Optically Transparent Hydrogels from an Auxin-Amino-Acid Conjugate Super Hydrogelator and its Interactions with an Entrapped Dye

Low-molecular-weight organic hydrogelators (LMHGs) that can rigidify water into soft materials are desirable in various applications. Herein, we report the excellent hydrogelating properties of a simple synthetic auxinamino-acid conjugate, naphthalene-1-acetamide of L-phenylalanine (1-NapF, Mw=333.38 Da), which gelated water even at 0.025 wt?%, thereby making it the most-efficient LMHG known. Optically transparent gels that exhibited negligible scattering in the range 350900 nm were obtained. A large shift from the theoretical pKa value of the gelator was observed. The dependence of the minimum gelator concentration (MGC) and the gel-melting temperatures on the pH value indicated the importance of H-bonding between the carboxylate groups on adjacent phenylalanine molecules in the gelator assembly. FTIR spectroscopy of the xerogels showed a beta-sheet-like assembly of the gelator. Variable-temperature 1H NMR spectroscopy demonstrated that p stacking of the aromatic residues was also partly involved in the gelator assembly. TEM of the xerogel showed the presence of a dense network of thin, high-aspect-ratio fibrillar assemblies with diameters of about 5 nm and lengths that exceeded a few microns. Rheology studies showed the formation of stable gels. The entrapment of water-soluble dyes afforded extremely fluorescent gels that involved the formation of J-aggregates by the dye within gel. A strong induced-CD band established that the RhoB molecules were interacting closely with the chiral gelator aggregates. H-bonding and electrostatic interactions, rather than intercalation, seemed to be involved in RhoB binding. The addition of chaotropic reagents, as well as increasing the pH value, disassembled the gel and promoted the release of the entrapped dye with zero-order kinetics.