Mon objectif a consisté à élucider à l'échelle moléculaire le fonctionnement des systèmes enzymatiques complexes impliqués dans des processus de conversion d'énergie chez les êtres vivants. Je m'intéresse en particulier à la compréhension de deux étapes clés du fonctionnement commun à un grand nombre de ces systèmes, à savoir (i) les étapes d'interaction de ces complexes avec les quinones membranaires et (ii) les mécanismes catalytiques au niveau des sites actifs à molybdène. Le système modèle que j'étudie est la nitrate réductase A issue de la bactérie E. coli, en collaboration avec l'équipe du Dr. Axel Magalon (LCB, Marseille). Il permet la respiration anaérobie en catalysant la réduction du nitrate en nitrite et joue un rôle important dans le cycle biogéochimique de l'azote. Ma recherche vise en particulier à identifier les facteurs moléculaires qui permettent d'ajuster la réactivité de ces systèmes. Cela nécessite l'obtention d'informations structurales à l'échelle atomique sur ces complexes macromoléculaires. La stratégie utilisée a consisté dans un premier temps à générer des intermédiaires paramagnétiques clefs du fonctionnement de ces systèmes (radicaux semiquinones ou ion MoV). Puis j'ai caractérisé leurs propriétés rédox par potentiométrie suivie par spectroscopie RPE. Enfin, j'ai utilisé les techniques de spectroscopie RPE impulsionnelle à haute résolution, notamment la spectroscopie de corrélation des sous niveaux hyperfins (HYSCORE) pour sonder l'environnement magnétique local de ces intermédiaires à travers la détection des interactions nucléaires hyperfines et quadripolaires qui sont trop faibles pour être visibles par spectroscopie RPE classique. / The aim of my work is to elucidate at the molecular level the structure and the function of enzymes involved in energy conversion processes in living organisms. In particular, it is focused on the understanding of two important steps found in many of these systems, namely (i) their interaction with membrane quinones acting as electron/proton shuttles and (ii) the catalytic mechanism at the molybdenum active site. The nitrate reductase A (NarGHI) from the bacterium Escherichia coli is used as a model for these studies. This membrane-bound complex reduces nitrate into nitrite during anaerobic respiration and plays therefore an important role in the global nitrogen cycle. The goal of my research is mainly devoted to the identification of the molecular factors tuning the reactivity of this system at the two active sites. For this purpose, I mainly relied on the structural characterization of key paramagnetic intermediates e.g. semiquinone radicals or Mo(V) ion using electron paramagnetic resonance (EPR) spectroscopy in combination with rédox potentiometry. High resolution pulse EPR methods, especially Hyperfine Sublevel Correlation (HYSCORE) spectroscopy, were used to probe their local environment through the detection of hyperfine (and eventually quadrupole interactions) to nearby magnetic nuclei that are otherwise too weak to be measurable in conventional continuous wave EPR spectroscopy.

