Understanding the impact of the coordination sphere on metal ions for the design of small molecules in therapeutics and diagnostics

Abstract

Assessing, controlling, and applying ligand field effects to metal complexes continues to be a focus for inorganic chemists. Within the field of bioinorganic chemistry, this theme is found heavily in diagnostics through the design of contrast agents focusing on the coordination sphere as well as evaluating metal ion chemistry as a result of environmental stimuli. Tetraazamacrocycles, in particular, are well-known for tunability, thermodynamic stability, kinetic inertness, and metal ion discrimination. Recently, a new pyridine-based terdentate ligand series has been synthesized conserving functionalities well-known within the Green group. Long term goals for this new pincer library will be evaluated for (1) stability, (2) metal binding affinity, (3) catalytic activity, and (4) antioxidant capability to the library of macrocyclic ligands. In addition, this approach was carried out with the development of ferrocene-based biosensors utilizing an on-on type mechanism.^The new biosensor is designed with a focus on changes to the metal ion as a result of an enzymatic reaction, specifically caspase-3. Within this presentation, a new library of NNN-pincer molecules (2.3 - 2.6, Figure 1) will be introduced. These molecules were inspired by tetraazamacrocyclic congeners recently produced within our group. The syntheses of the ligand series as well as characterization upon complexation with Cu(ClO4)2 will be described. Pincers 2.1 and 2.2 have been previously reported in the literature and provide the fundamental basis for comparison to new molecules 2.3 - 2.6.13 An X-ray quality crystal for Cu2.6 was isolated, solid-state analysis completed, and compared to the copper tetraazamacrocycle congener. In addition, Cu2.2, Cu2.3, and Cu2.6 were evaluated electrochemically to observe the effects of the donating or withdrawing capacity of the ligand on the redox chemistry of the Cu2+-ion.^The remainder of the presentation will focus on the continued development of an on-on-type ferrocene biosensor for the caspase-3 enzyme. Biotin-ferrocene biosensors have been previously developed and used to quantify avidin in solution using cyclic voltammetry as a result of current depletion upon avidin-biotin interaction (on-off-type sensor). The fundamental design of the sensor showed the versatility of ferrocene as a redox center and the ease of modification in attaching various substrates. However, the on-off sensor design still has limitations in analyte detection as well as sensor design as false (+) or false (-) are possible. With the success of this model system in solution, a new library of caspase-3 enzyme-responsive sensors was designed to contain a D-E-V-D (Asp-Glu-Val-Asp) substrate covalently attached to a ferrocene redox core. This system was immobilized onto a gold surface by a thiolate linker unit and studied through cyclic voltammetry (Scheme 1).^To ensure that this enzyme-responsive system is suitable for the clinic, a series of experiments were completed to optimize (1) the experimental parameters for data acquisition, (2) reproducibility with sensor loading on the surface, and (3) evaluating sensor stability on the surface of the gold. Introduction of the target enzyme caspase-3 cleaves the D-E-V-D substrate from the biosensor. An amine terminus is left on the immobilized ferrocene and the cleaved D-E-V-D diffuses into solution. As a result of the substrate-enzyme reaction or interaction, a caspase-3 concentration dependent shift in ferrocene potential (mV) is observed by cyclic voltammetry. These results indicate that the design provides an on-on-type sensor platform capable of measuring 0.7 nM concentrations to date comparable to biological conditions.