Bioinorganic coordination chemistry
Nature has elaborated a wide variety of metalloproteins, which are involved in crucial steps in the living world. The majority of them contain one or more transition metal ions embebed within the protein matrix and are tethered through different amino acids from the protein backbone. These molecular industries have been designed to function in mild physiological medium and performing at the same time highly specific reactions. For instance, it is mind -boggling to see how nature perfom the photo -oxidation of water using sunlight and water as sole ingredients and at the same time reduction of protons are carried within such a small potential window. While in front of such a performance, chemists are currently using drastic and often polluting conditions to drive these reactions. Hence, the basics of bioinorganic chemistry are to understand the functioning of metalloproteins systems at a molecular level and to propose simplified synthetic models capable of performing the corresponding reactions. The detailed scrutiny of the "real thing" demands a plethora of expertise, going from spectroscopic analysis, structural techniques to biochemical methods. The central mission for the chemists who are considered as the master manipulators of matter, is to translate into simple chemical models the main structural and electronic parameters of the investigated active site. However, because of the intricate nature of active sites of enzymes, often the spectroscopic data from synthetic models are useful to distentangle the electronic properties of natural active sites and thus giving new insights in the structure -function relationship.
In our laboratory, we are involved in one of the most challenging task for chemists, i.e converting water into its constitutive elements (H2 and 02) using sunlight. This task can be pided in two parts:
1. oxidation of water 2H2O ->O2 + 4e - + 4H+
2. reduction of protons 2H+ + 2e --> H2
Our approach to do so is inspired from the nature. The first reaction is carried within the Photosystem II in plants and the second by microbial enzymes, the Hydrogenases.
PSII, perhaps the most fascinating enzyme in the living world, drives the photooxidation of water using sunlight. The X -ray cristallographic structure showing almost all the essential cofactors has been elucidated by the group of Barber (Science 2004). This enzyme can be pided into two parts, a photochemical apparatus (the P680 pigment) where light energy is converted into chemical energy through a charge separated state and a catalytic centre (the Oxygen Evolving Complex: OEC) made up of a tetranuclear manganese complex, relayed electronically to the P680 via a pair of amino acids, the TyrosineZ and Histidine 190. In our synthetic proposal for modeling the PSII as a whole, a photoactive chromophore for instance Ruthenium(II) trisbipyridine synthon is covalently linked to different manganese complexes (see Figure 1).
Results from these systems are encouraging but still a lot of scientific challenges are to be tackled. Among those are :
- the control of the intermetallic distances
- vectorialisation of electron transfer processes
- modulation of the eletrochemical properties of manganese complexes
- stability of coordinating cavities for highly oxidised manganese ions
Recently we have synthesised a novel family of ligands where the distance between the lumophore and the cavity for manganese ions can be modulated (see Figure 2).
These systems are now under investigation in our group. In view of the new X -ray structure of PSII it seems that the TyrZ -His190 pair is playing the role of an electronic relay between the oxidised P680 and the OEC. We have developed a synthetic model mimicking the first photoinduced electron trade in PSII (Figure 3).
More elaborate supramolecular systems are now underway in our group. This work is supported by the CNRS, CEA and the European Commission for STRP Solar -H program. In this quest, chemists in our group have solid organic background and inorganic synthetic skills. However, they need a large panel of spectroscopic methods (NMR, IR, UV -Vis, Electrochemistry, Magnetic properties, Electron Paramagnetic Resonance, …) for the characterisation of their target complexes. In order to comfort our experimental data, Dr. Marie -France Charlot is an essential asset in our group, in that she performs DFT (Density Functional Theory) and TD -DFT (Time Dependent) calculations to underpin our findings. Finally, all the photophysical properties of our compound are being studied with nano and picosecond flash spectroscopy (with the fruitful collaboration of the group of Dr. Bill Rutherford and Dr Winfried Leibl in CEA Saclay at the Service Bioénergétiques.
"Mighty" manganese ions have been chosen by nature to perform the four electron oxidation of water to oxygen. Up to date, very few synthetic manganese complexes are known to reproduce this activity. In this field, we are developing a series of manganese complexes where the metal ions are disposed in a good topology to expect a synergy between them towards the redox activity. As an exemple of such complex we recently synthesised a dinuclear pillared Mn(III) (see Figure 4). The idea behind such construction is to expect a concerted mechanism for the formation of the oxygen -oxygen bond. Tetranuclear manganese cluster behaving like a four equivalent oxidising reservoir and capable to oxidise water is our grail. Dr Elodie Anxolabéhère -Mallart is constantly pursuing the characterisation of manganese in high oxidation states and more precisely the Mn=O motif. This species are coined to play a crucial role in the oxidation of water.
Reduction of protons.
As we mentioned above, oxidation of water is just the first step towards the production of Hydrogen from water. Indeed, in parallel to our target to strip electrons and protons from water, we are involved in the design and synthesis of catalysts towards reduction of protons. This research is ongoing in our lab in collaboration with CETH as industrial partner. This research is fueled by the discovery of novel non -noble metal (Fe, Co, Ni) complexes to replace platinum. At the Institute we worked together with the group of Dr. Pierre Millet in the design of novel electrocatalysts and the fabric of modified electrodes for reduction of protons to H2.
Students joining our group:
Students who are willing to work in this research theme can join us from different venues:
- Synthetic organic chemistry
- Synthetic inorganic chemistry
- Spectroscopic characterisations (Electrochemistry, Spectro -electrochemistry, Photophysical studies)
- European Commission STRP NEST SOLAR -H N° 516510 (2004 -2007)
- Chaire Blaise Pascal Pr. Tom Moore Arizona State University USA (2005 -2007)
- ANR Blanc HYPHO (2005 -2008)
- ANR Blanc Photobiohydrogène (2005 -2008)
- Laboratoire de Recherche Correspondant (LRC) avec la Section de Bioénergétique, Département de Biologie Cellulaire et Moléculaire, CEA Saclay.(2000 - )