LOB, Ecole Polytechnique, CNRS, INSERM, Université Paris-Saclay , Palaiseau 91128 Cedex, France.
J Am Chem Soc. 2017 Aug 23;139(33):11500-11505. doi: 10.1021/jacs.7b04586. Epub 2017 Aug 8.
Tyrosine (TyrOH) and tryptophan radicals play important roles as intermediates in biochemical charge-transfer reactions. Tryptophanyl radicals have been observed both in their protonated cation form and in their unprotonated neutral form, but to date, tyrosyl radicals have only been observed in their unprotonated form. With a genetically modified form of the flavoenzyme TrmFO as a suitable model system and using ultrafast fluorescence and absorption spectroscopy, we characterize its protonated precursor TyrOH, and we show this species to have a distinct visible absorption band and a transition moment that we suggest to lie close to the phenol symmetry axis. TyrOH is formed in ∼1 ps by electron transfer to excited flavin and decays in ∼3 ps by charge recombination. These findings imply that TyrOH oxidation does not necessarily induce its concerted deprotonation. Our results will allow disentangling of photoproduct states in flavoproteins in often-encountered complex situations and more generally are important for understanding redox chains relying on tyrosyl intermediates.
酪氨酸（TyrOH）和色氨酸自由基作为生化电荷转移反应的中间体起着重要作用。色氨酸自由基既存在于其质子化的阳离子形式中，也存在于未质子化的中性形式中，但迄今为止，酪氨酸自由基仅存在于未质子化的形式中。利用经过基因改造的黄素酶 TrmFO 作为合适的模型系统，并结合超快荧光和吸收光谱，我们对其质子化前体 TyrOH 进行了表征，结果表明该物质具有独特的可见吸收带和跃迁矩，我们推测跃迁矩接近于苯酚的对称轴。TyrOH 通过向激发态黄素的电子转移在 ∼1 ps 内形成，并通过电荷复合在 ∼3 ps 内衰变。这些发现表明，TyrOH 的氧化不一定会诱导其协同去质子化。我们的研究结果将有助于在经常遇到的复杂情况下分离黄素蛋白中的光产物状态，并且更普遍地有助于理解依赖酪氨酸中间体的氧化还原链。
