Calixarene based artificial metalloenzyme and fluorescent sensor for soft-metal cations

Öztürk, Gülsüm
Supramolecular chemistry greatly facilitates the development of receptor molecules for artificial enzymes, photoresponsive host-guest systems and organic conductors among others. In recent years, there has been a growing interest in the rational design of synthetic catalysts capable of achieving significant results in terms of reaction rate and selectivity. No doubt, the admirable properties of enzymes have attracted the attention of chemists to the synthesis of novel enzyme models. The several features possessed by the calixarenes, notably their ability to form host-guest complexes and the possibility of introducing a large variety of functions by means of selective derivatization, helps them to act as enzyme mimics or, more in general, as synthetic catalysts. In the first part of the study, functionalized calix[4]arene with the one and two cyclenyl groups has been synthesized (compounds 2.11 and 2.16 respectively). These calix[4]arenes were used as the starting compounds for the 111syntheses of the mononucleating ligand MCC-[Zn2+] (2.20) and dinucleating ligand DCC-[Zn2+]2 (2.21). Kinetic studies of mononucleating ligand and dinucleating ligand were carried out spectrophotometrically. The hydrolyses of the model ester substrates bis-p-nitrophenylcarbonate (BPNPC) (2.17), /7-nitrophenylacetate (PNPA) (2.18), and p-nitrophenylstearate (PNPS) (2.19) generating /?-nitrophenolate ion which has an intense absorbance at 400 nm were studied at different pH. The greatest rate enhancement of 400-fold in the hydrolysis of the substrate PNPS catalyzed by the MCC-[Zn2+] at pH 8.5 was observed. Selectivity of enzyme model 2.20 to the /»-nitrophenylstearate (PNPS) among these substrates has a great importance in hydrolysis of long-chain fatty acid esters. The design of sensors to monitor the activity of chemical and biochemical species in various environments is an important and growing field of science. The measurement of the concentrations of transition-metal ions is very important, since many of these are also pollutants in environmental systems and essential trace elements in biological systems. In the second part of the study, tetradansylamidothiacalix[4]arene as a fluorescent sensor have been synthesized and characterized. The complexation of the dansyl (a fluorophore) modified thiacalix[4]arene with metal cations (Cu(II), Co(H), K(I), Cd(II), Ca(II), Li(I), Mg(II), Na(I), Ag(I), Zn(II), and Hg(II)) in acetonitrile has been studied with UV-Vis and Fluoresence spectroscopy. Supramolecular photochemistry, like catalysis, may involve the binding of substrate and receptor, and mediating a photochemical process. In this study, a fluorescent sensor was made by affixing moieties of dansyl to the upper rim of thiacalix[4]arene, providing a system in which the dansyl fluorescence is quenched in the presence of Cu(II), Co(II), Zn(II), and Hg(II).
Citation Formats
G. Öztürk, “Calixarene based artificial metalloenzyme and fluorescent sensor for soft-metal cations,” Ph.D. - Doctoral Program, Middle East Technical University, 2002.