Bioessential metal cations, such as Fe(III), Cu(II) and Mo(VI), play a key role in biological processes, such as electron transfer, oxygen transport and catalysis. We are interested in the strategies used by nature to bind these essential metal ions and to tune their reactivity. Once these strategies are understood, they can be applied to other areas of Biological Inorganic Chemistry. Current projects include:
Microorganisms produce and secrete siderophores to solubilise essential iron for uptake into the cell. We are interested in the chemical design features that allow siderophores of the 2, 3-dihydroxybenzamide type (Fig. 1) to bind Fe(III) and other essential metal ions, such as Mo(VI). The importance of the denticity, chirality and backbone structure of these chelators and the interactions with their cognate binding proteins are the focus of our research.1, 2, 3
Figure 1. Selected 2, 3-dihydroxybenzamide siderophores.
By co-crystallising the periplasmic binding protein CeuE of the food-borne pathogen Campylobacter jejuni with ferric complexes of both hexadentate and tetradentate siderophores, we were able to identify structural changes that allow this bacterial protein to capture more than one type of ferric siderophore (Fig. 2, collaboration with Prof. K. S. Wilson).1, 2, 3
Figure 2. Fe(III) (orange) coordinated to the siderophore mimics MECAM6- and 4-LICAM4- (coloured by atom type) with key amino acid residues in the binding pocket of CeuE shown (green cylinders as carbon atoms).
Artificial metalloenzymes combine the beneficial features of both, reactive synthetic catalysts and highly selective protein scaffolds. In this way, synthetic reactions can be performed with new levels of selectivity and in a biocompatible environment. Inspired by the way bacteria use siderophores for the uptake of essential iron, we are interested in using siderophores as new redox-reversible anchoring groups for synthetic catalysts. In the presence of iron(III) a siderophore-based anchor can be used to strongly connect a catalyst to a protein scaffold, creating an artificial enzyme, but on reduction to iron(II), dissociation takes place and the artificial enzyme disassembles (Fig. 3, collaboration with Prof. K. S. Wilson). This reversible anchoring approach allows high value components, in particular the protein, to be reclaimed and reused.4
Figure 3. A redox reversible iron-siderophore anchor connects a synthetic catalyst to a protein scaffold, thereby allowing the controlled assembly and disassembly of an artificial metalloenzyme.
Due to an increase in bacterial resistance to antibiotics, there is an urgent need for novel ways of combating bacterial infections. By linking antimicrobial agents to ferric siderophores that bacterial cells needs to survive, we aim to promote the active uptake of the antimicrobial and overcome resistance arising from changes to cell permeability and drug efflux (Trojan horse approach). Current work exploits mainly citrate-based siderophores, such as staphyloferrin A, in combination with fluoroquinolone antimicrobials (Fig. 4, collaboration with Drs. A. Routledge and G. H. Thomas).5, 6
Figure 4. Trojan horse antimicrobial agent consisting of staphyloferrin A and a fluoroquinolone (left). Structure of the Fe(III) complex of (S),(R),(R)-staphyloferrin A (right).
Nature uses molybdenum-containing enzymes to catalyse oxygen atom transfer (OAT) from water to organic substrates. In these enzymes, the two electrons that are released are removed by spatially-separated electron transfer units. We are particularly interested in the importance of these spatially-separated one-electron transfer units and the participation of non-innocent sulfur-containing ligands in OAT catalysis.7, 8 Inspired by the molybdoenzymes, we have developed Ru(II)-Mo(VI) dyads that allow the rate-determining step in the molybdenum-catalysed reaction, the transfer of an oxo ligand to PPh3, to be accelerated by photo-induced electron transfer (Fig. 5, collaboration with Prof. R. N. Perutz).In the catalytic cycle, methyl viologen (MV2+) acts as a mediator; it is regenerated and only required in catalytic rather than sacrificial amounts.
Figure 5. Structure of the dyad [Ru(bpy)2(L)MoO2(MeOH)](BF4)2 (left) and photographs taken of samples after irradiation (right). Sample A: [Ru(bpy)2(L)MoO2(solv)]2+, PPh3 and MV2+; sample B: control kept in the dark; sample C: [Ru(bpy)2(H2-L)]2+, PPh3 and MV2+; sample D: [Ru(bpy)2(L)MoO2(solv)]2+, PPh3 but no MV2+.