The Development of New Chelates for Magnesium, Calcium and Zinc Ions

Abstract

Metal cations such as Mg2+, Ca2+ and Zn2+ play a range of fundamental roles in biology. In particular, changes in their concentration are thought to play an important role in normal cellular function. The ability to measure and manipulate these concentrations is vital for furthering our basic understanding of biochemistry and to controlling it. Access to selective chelants for these cations is therefore highly desirable. Currently available chelants for Mg2+, Ca2+ and Zn2+ have various problems, mainly relating to selectivity.
In Chapter 2, the development of a new synthetic route to o-aminophenol-N,N,O-diiacetic acid phosphinate (APDAP) ligands is presented. They are analogues of the famous o-aminophenol-N,N,O-triacetic (APTRA) ligand. An improved synthesis of the methylphosphinate-containing ligand has been developed, which also gives access to novel phenyl and naphthyl derivatives. The phenyl-containing ligand showed improved sensitivity (Kd ~ 2 mM) for Mg2+ while retaining the same relative selectivity for this cation over Ca2+ and Zn2+. The naphthalene derivative displayed a 5-fold turn-on in emission intensity when chelated to Mg2+.
In Chapter 3, the chelates developed in Chapter 2 were conjugated to a range of luminescent reporter groups. Attachment of naphthalene, naphthalimide and 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) groups to the aminophenol ring of APDAP ligands was achieved. The fundamental photophysical properties of these ligands were studied, as well as their responses to Mg2+, Ca2+ and Zn2+. Depending on the fluorophore, some of the sensors obtained show promise as probes for Mg2+ and others for Ca2+. A naphthalene-containing APDAP-based probe was synthesised that displayed a stronger emission response to Mg2+ (5-fold enhancement) than Ca2+ (2.5-fold enhancement). Meanwhile, by connecting the APTRA ligand to the β position of a BODIPY dye, a sensor with a stronger response to Mg2+ (2.5-fold enhancement) over Ca2+ (1.5-fold enhancement) was discovered.
Finally, the synthesis of a new set of sulfur-containing chelates is described in Chapter 4. Replacing the oxygen in APTRA by sulfur resulted in a new chelate, S-APTRA, which showed improved selectivity for Zn2+¬ and high binding affinity (Kd ~ 7 nM). A lower-denticity analogue, with a methyl rather than a carboxylate unit on sulfur, displayed excellent selectivity for Zn2+ and had a lower binding affinity (Kd ~ 8 μM). Oxidation of the sulfide in these compounds to sulfoxides resulted in ligands with Kd values in the high μM and low mM range.