The color-transitions of bilin photoreceptors

具有开链四吡咯（胆磷脂）的光感受器与被充分研究的光敏色素一样，形成了一大类依赖光的开关。典型光敏色素经历光化学双键异构化和介观软-硬转变，并伴有从红色到远红色的颜色转变。2001年，Ohmori和Ikeuchi发现了蓝细菌色素（CBCRs），在一些研究中也被称为cyanochromes或GAF-only蛋白，导致了十亿蛋白光感受器的光物理学和光化学范式的改变。突然间，很明显，经典的光感受器，光敏色素，只是一个特殊的情况下，巨大的未知的光化学参数的十亿光化学景观。这一事实提醒我们，尽管进行了大量的实验研究，但光敏色素颜色变化的起源也没有得到充分的了解。本文旨在对十亿蛋白光感受器的颜色物理和化学性质有一个共同的认识。为此，我们将实验核磁共振光谱与理论QM/MM模拟相结合，旨在纠缠控制十亿蛋白颜色转变的因素。基于x射线结构和核磁共振数据，我们将首先重建典型Cph1光敏色素和CBCR AnPixJg2的Pr态的分子轨道结构。接下来，我们将重建光产物Cph1 Pfr（远红色吸收）和AnPixJg2 Pg（绿色吸收）的结构和轨道结构。通过比较Pfr和Pg，我们期望获得对颜色调谐的概念性理解。

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.