Nature is based on self-assembling systems that generate functional structures. Inspired by nature, programmable artificial architectures have been developed, often making use of synthetic peptides and nucleic acids. In contrast, only limited examples of programmable carbohydrate assemblies have been reported. This can be partially ascribed to the limited knowledge of carbohydrates’ structure. Structural complexity, heterogeneity of natural carbohydrate samples, and the lack of suitable analytic techniques have prevented the molecular level description of carbohydrate materials. The introduction of synthetic model systems, able to generate chemically defined assemblies, could help the understanding of carbohydrate materials. Here, simple and well-defined oligosaccharides were employed to create model systems to study supramolecular carbohydrate-based assemblies and produce useful data for the formation of tailor-made materials. This approach also shined light on the interactions involved in the formation of natural systems, such as bacterial biofilms, where carbohydrate and peptides interact to form valuable nanocomposites.
In chapter 2, I investigated the supramolecular structure formation of synthetic oligosaccharides. Systematic variation in their chain length, substitution pattern, and glycosidic linkages, generated distinctive morphologies, including spherical particles or fiber-like structures. The compounds showed unique intrinsic optical properties (e.g. red edge excitation shift), highly dependent on their aggregation status. Potential applications of glycomaterials in bioimaging and optical devices are envisioned.
Among these six compounds, disaccharide 13 (13-D in chapter 3), growing into needle-like structures, offered the perfect model system to explore details of oligosaccharide assemblies and optimize analytical techniques to study carbohydrate materials. The stability of the assembly enabled the implementation of microcrystal electron diffraction (MicroED) for oligosaccharide samples. This technique allowed for the reconstruction of the crystal unit cell and permitted correlating the local molecular organization with the supramolecular assembly. Synthetic analogous of compound 13 with specific single-site modifications were designed to identify key stabilizing interactions. The combination of organic chemistry and electron diffractions methods will be implemented to reveal molecular details of natural polysaccharide assemblies.
In chapter 4, a new model system was introduced to study bacterial biofilms, nanocomposites of cellulose and proteins (e.g. curli fibers). Specific E. Coli strains produce phosphoethanolamine (pEtN) cellulose as part of their protective biofilms, providing increased adhesion. I employed synthetic peptides and oligosaccharides to generate artificial biofilm and study the role of pEtN cellulose in biofilm formation. Different amounts and patterns of pEtN substitution in the oligosaccharide modulated the length and aggregation tendency of the peptide fibers. The mechanical properties of the protein-carbohydrate network were affected by the chemical nature of the carbohydrate component, with high adhesion measured for highly substituted pEtN cellulose analogues. Synthetic oligosaccharides able to interrupt fibrillary assembly were identified and could serve as promising drug candidates for the treatment of neurological diseases or as antibacterial agents.
Overall, the synthetic oligosaccharide models presented in this thesis will establish the foundation of our understanding of carbohydrate interactions in nature and will promote several applications of carbohydrate materials in nanobiotechnology