Metal-organic framework (MOF) compounds as a new class of porous coordination polymers consists of metal ions or clusters linked by organic molecules. They have gained recent interest because of their large surface areas and huge variety of the porous network structures. They exhibit interesting adsorption properties and therefore are potential candidates for various technical applications. In this work, continuous wave (cw) and pulsed electron paramagnetic resonance (EPR) methods such as pulsed electron-nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopy are applied to study metal-organic frameworks with respect to different aspects of their properties: The host-guest interactions between Cu2+ ions in [Cu3(btc)2]n (HKUST-1; btc: 1,3,5-benzenetricaboxylate) with adsorbed methanol (CH3OH), 13C enriched carbon monoxide and dioxide (13CO, 13CO2), hydrogen (H2), deuterium (D2) and mixed isotopic HD. In [Cu3(btc)2]n, the Cu2+ ions are connected to binuclear Cu/Cu paddle wheel units. Since the Cu2+ ions in the [Cu3(btc)2]n are antiferromagnetically coupled, the new compound [Cu2.97Zn0.03(btc)2]n is synthesized by isomorphous substitution containing about 1 % paramagnetic Cu/Zn paddle wheel units. The modified Cu/Zn paddle wheel units prove to be a very sensitive probe for the interactions with the adsorbed molecules. Secondly, the exchange interactions of antiferromagnetically coupled Cu/Cu paddle wheel units as well as additional inter-paddle wheel exchange interactions between the Cu/Cu pairs are studied in [Cu2(bdc)2(dabco)]n, a layered MOF with 1,4-benzenedicaboxylate (bdc) as linker and 1,4-diazabicyclo[2.2.2]octane (dabco) acting as pillars between the layers. In comparison to [Cu3(btc)2]n, the additional inter-paddle wheel exchange interactions are much easier disturbed by incorporation of Zn2+ ions into the framework structure. Third, the structural dynamics of the framework is investigated in the compound [Al(OH)(bdc)]n (MIL-53) which was isomorphously substituted by V(III)/V(IV) species. The 51V hyperfine structure revealed to be sensitive to the so-called breathing effect, a flexible structural behaviour upon guest adsorption/desorption or upon thermal treatment. It is shown that the aluminum ions can be substituted by vanadium but the octahedral coordination environment changes slightly to a pseudo-octahedral or a square-pyramidal coordination. Based on the hyperfine interactions between the electron spin and the nuclear spins of the surrounding atoms, structural models can be derived from orientation-selective measurements. In such a way, structural information of materials like powder samples and adsorbate complexes can be obtained which are hardly or even not accessible by other methods.

Metal-organic framework (MOF) compounds as a new class of porous coordination polymers consists of metal ions or clusters linked by organic molecules. They have gained recent interest because of their large surface areas and huge variety of the porous network structures. They exhibit interesting adsorption properties and therefore are potential candidates for various technical applications. In this work, continuous wave (cw) and pulsed electron paramagnetic resonance (EPR) methods such as pulsed electron-nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopy are applied to study metal-organic frameworks with respect to different aspects of their properties: The host-guest interactions between Cu2+ ions in [Cu3(btc)2]n (HKUST-1; btc: 1,3,5-benzenetricaboxylate) with adsorbed methanol (CH3OH), 13C enriched carbon monoxide and dioxide (13CO, 13CO2), hydrogen (H2), deuterium (D2) and mixed isotopic HD. In [Cu3(btc)2]n, the Cu2+ ions are connected to binuclear Cu/Cu paddle wheel units. Since the Cu2+ ions in the [Cu3(btc)2]n are antiferromagnetically coupled, the new compound [Cu2.97Zn0.03(btc)2]n is synthesized by isomorphous substitution containing about 1 % paramagnetic Cu/Zn paddle wheel units. The modified Cu/Zn