Cyanobacteriochromes (CBCRs) are photoreceptor proteins that photoconvert between two parent states and thereby regulate various biological processes. An intriguing property is their variable ultraviolet-visible (UV-vis) absorption that covers the entire spectral range from the far-red to the near-UV region and thus makes CBCRs promising candidates for optogenetic applications. Here, we have studied Slr1393, a CBCR that photoswitches between red- and green-absorbing states (Pr and Pg, respectively). Using UV-vis absorption, fluorescence, and resonance Raman (RR) spectroscopy, a further orange-absorbing state O600 that is in thermal equilibrium with Pr was identified. The different absorption properties of the three states were attributed to the different lengths of the conjugated π-electron system of the phycocyanobilin chromophore. In agreement with available crystal structures and supported by quantum mechanics/molecular mechanics (QM/MM) calculations, the most extended conjugation holds for Pr whereas it is substantially reduced in Pg. Here, the two outer pyrrole rings D and A are twisted out of the plane defined by inner pyrrole rings B and C. For the O600 state, the comparison of the experimental RR spectra with QM/MM-calculated spectra indicates a partially distorted ZZZssa geometry in which ring A is twisted while ring D and the adjacent methine bridge display essentially the same geometry as Pr. The quantitative analysis of temperature-dependent spectra yields an enthalpy barrier of ∼30 kJ/mol for the transition from Pr to O600. This reaction is associated with the movement of a conserved tryptophan residue from the chromophore binding pocket to a solvent-exposed position.

中文翻译：

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红色，橙色，绿色：蓝细菌色素中取决于光和温度的颜色调整。

蓝细菌色素（CBCRs）是在两个亲本状态之间进行光转换，从而调节各种生物学过程的感光蛋白。令人着迷的特性是其可变的紫外线-可见光（UV-vis）吸收范围涵盖了从远红外到近紫外区域的整个光谱范围，因此使CBCR成为光遗传学应用的有希望的候选者。在这里，我们研究了Slr1393，这是一种在红色和绿色吸收状态（分别为Pr和Pg）之间进行光开关的CBCR。使用紫外可见吸收，荧光和共振拉曼（RR）光谱，鉴定了另一种与Pr热平衡的橙色吸收状态O600。三种状态吸收特性的不同归因于藻蓝素生色团的共轭π电子系统的长度不同。与可用的晶体结构一致，并得到量子力学/分子力学（QM / MM）计算的支持，Pr的结合扩展最广，而Pg则大大降低。在此，两个外吡咯环D和A都扭曲到由内吡咯环B和C限定的平面内。对于O600状态，实验RR光谱与QM / MM计算光谱的比较表明ZZZssa几何形状部分失真其中，环A扭曲，而环D和相邻的次甲基桥具有与Pr基本上相同的几何形状。对温度相关光谱的定量分析产生了从Pr到O600跃迁的约30 kJ / mol的焓垒。