesion.

We have recently demonstrated, by employing azobenzene glycosides, that bacterial adhesion to surfaces can be switched through reversible reorientation of the carbohydrate ligands. To investigate this phenomenon further, we have turned here to more complex-that is, multivalent-azobenzene glycoclusters. We report on the synthesis of a photosensitive trivalent cluster mannoside conjugated to an azobenzene hinge at the focal point. Molecular dynamics studies suggested that this cluster mannoside, despite the conformational flexibility of the azobenzene-glycocluster linkage, offers the potential for reversibly changing the glycocluster's orientation on a surface. Next, the photoswitchable glycocluster was attached to human cells, and adhesion assays with type 1 fimbriated Escherichia coli bacteria were performed. They showed marked differences in bacterial adhesion, dependent on the light-induced reorientation of the glycocluster moiety. These results further underline the importance of orientational effects in carbohydrate recognition and likewise the value of photoswitchable glycoconjugates for their study.

Pseudoenantiomeric glycoclusters: synthesis and testing of heterobivalency in carbohydrate-protein interactions.

Multivalent carbohydrate-protein interactions are key events in cell recognition processes and have been extensively studied by means of synthetic glycomimetics. To date, frequently the valency, i.e. the multiplicity of the ligand attached to a polyvalent scaffold, has been considered in the design of multivalent structures but these studies have not led to a conclusive understanding of glycan recognition at the molecular level. In this work, we add a new aspect to carbohydrate-lectin recognition studies by designing the first heterobivalent diastereomeric glycoclusters in order to investigate the influence of both heteromultivalency and relative ligand orientation. Two enantiomeric scaffolds, derived from d- and l-serine, respectively, were glycosylated with two distinct carbohydrate ligands to obtain a library of pseudoenantiomeric glycoclusters. They all have an α-d-mannosyl residue in common as a specific ligand for lectins FimH and ConA, while they differ in the second carbohydrate portion, consisting of a ß-d-glucosyl, a ß-d-galactosyl or a ß-d-glucosaminyl residue as unspecific ligands. The synthesised heteroclusters were tested in standard binding-inhibition assays investigating FimH-mediated bacterial adhesion and ConA binding to mannosylated surfaces. A striking difference was observed between the potencies of the two pseudoenantiomeric glucose-containing glycoclusters as inhibitors of FimH-mediated bacterial adhesion. For the other diastereomeric glycocluster pairs smaller inhibitory potency differences were detected in the bacterial adhesion assay. In contrast, the assays with ConA showed no significant variation for all tested cluster pairs. The results obtained with the diastereomeric glucose-mannose glycocluster pair were rationalised by molecular docking. Binding energies for the FimH carbohydrate recognition domain were calculated for both diastereomers and are in agreement with experimental data obtained in the bacterial adhesion assays.

En route from artificial to natural: Evaluation of inhibitors of mannose-specific adhesion of E. coli under flow.

We investigated the properties of six Escherichia coli adhesion inhibitors under static and under flow conditions. On mannan-covered model substrates and under static conditions, all inhibitors were able to almost completely abolish lectin-mediated E. coli adhesion. On a monolayer of living human microvascular endothelial cells (HMEC-1), the inhibitors reduced adhesion under static conditions as well, but a large fraction of bacteria still managed to adhere even at highest inhibitor concentrations. In contrast, under flow conditions E. coli did not exhibit any adhesion to HMEC-1 not even at inhibitor concentrations where significant adhesion was detected under static conditions. This indicates that the presence of shear stress strongly affects inhibitor properties and must be taken into account when evaluating the potency of bacterial adhesion inhibitors.

Photocontrol over Molecular Shape: Synthesis and Photochemical Evaluation of Glycoazobenzene Macrocycles.

Reversible shape switching due to external stimuli is an attractive property for the control of molecular features. Hence, we aimed at macrocycles to investigate photoswitching of molecular shape. We prepared the first carbohydrate-based macrocycles comprising a photoresponsive azobenzene hinge. These macrocycles were readily obtained by cyclization of isothiocyanate-armed bis-azobenzene glycosides with piperazine. The unprotected macrocycles exhibit favorable photochromic properties in water and DMSO. Notably, the efficient trans→cis isomerization results in a remarkable shape transformation of the molecule. Additionally, the structure is characterized by restricted conformational freedom of the backbone, resulting in a single main conformer in each geometrical state (trans or cis). Measurement of optical rotation values and circular dichroism spectra revealed a tremendous change in chirality upon photoisomerization, with a strong helical induction in the cis state. These findings highlight the applicability of the new macrocycles as chiroptical molecular switches.

