The Design and Synthesis of New Magnesium and Zinc Fluorescent Probes

Whilst Mg(II), Ca(II) and Zn(II) are involved in a multitude of biological processes, the ability to
detect either Mg(II) or Zn(II) using luminescent probes – without interference from endogenous
molecules – remains challenging. o-Aminophenol-N,N,O-triacetate (APTRA) is a Mg(II) binding
unit but it suffers from Ca(II) and Zn(II) interference. This thesis focuses on modifying the
structure of APTRA to develop new Mg(II)
- and Zn(II)
-selective fluorescent probes.

Chapter 2 describes the development of a series of aryl-alkynyl APTRA probes. Charge
transfer states dominate both the absorption and emission spectra of ester proligands that
contain electron-withdrawing substituents. Ligand-metal binding has been quantified using
UV-visible absorbance titrations. Electron-withdrawing substituents reduced the Ca(II) affinity
more than Mg(II), particularly when the alkyne was positioned para to the APTRA nitrogen atom,
thus improving Mg(II) selectivity.

Walter et al. demonstrated that the replacement of the phenolic APTRA carboxylate by a
phosphinate group, to give o-aminophenol-N,N-diacetate-O-methylene-methyl phosphinate
(APDAP), improves the Mg(II) selectivity. In Chapter 3, the synthesis of new APDAP derivatives
is described, where the methyl group at the phosphorus atom is replaced by a substituted aryl
ring or an alkyl group. The influence on Mg(II) and Ca(II) affinities is explored. The work has
subsequently been extended to fluorescent systems in which a BODIPY fluorophore is
attached at the phosphorus atom, as described in Chapter 4.

Previous work has shown that the substitution of the phenolic oxygen atom of APTRA by a
sulfur atom, to give S-APTRA, favours binding of the softer Zn(II) ion over Mg(II) and Ca(II). In
Chapter 5, this binding unit is incorporated into S-APTRA-Rosamine. It binds strongly to Zn(II)
with a “turn-on” emission response, whilst binding Ca(II) only weakly. The replacement of the
oxygen-bound carboxylate of S-APTRA-Rosamine by a methyl group reduced the Zn(II) affinity
by a factor of 1000, whilst oxidation of the thioether to the sulfoxide prevented Zn(II) binding.
As discussed in Chapter 6, a new binding unit incorporating a picolyl group at the sulfur atom
was developed. This binding unit dispalyed nM Zn(II) affinity, with no interference from Mg(II) or
Ca(II), rendering it potentially very attractive for further future development, for example
incorporation into a fluorescent probe.

These new, APTRA-derived Mg(II) and Zn(II) binding units and fluorescent probes offer improved
metal ion selectivity and reduced interference from endogenous molecules, making them
attractive complementary additions to existing ligands and probes.