The role of the local environment in ultrafast non-adiabatic dynamics: primary events in the photocycle of the yellow protein

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In this project, theory - including electronic structure and dynamical calculations, as well as theory formulation and modelling - will be combined with femtosecond time-resolved experiments to understand the initial photoevents for the photoactive yellow protein (PYP) at the molecular level. We focus on the very earliest events observed in the photocycle (~700 fs) that are believed to involve a partial trans-to-cis isomerization of the ethylenic double bond in the p-hydroxycinnamyl anion chromophore, along with a twisting motion of the thioester linkage by which the chromophore is bound to the protein. The dynamics is strongly dependent on the local constraints of the protein environment (primarily due to electrostatic effects and hydrogen bonding): in fact, the dynamics of either the denatured PYP or a slightly altered chromophore in solution is much slower (10 ps). Our theoretical analysis will be based on electronic structure calculations for the chromophore, and analysis of the non-adiabatic ultrafast dynamics which is presumably related to the presence of a conical intersection. The theory will be informed by previous experiments on the native protein and new experimental investigations of the PYP chromophore in solution and in modified nanospace environments. Special emphasis will be placed on extracting the key microscopic features which make these events so sensitive to the local environment and -- very importantly -- which could be more generally applicable to other biological photosystems. Indeed, related questions are of importance in a wide range of biologically relevant photophysical processes, in particular the photocycles of visual rhodopsin and bacteriorhodopsin. An essential aspect of the project is the necessary development of a theoretical framework to describe, and to calculate, the dynamics of the strongly coupled quantum chromophore/environment supermolecular system. This will require a strongly interacting combination of expertises on the theory side (gas phase dynamical theory, condensed phase reaction theory) from the French and German teams involved in this project.