Professor Anne-Kathrin Duhme-Klair

01904 322587
Email: anne.duhme-klair@york.ac.uk

Metal Ions in Biology and Medicine

Metal ions, such as Fe(III), Cu(II) and Mo(VI), play a key role in biological processes, such as electron transfer, oxygen transport and catalysis. In this context 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 co-ordination chemistry. Current projects include:

1. New insights into siderophore-mediated iron uptake

Micro-organisms 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 (for examples see 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 is a major focus of our research.1

Selected dihydroxybenzamide siderophores

Figure 1. Selected 2, 3-dihydroxybenzamide siderophores.

Recent work involves the investigation of interactions between ferric siderophores and bacterial binding proteins (collaboration with Professor K. S. Wilson). 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).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).

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).

2. The Development of New Antimicrobials

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 the bacterial cell 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 citrate-based siderophores, such as staphyloferrin A, in combination with fluoroquinolone antimicrobials (Fig. 3, collaboration with Dr. A. Routledge).3

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).

Figure 3. 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).

3. The Development of Luminescent Sensors for Oxometalates

Siderophore-inspired chelating units can be used as receptor components in luminescent sensors for bioessential oxometalates, such as molybdate, tungstate and vanadate. The resulting sensors are highly specific, sensitive and do not require extensive pre-treatment of the analyte solution.

Structure and function of the molybdate (MoO42-) sensor [Re(bpy)(CO)3(H2-L1)]+.

Figure 4. Structure and function of the molybdate (MoO42-) sensor [Re(bpy)(CO)3(H2-L1)]+.

Recent work uses biomimetic 2, 3-dihydroxybenzamide units as receptors in combination with metal-based luminophores as signalling units.4 The crystal structure of an example, [Re(bpy)(CO)3(H2-L1)]+ (Fig. 4), with molybdenum bound is shown in Figure 5. In collaboration with Professor R. N. Perutz we are investigating the mechanism of the molybdenum-induced luminescence quenching by time-resolved absorption-, emission- and IR-spectroscopy.5

The crystal structure of [MoO2{Re(bpy)(CO)3(L1)}2] (left) and a comparison of the emission intensity of [Re(bpy)(CO)3(H2-L1)]+ in the absence (grey) and presence (light blue) of the indicated ions and in the presence of the indicated ions and molybdate (dark blue).

Figure 5. The crystal structure of [MoO2{Re(bpy)(CO)3(L1)}2] (left) and a comparison of the emission intensity of [Re(bpy)(CO)3(H2-L1)]+ in the absence (grey) and presence (light blue) of the indicated ions and in the presence of the indicated ions and molybdate (dark blue).

4. The Development of Luminescent Probes for Integral Membrane Proteins

As gateways to the cell, integral membrane proteins are important drug targets, however, these proteins are difficult to express, isolate and handle. To facilitate the visualisation and investigation of integral membrane proteins, we are developing luminescent probes that take advantage of the photophysical properties of Tb(III). The terbium luminescence of the lipid-analogue probes is sensitised by tryptophan and tyrosine residues that form a 'collar' at the hydrophilic/hydrophobic interface of the integral membrane protein. In amphiphilic environments, where the polar head groups of the probes are located in close proximity to the aromatic collar of an integral membrane protein, intermolecular energy transfer (ET) at the interface takes place, giving rise to long-lived terbium-based emission (Fig. 6).6

Schematic illustration of the intermolecular sensitization process.

Figure 6. Illustration of the intermolecular sensitization process. The aromatic collar of maltoporin (PDB-ID: 2MPR, tryptophan in red, tyrosine in orange) is shown in space-fill representation. Right: Absorption (red), excitation (black, monitored at 550 nm) and emission spectrum (green, λexc = 285 nm) of an aqueous system containing an integral membrane protein and the probe [Tb(DTPA-2C16)(H2O)]. Control: analogous system containing [Tb(DTPA)]2-.

5. The Development of Xanthine Oxidase Inhibitors.

Xanthine oxidase is implicated in hyperuricemia and gout and also a target for drugs designed to alleviate tissue damage caused by oxidative stress. To identify new inhibitors, we are synthesising structurally related Schiff bases by condensing a range of hydroxy-substituted benzaldehydes with suitable aromatic NH2-containing components.7,8 To mimic potential interactions of the Schiff bases with the molybdenum-centre in the active site of xanthine oxidase, representative examples are co-ordinated to cis-dioxo-molybdenum(VI) and the resulting complexes are studied by X-ray crystallography (Fig 7).

Crystal structures of selected molybdenum(VI) complexes.

Figure 7. Crystal structures of selected molybdenum(VI) complexes.

Selected publications

  1. From Siderophores and Self-assembly to Luminescent Sensors: the Binding of Molybdenum by Catecholamides,
    A-K Duhme-Klair, Eur. J. Inorg. Chem., 2009, 3689.
  2. Interactions of a Periplasmic Binding Protein with a Tetradentate Siderophore Mimic,
    D R Raines, O V Moroz, K S Wilson, A-K Duhme-Klair, Angew. Chem. Int. Ed., 2013, 52, 4595.
  3. Staphyloferrin A as Siderophore-component in Fluoroquinolone-based Trojan Horse Antibiotics,
    S J Milner, A Seve, A M Snelling, G H Thomas, K Kerr, A Routledge, A-K Duhme-Klair, Org. Biomol. Chem., 2013, 11, 3461.
  4. Synthesis, Characterization, Solid-State Structures and Spectroscopic Properties of Two Catechol-Based Luminescent Chemosensors for Biologically Relevant Oxometalates,
    H D Batey, A C Whitwood, A-K Duhme-Klair, Inorg. Chem., 2007, 46, 6516.
  5. Spectroscopic and Structural Investigations Reveal the Signaling Mechanism of a Luminescent Molybdate Sensor,
    V A Corden, A-K Duhme-Klair, S Hostachy, R N Perutz, N Reddig, H-C Becker, L Hammarström, Inorg. Chem., 2011, 50, 1105.
  6. Intermolecular Sensitization of a Terbium-containing Amphiphile by an Integral Membrane Protein,
    C L Davies, N G Housden and A-K Duhme-Klair, Angew. Chem., Int. Ed. Engl., 2008, 47, 8856.
  7. Synthesis, Activity Testing and Molybdenum(VI) Complexation of Schiff Bases Derived from 2, 4, 6-Trihydroxybenzaldehyde Investigated as Xanthine Oxidase Inhibitors,
    M Leigh, C E Castillo, D J Raines, A-K Duhme-Klair, ChemMedChem, 2011, 6, 612.
  8. Inhibition of Xanthine Oxidase by Thiosemicarbazones, Hydrazones and Dithiocarbazates Derived from Hydroxy-substitued Benzaldehydes,
    M Leigh, C Castillo, D J Raines, A-K Duhme-Klair, ChemMedChem, 2011, 6, 1107.