Synthesis of linear chitosan-block-dextran copolysaccharides with dihydrazide and dioxyamine linkers

Dihydrazide (ADH) and dioxyamine (PDHA) were assessed for their efficacy in coupling chitosan and dextran via their reducing ends. Initially, the end-functionalization of the individual polysaccharide blocks was investigated. Under non-reducing conditions, chitosan with a 2,5-anhydro-D-mannose unit at its reducing end exhibited high reactivity with both PDHA and ADH. Dextran, with a normal reducing end, showed superior reactivity with PDHA compared to ADH, although complete conversion with ADH could be achieved under reductive conditions with NaBH3CN. Importantly, the oxime bond in PDHA conjugates exhibited greater stability against hydrolysis compared to the hydrazone bond in ADH conjugates. The optimal block coupling method consisted in reacting chitosan with an excess of dextran pre-functionalized with PDHA. The copolysaccharides could be synthesized in high yields under both reducing and non-reducing conditions. This methodology was applied to relatively long polysaccharide blocks with molecular weight up to 14,000 g/mol for chitosan and up to 40,000 g/mol for dextran. Surprisingly, block copolysaccharides did not self-assemble at neutral or basic pH; rather, they precipitated due to hydrogen bonding between neutralized amino groups of chitosan. However, nanoparticles could be obtained through a nanoprecipitation approach.

Influence of chain length and branching on poly(ADP-ribose)-protein interactions

Hundreds of proteins interact with poly(ADP-ribose) (PAR) via multiple PAR interaction motifs, thereby regulating their physico-chemical properties, sub-cellular localizations, enzymatic activities, or protein stability. Here, we present a targeted approach based on fluorescence correlation spectroscopy (FCS) to characterize potential structure-specific interactions of PAR molecules of defined chain length and branching with three prime PAR-binding proteins, the tumor suppressor protein p53, histone H1, and the histone chaperone APLF. Our study reveals complex and structure-specific PAR-protein interactions. Quantitative Kd values were determined and binding affinities for all three proteins were shown to be in the nanomolar range. We report PAR chain length dependent binding of p53 and H1, yet chain length independent binding of APLF. For all three PAR binders, we found a preference for linear over hyperbranched PAR. Importantly, protein- and PAR-structure-specific binding modes were revealed. Thus, while the H1-PAR interaction occurred largely on a bi-molecular 1:1 basis, p53-and potentially also APLF-can form complex multivalent PAR-protein structures. In conclusion, our study gives detailed and quantitative insight into PAR-protein interactions in a solution-based setting at near physiological buffer conditions. The results support the notion of protein and PAR-structure-specific binding modes that have evolved to fit the purpose of the respective biochemical functions and biological contexts.

Enzymatic elaboration of oxime-linked glycoconjugates in solution and on liposomes

Oxime formation is a convenient one-step method for ligating reducing sugars to surfaces, producing a mixture of closed ring α- and β-anomers along with open-chain (E)- and (Z)-isomers. Here we show that despite existing as a mixture of isomers, N-acetylglucosamine (GlcNAc) oximes can still be substrates for β(1,4)-galactosyltransferase (β4GalT1). β4GalT1 catalysed the galactosylation of GlcNAc oximes by a galactose donor (UDP-Gal) both in solution and in situ on the surface of liposomes, with conversions up to 60% in solution and ca. 15-20% at the liposome surface. It is proposed that the β-anomer is consumed preferentially but long reaction times allow this isomer to be replenished by equilibration from the remaining isomers. Adding further enzymes gave more complex oligosaccharides, with a combination of α-1,3-fucosyltransferase, β4GalT1 and the corresponding sugar donors providing Lewis X coated liposomes. However, sialylation using T. cruzi trans-sialidase and sialyllactose provided only very small amounts of sialyl Lewis X (sLex) capped lipid. These observations show that combining oxime formation with enzymatic elaboration will be a useful method for the high-throughput surface modification of drug delivery vehicles, such as liposomes, with cell-targeting oligosaccharides.

