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2.2 The Chelator – Inorganic Aspects Combined with Modern Drug Design
Selectivity is an essential requirement for all drugs and biomarkers. Especially metal targeting drugs have to be selective, not only for the targeted metal, but also for the specific protein, since otherwise serious consequences are looming. Metals are not only used in the active centre of various proteins, but are also involved in many biological activities like signal transduction, electron transfer, structure factors and others. Dysregulation of the natural metal homeostasis, which is a tightly regulated process, can therefore lead to several disease patterns and also to cell death. Thus, for pharmaceutics and compounds with biological applications is selective coordination essential. The overriding aim of this study is to achieve a more profound understanding of the Cu+ chemistry in AD. A multifunctional tool, which not only selectively binds Cu+ with a defined stability constant, but comprises selectivity for Aβ plaques, should therefore be developed.
Coordination of a metal depends on the donor strength of the ligand, including the size of the binding pocket, the charge, electronegativity, bond lengths and angles in the ligand but also on the concentrations, of the reaction partners as well as of co-ligands. Especially the latter is an important factor for in vivo studies, complicating simple metal-ligands reactions. A better description of ligation reactions would therefore be the equilibrium between the aqueous M(H2O)Xn+
complexes and the chelated LM complexes. Thus, basic concepts like the principle of Pearson are of limited application.[232] More precise is the Irving-Williams series, which directly considers the aqueous milieu and compares equilibrium constants for most donor groups. The Irving-Williams series shows that the most stable Cu2+ complexes are formed with a mixed {NS-} donor set.[233] Although, Cu2+ has different chemical behaviours than Cu+, this empirical finding was used as starting point for the synthesis of Cu+ selective ligands and was refined with the HSAB (hard and soft acids and basis) concept from Pearson.[232] After Pearson, Cu+ has a softer character, since it is big, has a low charge and can easily be polarised. Cu+ should therefore prefer soft donor atoms like sulphur. Thus, a {NS2} and {N2S2} donor set was chosen in the ligands (Figure 10). Conversely, Lewis acids like Mg2+ or Ca2+ will not be coordinated and according to the Irving-Williams series also the chelation of Mn2+, Fe2+, Co2+ and Ni2+ is disfavoured in the presence of copper. The second aspect considered in the ligand design is the complex geometry.
Cu+ comprises due to its closed d10 electron shell no ligand field stabilisation energy (LFSE). The complex geometry is therefore defined by the steric demand of the ligands, which results for four donor atoms in a preferred tetrahedral geometry. In contrast to that, Cu2+ forms square planar complexes with four and square pyramidal or trigonal pyramidal complexes with five ligands. A tetrahedral complex formation can be achieved by a strong pre-organisation of the ligand. Complex formation with Cu2+ should therefore be disfavoured and selectivity towards Cu+ should be given.
The increased copper levels involved in AD could have their origin in a dyshomeostasis forming a labile copper pool.[98] Taken this into account rather weak ligand systems are required, which do not remove the excessive copper, but can relocate it and thus restore normal copper levels. To be more precise, the chelator should have a higher affinity than the Aβ protein but low enough, that the Cu+ can be released again to metal transport proteins. Under consideration of this, the first generation of ligands developed in this project features only a tridentate {NS2} donor set.
The corresponding Cu+ complexes should therefore be thermodynamically metastable and be
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able to release the metal again. In a second generation more stable tetradentate ligands with a {N2S2} donor set, to ensure tight Cu+ coordination, were synthesised. As nitrogen donors pyridine and imidazole groups were chosen. Both are good σ-donors and can tightly coordinate to copper. The sulphur donor groups are realised by thioether functionalities instead of thiolate groups. This offers various advantages to the ligand system: i) less hydrogen bond donors enhance the chance to cross the BBB and to permeate cell membranes due to the hydrophobic character, ii) thiolate groups have a high tendency to form thiolate bridges between two metal ions[234–236], iii) possible cell toxicity due to the formation of disulphide bridges and thus generation of ROS is reduced,[237–241] iv) lower binding affinities, since no charge is compensated.
The latter increases the chance for a copper release and also weakens coordination of other function was introduced in the ligands. The alcohol should also allow the attachment of other functional groups or molecules to confer chemical and biochemical properties (e.g. water solubility by coupling with an alkyl sulfonate) to the chelators. Overall, the ligands were designed as plain as possible to follow “Lipinski’s Rule of Five” and ensure their absorption and permeation of cell membranes.
One aspect in modern drug design is the distribution of the compound. In the previous chapter dyes like ThT and PIB were presented with the remarkable ability to selectively interact with Aβ plaques and with low clearance times from healthy tissues. By coupling of such a benzothiazole derivative with the evaluated ligands via a spacer a multifunctional compound can be created, which should be suitable for the desired application. This compound exhibits on one side a selective metal binding unit and on the other side a marker for Aβ plaques (Figure 10). Although, different pathways for the synthesis of benzothiazole derivatives are known, they lack flexibility in the variation of functional groups.[242,243] Herein, a synthetic route was created with Suzuki Coupling as the key step, which offers the possibility to combine various building blocks leading to new benzothiazoles.
Figure 10 Design of multifunctional systems for application in AD research.
Tripodal pre-organised ligand systems, like tris(pyrazol)borate (tpb) or tris(pyrazol-1-yl)methane (tpm), have been used for many years in model complexes in bioinorganic chemistry. Their specific binding properties were also utilised with respect to CO releasing molecules (CORMs), with remarkable results.[244–246] The compounds synthesised in this study provide with their soft {NS2} binding moiety a unique type of tripodal pre-organised chelator with unknown reactivity.
Thus, besides the applications in AD, the tripodal ligands were also used for the synthesis of a new class of CORMs.
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Chapter 3
Synthesis and Characterisation of Tridentate Ligands for
Applications in AD
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