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

Abstract

A histidine based amphiphile containing a C13 fatty acyl chain N-histidyl N-myristry ethyl amine (AM1, 14.7mM) forms hydrogels in the presence of Fe3+ (within the range 1.47 to 4.41mM) 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- and 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- and wide-angle powder X-ray diffraction and small-angle X-ray scattering data are in favour of a layered structure of the supramolecular assembly of the AM1 in 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 (n = 3, 4, 8). Ferric ion containing metallo-hydrogel exhibits higher catalytic activity than the corresponding AM1 aggregate in the absence of metal-ions.