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Modèles de molécules biologiques sondés vibrationnellement et électroniquement en phase gazeuse

Abstract : Photo-induced processes are largely involved in the biological world due to the intrinsic properties of the molecules that compose them, like the UV/Vis aromatic chromophores residues of the proteins, the heme of hemoglobin protein or the chlorophylls. The goal of this thesis was in part to document in details the electronic relaxation processes that follow the UV/Vis absorption by these molecules. The interest was focused on the formation of triplet states formation in these systems that are still poorly known despite their major importance. Indeed, the biradical character of the triplet states and their long lifetimes made them efficient intermediate states involved in photochemical deactivation channels, potentially harmful to their biological function. Furthermore, because these properties can be influenced by the molecules conformation, when they are flexible like proteins, the second part of this work was devoted to the structure characterization. The chosen approach, the vibrational or electronic pump-probe spectroscopy in gas phase, aims to obtain detailed spectroscopic information intrinsic to the molecule, allowing an easier comparison with theoretical modelisation, in particular with quantum chemistry. However, a precise control of the system, its conformation and its temperature in gas phase require the study of model molecules, a fraction of the targeted biological molecule, relevant for the studied process, isolating them either in a supersonic expansion or in helium droplets. Three approaches have been investigated. On the structural and thermodynamical sides, the side chain tautomerism of the histidine aminoacid was studied on the 4(5)-methylimidazole molecule. The IR spectroscopy in helium droplets allowed quantifying the molecules' tautomerization thermodynamics. Moreover small peptide chains that contain the phenylalanine residue, used as peptide chain models when probing triplet states, have been characterized in a supersonic expansion by double IR/UV resonance spectroscopy in the spectral domain of the NH stretch modes. These studies have both been supported by quantum chemistry calculations at the DFT-D level. Both triplet state formation and characterization induced by the photo-excitation of peptide chains containing phenylalanine amino acid have been studied in the framework of the protein photo-degradation after UV photo-excitation. Although the phenyl ring is known for its high intersystem crossing quantum yield after the first singlet state excitation, it is still not well know for protein chains. The triplet state formation for a series of model molecules protein chains containing the phenylalanine residue has been probed with a pump-probe method. With the help of quantum chemistry calculation, these studies have shown that the relaxation of the excited phenylalanine that follows the triplet states formation depends on the side chain and of the presence of peptide bonds (-CO-NH-) in its environment. A relaxation schema have been proposed, suggesting the competition between the formation of long lived triplets states ( µs) and at least one alternative channel leading to undetected dark states. Finally, triplet states of two copper porphyrins, heme models, have been detected in pump-probe experiments. Their detection allowed to first uncovering absorption properties of these molecules and then to characterize the relaxation modes leading to the ground state after the absorption of an UV photon.
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Submitted on : Wednesday, March 30, 2022 - 3:38:21 PM
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Floriane Grollau. Modèles de molécules biologiques sondés vibrationnellement et électroniquement en phase gazeuse. Chimie théorique et/ou physique. Université Paris-Saclay, 2022. Français. ⟨NNT : 2022UPASF013⟩. ⟨tel-03624951⟩

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