Synthesis and photophysical studies of anion-responsive lanthanide(III) probes

The design, synthesis and photophysical analysis of a series of emissive lanthanide(III) complexes is presented. The Ln(III) complexes were designed to reversibly bind and discriminate between polyphosphate anions in aqueous solution, with a view to utilise the complexes as polyphosphate sensors for monitoring kinase activity in vitro or detecting certain polyphosphate anions within the complex cellular environment. The Ln(III) complexes aimed to strike a balance between anion affinity, luminescence response, water solubility and cell permeability.

Chapter 2 describes the design and full synthesis of seven complexes, together with the partial synthesis of a further two complexes. For each complex, the ligand is based on a macrocyclic cyclen scaffold and two antennae (designed to sensitise the central Ln(III) ion) positioned in a trans relationship on the macrocycle. Variations in the structures of the Ln(III) complexes included modifying the Ln(III) ion (Tb, Eu, Gd), incorporation (or removal) of hydrogen bond donor/acceptor groups, addition of electron donating (methoxy) groups, and altering the peripheral charge of the ligand.

Photophysical characterisation of the complexes is described in Chapter 3, including determination of quantum yields, lifetime measurements, solvatochromic studies, pH dependence, and oxygen sensitivity studies to explore the structure-property relationships of the compounds. We demonstrated that it was possible to modulate the λmax of the complex through modifications of the antennae structure, importantly increasing λmax closer to 355 nm for use with standard laser instrumentation for cellular imaging. The Tb(III) complex was found to be very bright and was insensitive to oxygen which showed its potential for use in biological studies.

Chapter 4 explores the anion binding capabilities of six of the synthesised Ln(III) complexes, focussing on sensing of biologically relevant polyphosphate anions in buffered aqueous solution. The complexes containing no hydrogen bond donor groups within the quinoline antennae showed weaker binding compared with previously synthesised complexes possessing hydrogen bond donor groups. Interestingly, complexes based on a naphthyridine antennae did not bind to anions, demonstrating that small structural changes have a profound impact on the anion binding properties. Additionally, the complex with a negative peripheral charge was less sensitive to anions, indicating that a repulsion of negative charge was taking place. The importance of hydrogen bond donor (amide) groups was revealed to be key for stronger binding to ATP or ADP over other polyphosphates, the effect of electron donating groups increases λmax but reduces the overall brightness of the complex in water and the naphthyridine antennae can sensitise both Eu(III) and Tb(III) ions, but the resulting complexes do not bind anions.

Chapter 5 consists of density functional theory (DFT) calculations conducted using PCM(H2O)-wB97M-V/6-31G* level of theory allowed further understanding of the complexes’ structures and anion binding properties.

In Chapter 6, the potential biological use of the Tb(III) complex was explored, including cellular imaging studies in living cells, showing that the Tb(III) probe localises initially in the lysosomes and then displays time-dependent migration to the mitochondria. Finally, the Tb(III complex was utilised in combination with a structurally related Eu(III) complex for the successful ratiometric monitoring of a kinase reaction, by following the change in Tb/Eu emission ratio as a function of enzymatically generated ADP.