The Keto‐Enol Tautomerism of Biliverdin in Bacteriophytochrome: Could it Explain the Bathochromic Shift in the Pfr Form?,Photochemistry and Photobiology

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The Keto‐Enol Tautomerism of Biliverdin in Bacteriophytochrome: Could it Explain the Bathochromic Shift in the Pfr Form?
Photochemistry and Photobiology ( IF 2.6 ) Pub Date : 2020-11-03 , DOI: 10.1111/php.13341
Nery Villegas‐Escobar ₁ , Ricardo A. Matute _{1,

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Affiliation

Phytochromes are ubiquitous photoreceptors found in plants, eukaryotic algae, bacteria and fungi. Particularly, when bacteriophytochrome is irradiated with light, a Z‐to‐E (photo)isomerization takes place in the biliverdin chromophore as part of the Pr‐to‐Pfr conversion. This photoisomerization is concomitant with a bathochromic shift in the Q‐band. Based on experimental evidence, we studied a possible keto–enol tautomerization of BV, as an alternative reaction channel after its photoisomerization. In this contribution, the noncatalyzed and water‐assisted reaction pathways for the lactam–lactim interconversion through consecutive keto–enol tautomerization of a model BV species were studied deeply. It was found that in the absence of water molecules, the proton transfer reaction is unable to take place at normal conditions, due to large activation energies, and the endothermic formation of lactim derivatives prevents its occurrence. However, when a water molecule assists the process by catalyzing the proton transfer reaction, the activation free energy lowers considerably. The drastic lowering in the activation energy for the keto–enol tautomerism is due to the stabilization of the water moiety through hydrogen bonds along the reaction coordinate. The absorption spectra were computed for all tautomers. It was found that the UV–visible absorption bands are in reasonable agreement with the experimental data. Our results suggest that although the keto–enol equilibrium is likely favoring the lactam tautomer, the equilibrium could eventually be shifted in favor of the lactim, as it has been reported to occur in the dark reversion mechanism of bathy phytochromes.

中文翻译：

细菌光敏色素中胆绿素的酮-烯醇互变异构：能否解释 Pfr 形式的红移？

光敏色素是普遍存在于植物、真核藻类、细菌和真菌中的光感受器。特别是，当用光照射细菌植物色素时，作为 Pr 到 Pfr 转化的一部分，胆绿素发色团中会发生 Z 到 E（光）异构化。这种光异构化伴随着 Q 波段的红移。基于实验证据，我们研究了 BV 可能的酮-烯醇互变异构化，作为其光异构化后的替代反应通道。在这项贡献中，深入研究了通过模型 BV 物种的连续酮 - 烯醇互变异构化进行内酰胺 - 内酰胺互变的非催化和水辅助反应途径。发现在没有水分子的情况下，质子转移反应在正常条件下无法发生，由于活化能大，内酰胺衍生物的吸热形成阻止了它的发生。然而，当水分子通过催化质子转移反应来辅助该过程时，活化自由能会显着降低。酮-烯醇互变异构的活化能急剧降低是由于水部分通过沿反应坐标的氢键稳定。计算所有互变异构体的吸收光谱。发现紫外-可见吸收带与实验数据合理一致。我们的结果表明，尽管酮 - 烯醇平衡可能有利于内酰胺互变异构体，但平衡最终可能会转向有利于内酰胺，因为据报道它发生在深光敏色素的暗回复机制中。内酯衍生物的吸热形成阻止了它的发生。然而，当水分子通过催化质子转移反应来辅助该过程时，活化自由能会显着降低。酮-烯醇互变异构的活化能急剧降低是由于水部分通过沿反应坐标的氢键稳定。计算所有互变异构体的吸收光谱。发现紫外-可见吸收带与实验数据合理一致。我们的结果表明，尽管酮 - 烯醇平衡可能有利于内酰胺互变异构体，但平衡最终可能会转向有利于内酰胺，因为据报道它发生在深光敏色素的暗回复机制中。内酯衍生物的吸热形成阻止了它的发生。然而，当水分子通过催化质子转移反应来辅助该过程时，活化自由能会显着降低。酮-烯醇互变异构的活化能急剧降低是由于水部分通过沿反应坐标的氢键稳定。计算所有互变异构体的吸收光谱。发现紫外-可见吸收带与实验数据合理一致。我们的结果表明，尽管酮 - 烯醇平衡可能有利于内酰胺互变异构体，但平衡最终可能会转向有利于内酰胺，因为据报道它发生在深光敏色素的暗回复机制中。

更新日期：2020-11-03

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