Activation of enzymatically produced chitooligosaccharides by dioxyamines and dihydrazides

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.

Micellar Nanogels from Alginate-Based Diblock Copolysaccharides

Alginates are marine polysaccharides known for their ability to selectively bind calcium ions and form hydrogels. They are widely used in biomedical applications but are challenging to produce as nanogels. Here we introduce a self-assembly route to create stable alginate-based nanogels under near-equilibrium conditions. Guluronate (G) blocks, which interact with divalent cations such as Ca2+, Ba2+, and Sr2+, were extracted from alginates and covalently linked through their reducing end to the reducing end of dextran (Dex) chains, forming linear block copolymers that self-assemble into micellar nanogels with a core-corona structure in the presence of these ions. Real-time dynamic light scattering (DLS) and small-angle neutron scattering (SANS) were used to study the self-assembly mechanism of the copolymer during dialysis against divalent ions. For the G12-b-Dex51 copolymer, we achieved spherical micelles with an 8 nm radius and an aggregation number of around 20. Although the type of divalent cation affected micelle stability, it did not influence their size. Micellar nanogels are dynamic structures, capable of ion exchange, and can disassemble with chelating agents like ethylenediamine tetraacetic acid (EDTA).

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.

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.

Advances in the preparation and assessment of the biological activities of chitosan oligosaccharides with different structural characteristics

Chitosan oligosaccharides (COSs) are widely used biopolymers that have been studied in relation to a variety of abnormal biological activities in the food and biomedical fields. Since different COS preparation technologies produce COS compounds with different structural characteristics, it has not yet been possible to determine whether one or more chito-oligomers are primarily responsible for the bioactivity of COSs. The inherent biocompatibility, mucosal adhesion and nontoxic nature of COSs are well documented, as is the fact that they are readily absorbed from the intestinal tract, but their structure-activity relationship requires further investigation. This review summarizes the methods used for COS preparation, and the research findings with regard to the antioxidant, anti-inflammatory, anti-obesity, bacteriostatic and antitumour activity of COSs with different structural characteristics. The correlation between the molecular structure and bioactivities of COSs is described, and new insights into their structure-activity relationship are provided.

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.

Review: Advances in preparation of chitooligosaccharides with heterogeneous sequences and their bioactivity

Chitooligosaccharides has attracted increasing attention due to their diverse bioactivities and potential application. Previous studies on the bioactivity of chitooligosaccharides were mostly carried out using a mixture. The structure-function relationship of chitooligosaccharides is not clear. Recently, it is confirmed that chitooligosaccharides with different degrees of polymerization play different roles in many bioactivities. However, heterogeneous chitooligosaccharides with a single degree of polymerization is still a mixture of many uncertain sequences and it is difficult to determine which structure is responsible for biological effects. Therefore, an interesting and challenging field of studying chitooligosaccharides with heterogeneous sequences has emerged. Herein, we reviewed the current methods for preparing heterogeneous chitooligosaccharides, including chemical synthesis, separation techniques and enzymatic methods. Advances in the bioactivities of chitooligosaccharides with heterogeneous sequences are also reviewed.

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.