paddle wheel units prove to be a very sensitive probe for the interactions with the adsorbed molecules. Secondly, the exchange interactions of antiferromagnetically coupled Cu/Cu paddle wheel units as well as additional inter-paddle wheel exchange interactions between the Cu/Cu pairs are studied in [Cu2(bdc)2(dabco)]n, a layered MOF with 1,4-benzenedicaboxylate (bdc) as linker and 1,4-diazabicyclo[2.2.2]octane (dabco) acting as pillars between the layers. In comparison to [Cu3(btc)2]n, the additional inter-paddle wheel exchange interactions are much easier disturbed by incorporation of Zn2+ ions into the framework structure. Third, the structural dynamics of the framework is investigated in the compound [Al(OH)(bdc)]n (MIL-53) which was isomorphously substituted by V(III)/V(IV) species. The 51V hyperfine structure revealed to be sensitive to the so-called breathing effect, a flexible structural behaviour upon guest adsorption/desorption or upon thermal treatment. It is shown that the aluminum ions can be substituted by vanadium but the octahedral coordination environment changes slightly to a pseudo-octahedral or a square-pyramidal coordination. Based on the hyperfine interactions between the electron spin and the nuclear spins of the surrounding atoms, structural models can be derived from orientation-selective measurements. In such a way, structural information of materials like powder samples and adsorbate complexes can be obtained which are hardly or even not accessible by other methods.:1 Introduction 1.1 Electron paramagnetic resonance spectroscopy for investigation of porous materials 1.2 Metal-organic frameworks 1.3 Implementation of paramagnetism by isomorphous substitution 1.4 EPR spectroscopic methods 1.4.1 Spin Hamiltonian 1.4.2 cw EPR spectroscopy 1.4.3 Pulsed EPR spectroscopy 1.5 Description of the project 2 [Cu2.97Zn0.03(btc)2]n 2.1 Introduction: Monometallic [Cu3(btc)2]n (1) 2.1.1 Spin coupling 2.1.2 Adsorption of H2O 2.1.3 Adsorption of DTBN 2.2 Isomorphous substitution of Cu2+ by Zn2+ in [Cu3(btc)2]n 2.2.1 Synthesis and characterisation of [Cu2.97Zn0.03(btc)2]n (2) 2.2.2 cw EPR spectroscopy of 2 2.2.3 Pulsed EPR spectroscopy of 2 2.2.4 Summary: Zn2+ substitution 2.3 Adsorption of methanol (MeOH) on [Cu2.97Zn0.03(btc)2]n (2_MeOH) 2.3.1 cw EPR spectroscopy of 2_MeOH 2.3.2 Pulsed EPR spectroscopy of 2_MeOH 2.3.3 Discussion 2.3.4 Summary: adsorption of MeOH 2.4 Adsorption of 13CO2 and 13CO on [Cu2.97Zn0.03(btc)2]n (2_CO2, 2_CO) 2.4.1 cw EPR spectroscopy of 2_CO2 and 2_CO 2.4.2 Pulsed EPR spectroscopy of 2_CO2 and 2_CO 2.4.3 Discussion 2.4.4 Summary: adsorption of 13CO2 and 13CO 2.5 Adsorption of H2, D2 and HD on [Cu2.97Zn0.03(btc)2]n (2_HH, 2_DD and 2_HD) 2.5.1 cw EPR spectroscopy of 2_HH, 2_DD and 2_HD 2.5.2 Pulsed EPR spectroscopy of 2_HH, 2_DD and 2_HD 2.5.2.1 3p ESEEM spectroscopy of 2_HH, 2_DD and 2_HD 2.5.2.2 Davies-ENDOR spectroscopy of 2_HH 2.5.2.3 Davies-ENDOR spectroscopy of 2_HD 2.5.2.4 Davies-ENDOR spectroscopy of 2_DD 2.5.3 Discussion 2.5.4 Summary: adsorption of H2, D2 and HD 2.6 Conclusion: [Cu2.97Zn0.03(btc)2]n 3 [Cu2(bdc)2(dabco)]n (3) and [Cu(2-x)Zn(x)(bdc)2(dabco)]n (3_x) 3.1 [Cu2(bdc)2(dabco)]n (3) 3.2 [Cu1.9Zn0.1(bdc)2(dabco)]n (3_0.1) 3.3 [Cu(2-x)Zn(x)(bdc)2(dabco)]n (3_0.5, 3_1.0, 3_1.5 and 3_1.9) 3.4 Determination of the exchange coupling constant J 3.5 Discussion 3.6 Conclusions: [Cu(2-x)Zn(x)(bdc)2(dabco)]n (3_x) 4 [(AlOH)1-x(VO)x(bdc)]n (4) and [(AlOH)1-x(VO)x(ndc)]n (5) 4.1 Introduction 4.2 EPR spectroscopic investigations of mixed-metal bdc compounds 4.3 EPR spectroscopic investigations of mixed-metal ndc compounds 4.4: Conclucions: V(III)/V(IV) substitution in [Al(OH)(bdc)]n and [Al(OH)(ndc)]n 5 Summary and Conclusion 5.1 Host-guest interactions 5.2 Exchange couplings of Cu/Cu pairs 5.3 Structural dynamics of the bdc and the ndc framework 5.4 Conclusion 6 Appendix 6.1 Experimental details and additional spectra 6.2 Instrumental details 6.3 Curriculum vitae and publications