Molecular self-assembly is a bottom-up approach that uses non-covalent interactions (e.g. as hydrogen bonding, hydrophobic interactions, aromatic p-p stacking interactions, electrostatic interactions) between molecules to generate functional materials.1,2 It has emerged as a powerful tool in the field of biomaterials where peptides of different complexity (length and/or functionalization) have been used to assemble platforms with specific bioactivity.2 The main challenges in this filed are the design and synthesis of molecular building blocks that form assemblies with targeted biofunctionality, which are stable at physiological conditions.
Recently, we and others described short aromatic carbohydrate amphiphiles as viable choice in such bio-approach. The assembly of such amphiphiles is mainly driven by CH-p interactions.3,4
Herein, we investigated the effect of the carbohydrate stereochemistry and assembly conditions on the architecture and mechanical properties of the formed fibers and gels. We studied glucosamine (Glc), galactosamine (Gal) and mannosamine (Man) that were N-functionalized with fluorenylmathoxycarbonyl (Fmoc). FmocGlc and FmocGal were dissolved in water by heating (70-80°C) while FmocMan was insoluble at these conditions. The self-assembly of FmocGlc and FmocGal was triggered by cooling the obtained solutions and was confirmed by fluorescence spectroscopy. During the cooling step we observed formation of fibers that are entangled within a hydrogel. The process is reversible and the gels maintain the same mechanical properties up to 3 heating-cooling cycles when the gelation was carried at fast cooling regime (20 ° C/min) and up to 8 cycles when a slow cooling regime (5 ° C/min) was used. The chirality of the fibers and the gelation temperature (Tgel) depend on the stereochemistry of the carbohydrate and on the cooling rate. Fast cooling rate resulted in a positive signal in the circular dichroism (CD) spectra for both amphiphiles, while at slow cooling we observed an inversion of the CD signal for FmocGal. The sol-gel transition occurs within the temperature range 55-45°C, where Tgel (5 °C) > Tgel (20 °C) and Tgel (FmocGal) > Tgel (FmocGlc).
In a summary, we demonstrate that the choice of the carbohydrate and the assembly conditions are crucial for the properties and stability of the assembled nanofibers and the generated by them gels.