1-Naphthylacetic acid appended amino acids-based hydrogels: probing of the supramolecular catalysis of ester hydrolysis reaction

Abstract

A 1-naphthaleneacetic acid-appended phenylalanine-derivative (Nap-F) forms a stable hydrogel with a minimum gelation concentration (MGC) of 0.7% w/v (21 mM) in phosphate buffer of pH 7.4. Interestingly, Nap-F produces two-component [Nap-F + H = Nap-F_H, Nap-F + K = Nap-F_K and Nap-F + R = Nap-F_R], three-component [Nap-F + H + K = Nap-F_H–K, Nap-F + H + R = Nap-F_H–R and Nap-F + K + R = Nap-F_K–R] and four-component [Nap-F + H + K + R = Nap-F_H–K–R] hydrogels in water with all three natural basic amino acids (H = histidine, K = lysine and R = arginine) at various combinations below its MGC. Nap-F-hydrogel forms a nice entangled nanofibrillar network structure as evidenced by field emission scanning electron microscopy (FE-SEM). Interestingly, lysine-based co-assembled two- (Nap-F_K), three- (Nap-F_H–K and Nap-F_K–R) and four-component (Nap-F_H–K–R) xerogels exhibit helical nanofibrillar morphology, which was confirmed by circular dichroism spectroscopy, FE-SEM and TEM imaging. However, histidine and arginine-based two-component (Nap-F_H and Nap-F_R) and three-component (Nap-F_H–R) co-assembled xerogels exhibiting straight nanofibrillar morphology. In their co-assembled states, these two-, three- and four-component supramolecular hydrogels show promising esterase-like activity below their MGCs. The enhanced catalytic activity of helical fibers compared to obtained straight fibers (other than lysine-based assembled systems) suggests that the helical fibrillar nanostructure is involved in ordering the esterase-like although all supramolecular assemblies are chemically different from one another.