Photoisomerization of phytochrome chromophore models: An XMS-CASPT2 study

09 September 2022, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Phytochromes are a superfamily of photoreceptors that harbor linear tetrapyrroles as chromophores. Upon light illumination, the linear tetrapyrrole chromophore undergoes a double bond isomerization which starts a photocycle. In this work, we studied the photoisomerization of chromophore models designed based on the C- and D-rings of the phycocyanobilin (PCB) chromophore. In total, five different models with varying substitutions were investigated. Firstly, the vertical excitation energies were benchmarked using different computational methods to establish the relative order of the excited states. Based on these calculations, we computed the photoisomerization profiles using the extended multi-state (XMS) version of the CASPT2 method. The profiles were obtained for both the clockwise and counterclockwise rotations of the C15=C16 bond in the Z and E isomers using a linear interpolation of internal coordinates between the Franck-Condon and MECI geometries. In the minimal chromophore model that lacks the substitutions at the pyrrole rings, the isomerization involves both C14-C15 and C15=C16 bonds of the methine bridge between the C- and D-rings, resembling the hula-twist motion. The MECIs are characterized by a partial charge transfer between the two pyrrole rings pointing towards a twisted intramolecular charge transfer. Systematic introduction of substituents leads to an increase in the steric repulsion between the two pyrrole rings causing a pretwist of the dihedral around the C15=C16 bond, which creates a preference for the counterclockwise isomerization. Upon introduction of the carbonyl group at the D-ring the charge transfer has increased. This changes the isomerization mechanism from hula-twist to one-bond flip.

Keywords

linear tetrapyrrole
photoisomerization
conical intersection
XMS-CASPT2
phytochrome

Supplementary materials

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Title
Supplementary information of "Photochemistry of phytochrome chromophore models: An XMS-CASPT2 study"
Description
Vertical excitation energies, excited state relaxed scans and a summary of important parameters from LIICs can be found.
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