Photoinduced isomerization of the Photoactive Yellow Protein chromophore: a detailed theoretical investigation

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In this project, theory - including large-scale electronic structure and dynamical calculations - will be applied to provide detailed insight into the initial photoevents in a biological photoreceptor - the Photoactive Yellow Protein (PYP). We focus on the very earliest events in the PYP photocycle, related to the trans-to-cis isomerization of the -C=C- double bond of the PYP co-factor, the deprotonated p-hydroxycinnamoyl chromophore. The mechanism and dynamics of the isomerization process can be characterized as highly complex due to the involvement of several coordinates (two torsions and one bending mode) and the strong impact of the chromophore’s local environment. Our theoretical analysis will rest on quantum chemistry electronic structure and (semi-classical and quantum) dynamical calculations for the chromophore in the native (protein) environment and in a specific (model) environment. For the chromophore in the protein environment, the main emphasis will be placed on (i) establishing inhomogeneities in the dark-state of PYP and (ii) scrutinizing the initial phase of the chromophore’s isomerization. For the model environment, the main focus will be put on elucidating the role of the hydrogen out-of-plane (HOOP) coordinate in the isomerization process. Rationalization of these issues should illuminate such yet obscure aspects as the photoactive (isomerizable) form of PYP, implication/interplay of the isomerization coordinates, the reaction pathway and mechanism of the process. Acquiring control over the chromophore’s isomerization is considered an exciting perspective.