Design, synthesis and characterization of bifunctional ligands and fluorescent sensors for bioavailable Cu(II), Zn(II) and Fe(III)

Lea Nyiranshuti, University of New Hampshire, Durham


Metal ions are essential for growth and development of all living organisms; without their catalytic activity, many essential biochemical reactions would not take place. However, improper regulation of metal ion concentration can lead to harmful effects on aquatic organisms and human health. To this end, the work presented in this dissertation focuses on the design, synthesis and characterization of bifunctional ligands and fluorescent sensors for bioavailable Cu(II), Zn(II) and Fe(III).

Chapter 1 describes a novel bifunctional ligand, N-(3-((2-(pyridin-2-yl)ethyl)(pyridin-2-ylmethyl)aminopropyl acrylamide (PEPMA-C3-acrylamide) (I.6) for ratiometric fluorescent Cu(II) sensing. PEPMA-C3-acrylamide and its model version N-(3-((2-(pyridin-2-yl) ethyl)(pyridin-2-ylmethyl)aminopropyl isobutyramide (I.7) were synthesized in moderate yields. Owing to the reactivity of the vinyl protons of PEPMA-C3-acryalmide, the isobutyramide model version was used during potentiometric titration and X-ray diffraction studies. The thermodynamic stability constants of the isobutyramide ligand demonstrated log Kf of 11.72 and 5.45 for Cu(II) and Zn(II), respectively. X-ray crystallography analysis of the copper complex indicated two distinct complexes with trigonal bipyramidal geometry, and 1:1 binding stoichiometric ratio. Moreover, PEPMA-C3-acrylamide was copolymerized with N-isopropylacrylamide (NIPAm) and fluorophores in order to provide a polymeric sensor for monitoring environmental Cu(II). Poly(N-isopropylacrylamide) (PNIPAm) collapses from a coil to a globule at temperatures above its lower critical solution temperature (LCST). The change in the polymeric environment is translated into a shift in maximum emission wavelength of the dansyl fluorophore from 536 nm to 505 nm and an increase in the ratio of emission intensity (I505/I536) from 0.77 to 1.22. Upon coordination of Cu(II) to the polymerized PEPMA-C3, the emission intensity is diminished without having any significant effect on the phase transition of PNIPAm.

Chapter 2 describes a novel fluorescent Zn(II) sensor 1,1,1-tris (aminomethylethane) N,N’,N’’-(2-methylisoquinoline) (TAMEisoquin). A competition reaction between TAMEisoquin and the well-known Zn(II) chelator (N,N,N’,N’-tetrakis (2-pyridylmethyl) ethylenediamine) (TPEN) indicated a conditional dissociation constant for [Zn(TAMEisoquin)]2+ of Kd= 1.4×10-15 M at pH~7.2. The affinity of TAMEisoquin for Zn(II) is indicative of a ligand design exploiting ring size, preorganization, and chelate effects to potentially detect picomolar to femtomolar zinc concentrations. X-ray crystallographic analysis of [Zn(TAMEisoquin)]2+ showed four unique cations that preferred a distorted octahedral geometry with trigonal twist angles that range between 34(2)° and 43.1(8)°. The addition of Zn(II) to TAMEisoquin displayed 11-fold fluorescence enhancement, selective by comparison to divalent biometal ions and Cd(II), which induces only 16% of the Zn(II) response.

Chapter 3 focuses on the development of two fluorescent Fe(III) sensors, III.12 and III.13, based on an Fe(III) chelator 4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl)benzoic acid commonly known as deferasirox (ICL670) and tetramethyl pyrromethene fluoroborate (BODIPY) fluorophore. By incorporating a high affinity Fe(III) chelator conjugated to a BODIPY fluorophore, we aim to develop sensors that could potentially detect Fe(III) with high selectivity accompanied with a fluorescent turn-on event. To this end, two novel ICL670 derivatives, 2,2'-(1-(4-nitrophenyl)-1H-1,2,4-triazole-3,5-diyl)diphenol (ICL670-NO2) (III.5) and 2,2'-(1-(4-aminophenyl)-1H-1,2,4-triazole-3,5-diyl)diphenol (ICL670-NH2) (III.6) were successfully synthesized in 75% and 52% yields respectively. Lastly, various routes for coupling of BODIPY-aldehyde to ICL670-NH2, and synthetic approaches have been investigated for Fe(III) sensor III.12 and III.13, respectively.