Carbohydr Polym Special Issue Invited contribution: Click chemistry for block polysaccharides with dihydrazide and dioxyamine linkers - A review

Engineered block polysaccharides is a relatively new class of biomacromolecules consisting of chemical assembly of separate block structures at the chain termini. In contrast to conventional, laterally substituted polysaccharide derivatives, the block arrangement allows for much higher preservation of inherent chain properties such as biodegradability and stimuli-responsive self-assembly, while at the same time inducing new macromolecular properties. Abundant, carbon neutral, and even recalcitrant biomass is an excellent source of blocks, opening for numerous new uses of biomass for a wide range of novel biomaterials. Among a limited range of methodologies available for block conjugation, bifunctional linkers allowing for oxyamine and hydrazide 'click' reactions have recently proven useful additions to the repertoire. This article focuses the chemistry and kinetics of these reactions. It also presents some new data with the aim to provide useful protocols and methods for general use towards new block polysaccharides.

Rapid glycoconjugation with glycosyl amines

Conjugation of unprotected carbohydrates to surfaces or probes by chemoselective ligation reactions is indispensable for the elucidation of their numerous biological functions. In particular, the reaction with oxyamines leading to the formation of carbohydrate oximes which are in equilibrium with cyclic N-glycosides (oxyamine ligation) has an enormous impact in the field. Although highly chemoselective, the reaction is rather slow. Here, we report that the oxyamine ligation is significantly accelerated without the need for a catalyst when starting with glycosyl amines. Reaction rates are increased up to 500-fold compared to the reaction of the reducing carbohydrate. For comparison, aniline-catalyzed oxyamine ligation is only increased 3.8-fold under the same conditions. Glycosyl amines from mono- and oligosaccharides are easily accessible from reducing carbohydrates via the corresponding azides by using Shoda's reagent (2-chloro-1,3-dimethylimidazolinium chloride, DMC) and subsequent reduction. Furthermore, glycosyl amines are readily obtained by enzymatic release from N-glycoproteins making the method suited for glycomic analysis of these glycoconjugates which we demonstrate employing RNase B. Oxyamine ligation of glycosyl amines can be carried out at close to neutral conditions which makes the procedure especially valuable for acid-sensitive oligosaccharides.

A new method for carbohydrate-oxyamine ligation starting from glycosyl amines 1 instead of the commonly used reducing sugars 2 results in tremendously increased ligation rates without the need for a catalyst, such as aniline.

Development of a multifunctional neoglycoside auxiliary for applications in glycomics research

A novel, multifunctional, tetrazine-containing neoglycoside auxiliary has been synthesized in three steps and 28% overall yield. The oxyamine was conjugated with unprotected carbohydrates under aqueous conditions (pH = 4.7), with DMF as a cosolvent, to provide neoglycosides in yields ranging between 51% and 68%. This auxiliary displayed broad advantages in the isolation and purification of complex carbohydrate mixtures, compatibility during extension by glycosyltransferases, and direct conjugation to chemical probes. Furthermore, the auxiliary can be removed in 96% yield under acidic conditions (0.25% TFA in H2O) that leave glycosidic linkages intact. Thereby, the tetrazine-containing neoglycoside auxiliary can serve to facilitate future glycomics investigations.

Oxy-imino saccharidic derivatives as a new structural class of aldose reductase inhibitors endowed with anti-oxidant activity

Aldose reductase is a key enzyme in the development of long term diabetic complications and its inhibition represents a viable therapeutic solution for people affected by these pathologies. Therefore, the search for effective aldose reductase inhibitors is a timely and pressing challenge. Herein we describe the access to a novel class of oxyimino derivatives, obtained by reaction of a 1,5-dicarbonyl substrate with O-(arylmethyl)hydroxylamines. The synthesised compounds proved to be active against the target enzyme. The best performing inhibitor, compound (Z)-8, proved also to reduce both cell death and the apoptotic process when tested in an in vitro model of diabetic retinopathy made of photoreceptor-like 661w cell line exposed to high-glucose medium, counteracting oxidative stress triggered by hyperglycaemic conditions.

Functionalization of Chitosan Oligomers: From Aliphatic Epoxide to Cardanol-Grafted Oligomers for Oil-in-Water Emulsions

Hydrophobically modified chitooligosaccharides (COSs) were tested for suitability as an emulsifier in cationic bituminous emulsions. COSs with polymerization degrees (DPs) of 5, 10, 15, and 20 were obtained by nitrous acid deamination. A complete study on depolymerization and precise product and side product characterization was undergone. Chemical modification of COSs was performed to achieve amphiphilic structures using three fatty epoxides with a growing chain length butyl (C4), octadecyl (C9), and hexadecyl glycidyl ether (C16)). The grafting efficiency according to reaction conditions was established. Different substitution degrees (DSs) were obtained by modulating the ratio of fatty epoxy to NH₂. It was shown that after a certain DS, the oligomers thus formed were not water-soluble anymore. At the end, cardanol glycidyl ether was grafted on DP 5, 10, and 15 COSs, cardanol being a biobased compound extracted from cashew nut shell; this reaction led to a potentially fully biobased structure. Water-soluble candidates with a higher DS were used as surfactants to emulsify motor oil as a simulation of bitumen. Cardanol-chitosan-based surfactants led to direct oil-in-water emulsion (60/40 w/w) composed of particles of 15 μm average size that were stable at least for 24 h.

