Electroanalysis for Molecular Recognition and Reaction Mechanism Studies
Electrochemistry is used to monitor and/or favor a molecular recognition event between a redox-active receptor and a guest molecule/analyte. The receptor can be either in solution or grafted on an electrode surface. The principle of detection lies on the shift of the potential/current during the recognition event. Electrocatalytic processes using redox mediators or metals being part of the receptor may amplify the redox signal allowing for a better detection of the analyte. These studies can lead to a better understanding of the role of some biological molecules in biological processes or to investigate homogeneous catalytic systems.
For example, it is well known that quinones are key biological agents involved in the ADP oxidative phosphorylation or in photosynthesis process. They act as putative antioxidants and are the electron and proton carriers in the membranes of mitochondria, chloroplasts and aerobic bacteria. Electrochemical approach of the investigation of proton-coupled electron transfer (PCET) is highly suitable because the PCET reaction can be controlled through the imposed potential and non-destructive techniques such as cyclic voltammetry can be performed. We investigated the behavior of an ubiquinone, UQ2, incorporated in a self-assembled monolayer in buffered (PhysChemChemPhys, 2011) and unbuffered aqueous solutions (ChemElectroChem, 2014).
We initiated a new collaboration with the Centre des Sciences du Goût et de l’Alimentation de Dijon (CSGA) on the development of electrochemical biosensors with odorant binding proteins. The detection of odorant molecules has become an important challenge in different research area, such as the food industry, medical diagnostics and homeland security. Human olfactory system is able to discriminate thousands of different molecules thanks to biochemical mechanisms involving multiple protein receptor partners and a combinatorial coding. Among these biomolecules, odorant-binding proteins (OBP) are small soluble proteins present in nasal mucus at millimolar concentrations. Their hydrophobic binding pocket gives them the ability to reversibly bind odorant molecules. OBPs are robust and easy to produce and are thus good candidates for the design of biosensors. We focused on the detection of odorant molecules associating OBPs as a bioreceptor and electrochemistry as a transduction method (Bioelectrochem. 2015). Using site-directed mutagenesis, we have shown that substitution of a single amino acid in the binding pocket of the rat OBP, rOBP3, modulates its binding affinities towards odorants. We developed a qualitative and quantitative electrochemical method for the detection of volatile molecules using OBPs. We have shown that rOBP3 binds the electrochemical probe (2-methyl-1,4-naphtoquinone, MNQ). The amount of MNQ displaced from the binding pocket of rOBP3 by the model odorant 3-isobutyl-2-methoxypyrazine (IBMP), was measured using square-wave voltammetry. We determined the dissociation constants of the rOBP3/MNQ and rOBP3/IBMP complexes. These values measured by electrochemistry were confirmed by a competitive fluorescent assay and isothermal titration calorimetry (Bioelectrochem. 2015). By combining this new analytical method to rOBP3 variants with different and complementary binding profiles, we were able to selectively detect each of the components of a ternary mixture of odorants. This work, that combines the engineering of OBPs and electrochemistry, offers us interesting perspectives in the field of electronic noses.
We also explored multi-phosphine auxiliaries built on the ferrocene skeleton and synthesized in Pr. Hierso’s group. In association with palladium or copper as metal, these auxiliaries are used in carbon-carbon or carbon-heteroatom bond formation reactions. Electrochemistry, as a tool for analyzing and controlling the reactivity, is very useful to reach high activity levels (ultra-rapid catalysis, activation of poorly reactive bonds such as C-Cl or C-H, minimization of the catalyst quantity, …). Our studies have principally focused on the oxidative addition of aromatic halides on low-valent palladium complexes, which is a key step in the overall catalytic reaction. The influence of this step has been calibrated with respect to electronic and steric parameters of the reactants and catalyst (for instance, number of phosphine groups implanted in the ferrocene platform, electronic richness and steric hindrance of the halogenated reactant, …). This research program has demonstrated that electrochemistry is a valuable, practical and well-suited tool for discriminating the most favorable conditions for catalysis (Chem. Eur. J. 2011; Organometallics 2013 ; Inorg. Chem. 2013).