Organizing multivalency in carbohydrate recognition.

The interactions of cell surface carbohydrates as well as of soluble glycoconjugates with their receptor proteins rule fundamental processes in cell biology. One of the supramolecular principles underlying and regulating carbohydrate recognition is multivalency. Many multivalent glycoconjugates have therefore been synthesized to study multivalency effects operative in glycobiology. This review is focused on smaller multivalent structures such as glycoclusters emphasizing carbohydrate-centered and heteromultivalent glycoconjugates. We are discussing primary, secondary and tertiary structural aspects including approaches to organize multivalency.

FRET-Based Nanobiosensors for Imaging Intracellular Ca²âº and H⁺ Microdomains.

Semiconductor nanocrystals (NCs) or quantum dots (QDs) are luminous point emitters increasingly being used to tag and track biomolecules in biological/biomedical imaging. However, their intracellular use as highlighters of single-molecule localization and nanobiosensors reporting ion microdomains changes has remained a major challenge. Here, we report the design, generation and validation of FRET-based nanobiosensors for detection of intracellular Ca(2+) and H⁺ transients. Our sensors combine a commercially available CANdot(®)565QD as an energy donor with, as an acceptor, our custom-synthesized red-emitting Ca(2+) or H⁺ probes. These 'Rubies' are based on an extended rhodamine as a fluorophore and a phenol or BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid) for H⁺ or Ca(2+) sensing, respectively, and additionally bear a linker arm for conjugation. QDs were stably functionalized using the same SH/maleimide crosslink chemistry for all desired reactants. Mixing ion sensor and cell-penetrating peptides (that facilitate cytoplasmic delivery) at the desired stoichiometric ratio produced controlled multi-conjugated assemblies. Multiple acceptors on the same central donor allow up-concentrating the ion sensor on the QD surface to concentrations higher than those that could be achieved in free solution, increasing FRET efficiency and improving the signal. We validate these nanosensors for the detection of intracellular Ca(2+) and pH transients using live-cell fluorescence imaging.

From chitin to bioactive chitooligosaccharides and conjugates: access to lipochitooligosaccharides and the TMG-chitotriomycin.

The direct and chemoselective N-transacylation of peracetylated chitooligosaccharides (COSs), readily obtained from chitin, to give per-N-trifluoroacetyl derivatives offers an attractive route to size-defined COSs and derived glycoconjugates. It involves the use of various acceptor building blocks and trifluoromethyl oxazoline dimer donors prepared with efficiency and highly reactive in 1,2-trans glycosylation reactions. This method was applied to the preparation of the important symbiotic glycolipids which are highly active on plants and to the TMG-chitotriomycin, a potent and specific inhibitor of insect, fungal, and bacterial N-acetylglucosaminidases.

Connecting dysbiosis, bile-acid dysmetabolism and gut inflammation in inflammatory bowel diseases.

OBJECTIVE: Gut microbiota metabolises bile acids (BA). As dysbiosis has been reported in inflammatory bowel diseases (IBD), we aim to investigate the impact of IBD-associated dysbiosis on BA metabolism and its influence on the epithelial cell inflammation response. DESIGN: Faecal and serum BA rates, expressed as a proportion of total BA, were assessed by high-performance liquid chromatography tandem mass spectrometry in colonic IBD patients (42) and healthy subjects (29). The faecal microbiota composition was assessed by quantitative real-time PCR. Using BA profiles and microbiota composition, cluster formation between groups was generated by ranking models. The faecal BA profiles in germ-free and conventional mice were compared. Direct enzymatic activities of BA biotransformation were measured in faeces. The impact of BA on the inflammatory response was investigated in vitro using Caco-2 cells stimulated by IL-1ß. RESULTS: IBD-associated dysbiosis was characterised by a decrease in the ratio between Faecalibacterium prausntizii and Escherichia coli. Faecal-conjugated BA rates were significantly higher in active IBD, whereas, secondary BA rates were significantly lower. Interestingly, active IBD patients exhibited higher levels of faecal 3-OH-sulphated BA. The deconjugation, transformation and desulphation activities of the microbiota were impaired in IBD patients. In vitro, secondary BA exerted anti-inflammatory effects, but sulphation of secondary BAs abolished their anti-inflammatory properties. CONCLUSIONS: Impaired microbiota enzymatic activity observed in IBD-associated dysbiosis leads to modifications in the luminal BA pool composition. Altered BA transformation in the gut lumen can erase the anti-inflammatory effects of some BA species on gut epithelial cells and could participate in the chronic inflammation loop of IBD.

Toward libraries of biotinylated chondroitin sulfate analogues: from synthesis to in vivo studies.