Alginate-based diblock polymers: preparation, characterization and Ca-induced self-assembly

Renewable resources can provide a range of different polysaccharide blocks that can be used to prepare new types of stimuli-responsive polysaccharide-based block copolymers.

Protecting Group Free Synthesis of Glyconanoparticles Using Amino-Oxy-Terminated Polymer Ligands

Glycomaterials display enhanced binding affinity to carbohydrate-binding proteins due to the nonlinear enhancement associated with the cluster glycoside effect. Gold nanoparticles bearing glycans have attracted significant interest in particular. This is due to their versatility, their highly tunable gold cores (size and shape), and their application in biosensors and diagnostic tools. However, conjugating glycans onto these materials can be challenging, necessitating either multiple protecting group manipulations or the use of only simple glycans. This results in limited structural diversity compared to glycoarrays which can include hundreds of glycans. Here we report a method to generate glyconanoparticles from unprotected glycans by conjugation to polymer tethers bearing terminal amino-oxy groups, which are then immobilized onto gold nanoparticles. Using an isotope-labeled glycan, the efficiency of this reaction was probed in detail to confirm conjugation, with 25% of end-groups being functionalized, predominantly in the ring-closed form. Facile post-glycosylation purification is achieved by simple centrifugation/washing cycles to remove excess glycan and polymer. This streamlined synthetic approach may be particularly useful for the preparation of glyconanoparticle libraries using automation, to identify hits to be taken forward using more conventional synthetic methods. Exemplar lectin-binding studies were undertaken to confirm the availability of the glycans for binding and show this is a powerful tool for rapid assessment of multivalent glycan binding.

Conjugation of Oligonucleotides to Peptide Aldehydes via a pH-Responsive N-Methoxyoxazolidine Linker

The formation of N-methoxyoxazolidines in the preparation of oligonucleotide-peptide conjugates was evaluated. The reaction occurred between unprotected 2'-N-(methoxy)amino-modified oligonucleotides and peptide aldehydes in reasonable yields when isolated. The reaction is reversible under slightly acidic conditions, and it is pH-responsive. The rate and the equilibrium constant may be varied with structurally different aldehydes, allowing an optimization of the ligation and cleavage rate of the resultant conjugates. Therefore, this concept can be considered a cleavable linker.

2,5-Anhydro-d-Mannose End-Functionalized Chitin Oligomers Activated by Dioxyamines or Dihydrazides as Precursors of Diblock Oligosaccharides

Diblock oligosaccharides based on renewable resources allow for a range of new but, so far, little explored biomaterials. Coupling of blocks through their reducing ends ensures retention of many of their intrinsic properties that otherwise are perturbed in classical lateral modifications. Chitin is an abundant, biodegradable, bioactive, and self-assembling polysaccharide. However, most coupling protocols relevant for chitin blocks have shortcomings. Here we exploit the highly reactive 2,5-anhydro-d-mannose residue at the reducing end of chitin oligomers obtained by nitrous acid depolymerization. Subsequent activation by dihydrazides or dioxyamines provides precursors for chitin-based diblock oligosaccharides. These reactions are much faster than for other carbohydrates, and only acyclic imines (hydrazones or oximes) are formed (no cyclic N-glycosides). α-Picoline borane and cyanoborohydride are effective reductants of imines, but in contrast to most other carbohydrates, they are not selective for the imines in the present case. This could be circumvented by a simple two-step procedure. Attachment of a second block to hydrazide- or aminooxy-functionalized chitin oligomers turned out to be even faster than the attachment of the first block. The study provides simple protocols for the preparation of chitin-b-chitin and chitin-b-dextran diblock oligosaccharides without involving protection/deprotection strategies.

