Deoxyfluorination tunes the aggregation of cellulose and chitin oligosaccharides and highlights the role of specific hydroxyl groups in the crystallization process.

Cellulose and chitin are abundant structural polysaccharides exploited by nature in a large number of applications thanks to their crystallinity. Chemical modifications are commonly employed to tune polysaccharide physical and mechanical properties, but generate heterogeneous mixtures. Thus, the effect of such modifications is not well understood at the molecular level. In this work, we examined how deoxyfluorination (site and pattern) impact the solubility and aggregation of well-defined cellulose and chitin oligomers. While deoxyfluorination increased solubility in water and lowered the crystallinity of cellulose oligomers, chitin was much less affected by the modification. The OH/F substitution also highlighted the role of specific hydroxyl groups in the crystallization process. This work provides guidelines for the design of cellulose- and chitin-based materials. A similar approach can be imagined to prepare cellulose and chitin analogues capable of withstanding enzymatic degradation.

Synthesis of Fungal Cell Wall Oligosaccharides and Their Ability to Trigger Plant Immune Responses.

Oligosaccharide fragments of fungal cell wall glycans are important molecular probes for studying both the biology of fungi and fungal infections of humans, animals, and plants. The fungal cell wall contains large amounts of various polysaccharides that are ligands for pattern recognition receptors (PRRs), eliciting an immune response upon recognition. Towards the establishment of a glycan array platform for the identification of new ligands of plant PRRs, tri-, penta-, and heptasaccharide fragments of different cell wall polysaccharides were prepared. Chito- and ß-(1→6)-gluco-oligosaccharides were synthesized by automated glycan assembly (AGA), and α-(1→3)- and α-(1→4)-gluco-oligosaccharides were synthesized in solution using a recently reported highly α-selective glycosylation methodology. Incubation of plants with the synthesized oligosaccharides revealed i) length dependence for plant activation by chito-oligosaccharides and ii) ß-1,6-glucan oligosaccharides as a new class of glycans capable of triggering plant activation.

Progress and challenges in the synthesis of sequence controlled polysaccharides.

The sequence, length and substitution of a polysaccharide influence its physical and biological properties. Thus, sequence controlled polysaccharides are important targets to establish structure-properties correlations. Polymerization techniques and enzymatic methods have been optimized to obtain samples with well-defined substitution patterns and narrow molecular weight distribution. Chemical synthesis has granted access to polysaccharides with full control over the length. Here, we review the progress towards the synthesis of well-defined polysaccharides. For each class of polysaccharides, we discuss the available synthetic approaches and their current limitations.

Systematic Structural Characterization of Chitooligosaccharides Enabled by Automated Glycan Assembly.

Chitin, a polymer composed of ß(1-4)-linked N-acetyl-glucosamine monomers, and its partially deacetylated analogue chitosan, are abundant biopolymers with outstanding mechanical as well as elastic properties. Their degradation products, chitooligosaccharides (COS), can trigger the innate immune response in humans and plants. Both material and biological properties are dependent on polymer length, acetylation, as well as the pH. Without well-defined samples, a complete molecular description of these factors is still missing. Automated glycan assembly (AGA) enabled rapid access to synthetic well-defined COS. Chitin-cellulose hybrid oligomers were prepared as important tools for a systematic structural analysis. Intramolecular interactions, identified by molecular dynamics simulations and NMR analysis, underscore the importance of the chitosan amino group for the stabilization of specific geometries.

First total synthesis of trehalose containing tetrasaccharides from Mycobacterium smegmatis.

Total synthesis of three important trehalose containing tetrasaccharides isolated from Mycobacterium smegmatis is reported for the first time, using regioselective opening of benzylidene acetals and stereoselective glycosylations as key steps. The 1,2-cis stereoselectivity in the glycosylation reactions was achieved using anchimeric assistance from a remote participating group, steric effects and solvent participation. The synthetic strategy can also be utilized for the assembly of structurally related oligosaccharides from M. tuberculosis.

First Total Synthesis of Trehalose-Containing Branched Oligosaccharide OSE-1 of Mycobacterium gordonae (Strain 990).

The first total synthesis of the branched oligosaccharide OSE-1 of Mycobacterium gordonae (strain 990) is reported. An intramolecular aglycon delivery approach was used for constructing the desymmetrized 1,1'-α,α-linked trehalose moiety. A [3+2] glycosylation of the trisaccharide donor and trehalose acceptor furnished the right hand side pentasaccharide. Regioselective O3 glycosylation of L-rhamnosyl 2,3-diol allowed expedient synthesis of the left hand side tetrasaccharide. The nonasaccharide was assembled in a highly convergent fashion through a [4+5] glycosylation.