Journal
SOFT MATTER
Volume 5, Issue 7, Pages 1474-1482Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/b816664a
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Cyclo(L-Tyr-L-Lys) ( 1) and its 3-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 degrees 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. H-1 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.
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