Reducing-End Functionalization of 2,5-Anhydro-d-mannofuranose-Linked Chitooligosaccharides by Dioxyamine: Synthesis and Characterization

The nitrous acid depolymerization of chitosan enables the synthesis of singular chitosan oligosaccharides (COS) since their reducing-end unit is composed of 2,5-anhydro-d-mannofuranose (amf). In the present study, we describe a chemical method for the reducing-end conjugation of COS-amf by the commercially available dioxyamine O,O'-1,3-propanediylbishydroxylamine in high mass yields. The chemical structure of resulting dioxyamine-linked COS-amf synthesized by both oximation and reductive amination ways were fully characterized by ¹H- and ¹³C-NMR spectroscopies and MALDI-TOF mass spectrometry. The coupling of chemically attractive linkers such as dioxyamines at the reducing end of COS-amf forms a relevant strategy for the development of advanced functional COS-based conjugates.

High-throughput chemical and chemoenzymatic approaches to saccharide-coated magnetic nanoparticles for MRI

There is a need for biofunctionalised magnetic nanoparticles for many biomedical applications, including MRI contrast agents that have a range of surface properties and functional groups. A library of eleven adducts, each formed by condensing a reducing sugar with a catechol hydrazide, for nanoparticle functionalisation has been created using a high-throughput chemical synthesis methodology. The enzymatic transformation of an N-acetylglucosamine (GlcNAc) adduct into an N-acetyllactosamine adduct by β-1,4-galactosyltransferase illustrates how chemoenzymatic methods could provide adducts bearing complex and expensive glycans. Superparamagnetic iron oxide nanoparticles (8 nm diameter, characterised by TEM, DLS and SQUID) were coated with these adducts and the magnetic resonance imaging (MRI) properties of GlcNAc-labelled nanoparticles were determined. This straightforward approach can produce a range of MRI contrast agents with a variety of biofunctionalised surfaces.

The neoglycolipid (NGL) technology-based microarrays and future prospects

The neoglycolipid (NGL) technology is the basis of a state-of-the-art oligosaccharide microarray system, which we offer for screening analyses to the broad scientific community. We review here the sequential development of the technology and its power in pinpointing and isolating naturally occurring ligands for glycan-binding proteins (GBPs) within glycan populations. We highlight our Designer Array approach and Beam Search Array approach for generating natural glycome arrays to identify novel ligands of biological relevance. These two microarray approaches have been applied for assignments of ligands or antigens on glucan polysaccharides for effector proteins of the immune system (Dectin-1, DC-SIGN and DC-SIGNR) and carbohydrate-binding modules (CBMs) on bacterial hydrolases. We also discuss here the more recent applications to elucidate the structure of a prostate cancer- associated antigen F77 and identify ligands for adhesins of two rotaviruses, P[10] and P[19], expressed on an epithelial mucin glycoprotein.

DNA-Templated N(Me)-Alkoxyamine Glycosylation

The potential of N(Me)-alkoxyamine glycosylation as a DNA-templated ligation has been studied. On a hairpin stem-template model, a notable rate enhancement and an increased equilibrium yield are observed compared to the corresponding reaction without a DNA catalyst. The N-glycosidic connection is dynamic at pH 5, whereas it becomes irreversible at pH 7. The N(Me)-alkoxyamine glycosylation may hence be an attractive pH controlled reaction for the assembly of DNA-based dynamic products.

Reductive oxyamination: a method for the qualitative and quantitative analysis of monosaccharides with a new aminooxy reagent using high-performance liquid chromatography with fluorescence detection

Derivatization of carbohydrates with aminooxy agents to form oximes can be used for qualitative and quantitative analysis of carbohydrates; however, the formation of isomeric products limits its application. A new reductive oxyamination procedure developed for the analysis of monosaccharides with a novel fluorescent O-substituted aminooxy reagent, 4-((aminooxy)methyl)-6-chloro-7-hydroxycoumarin (AOCC), is reported. In this procedure, monosaccharides undergo an oxime formation reaction with AOCC and are then readily reduced with 2-picoline-borane, followed by analysis with high-performance liquid chromatography with fluorescence detection. Good separation of five monosaccharide derivatives was achieved within 40 min with acetonitrile-water-tetrahydrofuran as the mobile phase. The detection limits were on the order of femtomoles. The linear range was 0.2-4000 nM, with a good correlation coefficient (R ≥ 0.9985). Furthermore, the method was applied for analysis of real samples, such as bovine milk powder, without complicated and tedious sample treatment. This reductive oxyamination method circumvents the problem caused by oxime isomers and can be used for the highly sensitive and selective analysis of monosaccharides with high accuracy, providing an effective and promising method for the analysis of carbonyls with aminooxy agents.