Chondroitin sulfate-E (CS-E) oligosaccharidic analogues (di to hexa) were prepared from lactose. In these compounds, the 2-acetamido group was replaced by a hydroxyl group. This modification speeded up the synthesis, and large oligosaccharides were constructed in a few steps from a lactose-originated block. The protecting groups used were as follows; Fmoc for hydroxyl groups to be glycosylated, allyl group for anomeric position protection, and trichoroacetimidate leaving groups were used to prepare up to octasaccharides. We took advantage of the presence of allyl group to develop a click biotinylation, through its transformation into a 3-azido-2-hydroxyl propyl group in two steps (epoxidation and sodium azide epoxide opening). The biotinylating agent was a water-soluble propargylated and biotinylated triethylene glycol (PEG). By using surface plasmon resonance (SPR), it was shown that the di-, tetra-, and hexasaccharides display a binding affinity and selectivity toward HSF/GSF and CXCL12 similar to that of CS-E. A parallel study confirmed their mimicry of natural compounds, based on the hexasaccharide interaction with Otx2, a homeodomain protein involved in brain maturation, thus validating our simplification approach to synthesize bioactive GAG.

Biotin sulfone tagged oligomannosides as immunogens for eliciting antibodies against specific mannan epitopes.

Biotinylated tri and tetrasaccharide: α Man (1→3) α Man (1→2) α Man; α Man (1→3) α Man (1→2) α Man (1→2) α Man were prepared using methyl tertbutyl phenyl thioglycosides glycosyl donors (MBP) and biotin sulfone strategy. Three key mannosyl thioglycosidic donors have been prepared: one for 1→2 linkage and two for the 1→3 linkage (protected with a 4,6-O-benzylidene or a 4,6-di-O-benzyl). The benzyliden protected one was not found reactive enough, and the benzylated donor was preferred. These biotinylated oligomanosides were evaluated as antigen in Crohn disease diagnosis and used coupled to streptavidin as hapten for eliciting polyclonal antibodies in mice.

A compatibility study on the glycosylation of 4,4'-dihydroxyazobenzene.

Photoresponsive glycoconjugates based on the azobenzene photoswitch are valuable molecules which can be used as tools for the investigation of carbohydrate-protein interactions or as precursors of shape-switchable molecular architectures, for example. To access such compounds, glycosylation of 4,4'-dihydroxyazobenzene (DHAB) is a critical step, frequently giving heterogeneous results because DHAB is a challenging glycosyl acceptor. Therefore, DHAB glucosylation was studied using nine different glycosyl donors, and reaction conditions were systematically varied in order to find a reliable procedure, especially towards the preparation of azobenzene bis-glucosides. Particular emphasis was put on glucosyl donors which were differentiated at the primary 6-position (N3, OAc) for further functionalisation. The present study allowed us to identify suitable glycosyl donors and reaction conditions matching with DHAB, affording the bis-glycosylated products in fair yields and good stereocontrol.

A two-step approach to a glycoazobenzene macrocycle with remarkable photoswitchable features.

A combination of the Mitsunobu reaction and Glaser coupling was used to achieve a new glycoazobenzene macrocycle. This photosensitive macrocycle can be efficiently and reversibly switched between two stable conformational isomers, which are characterized with photoswitchable shape, chiroptical and solubility behaviour.

H-Rubies, a new family of red emitting fluorescent pH sensors for living cells.

Monitoring intracellular pH has drawn much attention due to its undeniably important function in cells. The widespread development of fluorescent imaging techniques makes pH sensitive fluorescent dyes valuable tools, especially red-emitting dyes which help to avoid the overcrowded green end of the spectral band. Herein, we present H-Rubies, a family of pH sensors based on a phenol moiety and a X-rhodamine fluorophore that display a bright red fluorescence upon acidification with pKa values spanning from 4 to 9. Slight structural modifications led to dramatic changes in their physicochemical properties and a relationship between their structures, their ability to form H-aggregates, and their apparent pKa was established. While molecular form H-Rubies can be used to monitor mitochondrial acidification of glioma cells, their functionalised forms were linked via click chemistry to dextrans or microbeads containing a near infrared Cy5 (Alexa-647) in order to provide ratiometric systems that were used to measure respectively the phagosomal and endosomal pH in macrophages (RAW 264.7 cells) using flow cytometry.

One-pot synthesis of D-glucosamine and chitobiosyl building blocks catalyzed by triflic acid on molecular sieves.

Combining triflic acid-catalyzed acetalation, benzylation, reductive ring opening of benzylidene acetal and glycosylation in one-pot transformations leads to a wide range of d-glucosamine building blocks for assembling oligomers.