Organo- and hydrogels derived from cyclo(L-Tyr-L-Lys) and its ε-amino derivatives

Cyclo(L-Tyr-L-Lys) (1) and its ε-amino derivatives (2a-2g) were synthesized and investigated as organo- and hydrogelators. Although 1 precipitates in water, it enables the gelation of a number of polar organic solvents including DMF and DMSO. Ultrasound promotes 1 to form a hydrogel when cooling its aqueous solution. Even at 56 °C the hydrogelation still occurs when its concentration reaches 1.5 wt% under the assist of sonication. Moreover, after N-acylation of 1 with alkyl carboxylic acids and anhydrides, 2a-2g exhibit chain length dependent hydrogelation ability. 2a-2c with short alkyl tails yield hydrogels at around 1.0 wt%. Rheological results showed that a robust thermoreversible hydrogel is created from 2b at 2.5 wt%. The self-assembled fibrillar networks in the gels were distinctly evidenced by TEM and SEM observations. ¹H NMR analyses suggested that intermolecular hydrogen-bonding between diketopiperazine rings plays an essential role in the self-aggregation of these gelators. Based on X-ray powder diffraction data, a possible molecular packing model was deduced for the xerogel of 1. Also the interdigitated bilayer structure was proposed for the self-assembly of 2a-2g in ethanol.