The color-transitions of bilin photoreceptors

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Photoreceptors with open-chain tetrapyrroles (bilins) form a large class of light-dependent switches as the well-studied phytochromes. The paradigmatic phytochromes undergo photochemical double-bond isomerisation and mesoscopic soft-to-hard transition accompanied by a color transition from red to far-red. The discovery of cyanobacteriochromes (CBCRs), in some studies also named as cyanochromes or GAF-only proteins, by Ohmori and Ikeuchi in 2001 led to a change of paradigm in photophysics and photochemistry of bilin photoreceptors. Suddenly it became evident that the classical photoreceptors, the phytochromes, constitute only a special case in remarkably huge landscape of unknown photochemical parameters of bilin photochemistry. That fact minded us that, although plenty of experimental studies, the origin of the color change in phytochromes is not sufficiently understood too. Here we aim for a common view on the color physics and chemistry of bilin photoreceptors. To this end, combining experimental NMR spectroscopy with theoretical QM/MM simulations, we aim to entangle the factors controlling the color transitions in bilin proteins. Based on the X-ray structures and NMR data, we will first reconstruct the molecular orbital architecture of Pr states of canonical Cph1 phytochrome and the CBCR AnPixJg2. In the following, we will reconstruct structure and orbital architecture of the photoproducts Cph1 Pfr (far-red absorbing) and AnPixJg2 Pg (green absorbing). By comparing Pfr and Pg, we expect to obtain conceptual understanding of the color tuning.