Amino-Acid-Based Metallo-Hydrogel That Acts like an Esterase

A histidine-based amphiphile containing a C14 fatty acyl chain, N-histidyl N′-myristry ethyl amine (AM1, 14.7 mM) forms hydrogels in the presence of Fe³⁺ (within the range 1.47 to 4.41 mM) and Hg²⁺ (within the range 3.67 to 11.02 mM) ions in aqueous dispersions at pH 6.6 (27 °C). The imidazole ring of the histidine residue plays a vital role to interact with these metal-ions. The thermal and mechanical stability of these metallo-hydrogels can be tuned by changing the proportion of amphiphile to metal ion ratio (1:0.1 to 1:0.3 for Fe³⁺-containing gel and 1:0.25 to 1:0.75 for Hg²⁺-containing gel). The metallo-hydrogels were characterized by different spectroscopic and microscopic techniques, low- A nd wide-angle powder X-ray diffraction, and small-angle X-ray scattering studies. FT-IR and NMR spectroscopic studies indicate the participation of the imidazole ring in metal-ion binding. Low- A nd wide-angle powder X-ray diffraction and small-angle X-ray scattering data are in favor of a layered structure of the supramolecular assembly of the AM1 in the presence of metal-ions. Both, the amphiphiles and the metal ion induced hydrogels reveal catalytic activity of p-nitrophenyl esters hydrolysis for the acetyl, n-butyl and n-octyl esters. Ferric ion containing metallo-hydrogel exhibits higher catalytic activity than the corresponding AM1 aggregate in the absence of metal ions.