Thèse quels Facteurs Déterminent le Destin de l'Énergie au Cours de la Photosynthèse H/F

Université Paris-Saclay GS Chimie

Établissement : Université Paris-Saclay GS Chimie
École doctorale : Sciences Chimiques : Molécules, Matériaux, Instrumentation et Biosystèmes
Laboratoire de recherche : Service de Bioénergétique, Biologie Structurale et Mécanismes
Direction de la thèse : Manuel LLANSOLA-PORTOLES ORCID 0000000280659459
Début de la thèse : 2026-10-01
Date limite de candidature : 2026-03-31T23:59:59

Ce projet de doctorat se situe à l'interface entre la biologie et la physique et vise à révéler la structure des états excités qui contrôlent le destin de l'énergie au cours des premières étapes de la photosynthèse. L'étudiant(e) utilisera le premier appareil au monde de Raman stimulé résolu en temps femtoseconde avec une résonance ajustable pour étudier la structure des états excités et leur dynamique dans les processus ultrarapides de la photosynthèse. Cette technique permettra d'observer de manière sélective les modes vibrationnels caractéristiques de chaque état excité afin de comprendre leur rôle dans le transfert d'énergie. Pour cela, l'étudiant(e) caractérisera la structure des états excités de pigments isolés, tels que les chlorophylles et les caroténoïdes, en solution. Une fois ces pigments isolés étudiés, il/elle examinera des complexes photosynthétiques bactériens, en particulier les protéines LH2 de Rhodobacter sphaeroides, utilisées comme modèle simplifié de protéines de récolte de lumière, pour comparer les paysages vibrationnels des chlorophylles liées à ceux des pigments isolés. Cela permettra de déterminer comment la structure des états excités optimise le transfert d'énergie d'excitation.
Ensuite, en étudiant des protéines photosynthétiques plus complexes capables de transférer ou de désactiver l'énergie d'excitation, nous décrirons comment la structure des états excités ajuste l'équilibre entre transfert d'énergie et photoprotection. Enfin, nous modéliserons le transfert d'énergie d'excitation à l'aide de calculs quantiques (DFT et TD-DFT) en intégrant les modes vibrationnels identifiés pour développer un premier modèle complet du transfert ultrarapide d'énergie dans la photosynthèse. L'étudiant(e) participera activement aux discussions et à l'élaboration de ce modèle, en collaboration avec le groupe du Prof. Valkunas (Université de Vilnius).
Ce projet a pour objectif final de produire une description structurale et mécanistique détaillée des premières étapes de la conversion énergétique dans la photosynthèse. Ces résultats seront d'une importance capitale pour orienter la prochaine génération de technologies solaires, en s'inspirant des stratégies optimisées par la nature pour une capture et une conversion efficaces de l'énergie solaire.

Photosynthesis converts solar energy into chemical energy, reducing CO2 to sugar. This process, vital for our planet, transforms 90 TW of solar energy into biomass and oxygen. Decoding the sophisticated molecular mechanisms of photosynthesis could revolutionize our solar energy technology. The initial stages of photosynthesis involve three ultrafast processes involving excited states: (i) Light-harvesting proteins absorb solar photons; (ii) excitation energy transfers to a reaction center within ~20 picoseconds; (iii) charge separation occurs and is stabilized in the reaction center. Under optimal conditions, this process involving hundreds of excitation transfer steps with a quantum yield approaching unity. This extremely efficient energy transfer occurs through the excited states of chlorophylls and carotenoids bind to specialized proteins. This exceptionally high quantum yield is related to the fact that the energy transfer is much faster than predicted by the most advanced quantum mechanical models. This was initially rationalized by the delocalization of excitation across multiple pigments and the appearance of quantum effects, suggesting that the wavelike nature of energy allows for simultaneous exploration of multiple pathways. However, this hypothesis has been gradually abandoned in favor of a vibrational coupling model, which proposes that specific vibrational modes can couple with electronic states, providing additional pathways for ultrafast energy flow. Conversely, there are not conclusive experimental evidences that can validate the vibrational assisted model. Crystallography and cryo-electron microscopy provide invaluable structural frameworks, but they lack the resolution to capture the dynamic subtleties whereas spectroscopic techniques provide ultrafast temporal resolution but limited structural information about excited states. This creates a knowledge gap between static structure and ultrafast dynamics, necessitating advanced multidimensional spectroscopic techniques and theoretical modeling to bridge our understanding of these processes.
We have designed the first apparatus specifically conceived to access to the structure of the excited states. We have taken advantage of femtosecond stimulated Raman spectroscopy (FSRS) and combined with a Raman pulse selection to achieve selectivity of the excited states. Brief, FSRS has been used with success leading to breakthroughs, but with limited information due to their current lack of selectivity. Whereas FSRS experiments are adequate to follow the vibrational modes of excited states, their interpretation is challenging (if not impossible) in complexes matrices. To overcome this problem, we have conceived a FSRS apparatus with the capabilities to tune the Raman pump wavelength. This is the only system in the world capable of taking advantage of the resonant phenomenon in Raman spectroscopy to observe selectively the excited states of interest among a manifold of carotenoids and chlorophylls excited states evolving simultaneously.

This project aims to reveal the role and structure of excited states in the photosynthetic energy transfer process. We will obtain a detailed picture of the ground and excited states wavefunction organization, which will set the foundations for the development of new solar energy capturing systems.

This project will take advantage of the natural biodiversity, comparing a wide range of photosynthetic antenna complexes from plants, purple bacteria and diatoms (e.g., LHCII, LHII, and FCP). These complexes will be isolated by biochemists within the LBMS group or in collaboration with international partners, such as Claudia Büchel's group at Goethe University Frankfurt. While the student will not perform the purifications, he/she will be trained in handling and manipulating these biological samples.
The student will be trained in performing experiments using mainly the tunable-FSRS apparatus, in addition to femtosecond transient absorption, and fs-to-ns time resolved fluorescence.
1.The student will characterize the vibrational modes of the manifold of excited states of isolated (bacterio)chlorophylls and carotenoids in solvents of different polarizability.
2.The student will characterize the structure of the excited states in LH2 proteins from Rhodobacter sphaeroides. They will compare the vibrational modes of this antenna with the vibrational modes of the isolated pigments, and analyze their involvement in the energy transfer process.
3.The student will carry out the detailed characterization of the excited states controlling the excitation energy fate of LHCII antennas (plants) and FCP antennas (diatoms) at different degrees of NPQ (photoprotection).
4.The student will participate in the discussion to create the model describing the excitation energy transfer using quantum calculations (DFT and TD-DFT) by introducing the identified vibrational modes participating in the process.