Phycocyanobilin (PCB)-binding proteins, including cyanobacteriochromes and phytochromes, function as photoreceptors and exhibit a wide range of absorption maximum wavelengths. To elucidate the color-tuning mechanisms among these proteins, we investigated seven crystal structures of six PCB-binding proteins: Anacy_2551g3, AnPixJg2, phosphorylation-responsive photosensitive histidine kinase, RcaE, Sb.phyB(PG)-PCB, and Slr1393g3. Employing a quantum chemical/molecular mechanical approach combined with a polarizable continuum model, our analysis revealed that differences in absorption wavelengths among PCB-binding proteins primarily arise from variations in the shape of the PCB molecule itself, accounting for a ∼150 nm difference. Remarkably, calculated excitation energies sufficiently reproduced the absorption wavelengths of these proteins spanning ∼200 nm, including 728 nm for Anacy_2551g3. However, assuming the hypothesized lactim conformation resulted in a significant deviation from the experimentally measured absorption wavelength for Anacy_2551g3. The significantly red-shifted absorption wavelength of Anacy_2551g3 can unambiguously be explained by the significant overlap of molecular orbitals between the two pyrrole rings at both edges of the PCB chromophore without the need to hypothesize lactim formation.

中文翻译：

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藻蓝蛋白结合蛋白之间吸收波长变化的分子起源


藻蓝蛋白 （PCB） 结合蛋白，包括蓝细菌色素和植物色素，起光感受器的作用，并表现出广泛的吸收最长范围。为了阐明这些蛋白质之间的颜色调节机制，我们研究了六种 PCB 结合蛋白的七种晶体结构：Anacy_2551g3、AnPixJg2、磷酸化反应性光敏组氨酸激酶、RcaE、Sb.phyB（PG）-PCB 和 Slr1393g3。采用量子化学/分子力学方法结合可极化连续体模型，我们的分析表明，PCB 结合蛋白之间吸收波长的差异主要是由PCB 分子本身，占 ∼150 nm 的差异。值得注意的是，计算的激发能量充分再现了这些蛋白质的吸收波长，范围跨度为 ∼200 nm，包括 Anacy_2551g3的 728 nm。然而，假设假设的 lactim 构象导致 Anacy_2551g3 与实验测量的吸收波长显着偏差。Anacy_2551g3 的显著红移吸收波长可以明确地解释为 PCB 发色团两个边缘的两个吡咯环之间的分子轨道显着重叠，而无需假设内酰胺的形成。