Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, USA.
Faraday Discuss. 2019 Jul 11;216(0):520-537. doi: 10.1039/c8fd00231b.
Eumelanin is a natural pigment with photoprotective and radical scavenging characteristics, which are vital for a multitude of living organisms. However, the molecular mechanisms behind these functions remain obscure, in part because eumelanin is a heterogeneous polymer composed of a complex assortment of structural and chemical domains. Despite uncertainty about its precise structure, the functional units of eumelanin are thought to include quinones in various oxidation states. Here, we investigate the photochemistry of a catechol : o-quinone heterodimer as a model system for uncovering the photoprotective roots of eumelanin. Ultrafast transient absorption measurements in the UV to near-IR spectral regions are used to identify the photochemical processes that follow selective excitation of the o-quinone in the heterodimer using 395 nm light. We find that both singlet and triplet o-quinone excited states induce hydrogen atom transfer from the catechol, forming semiquinone radical pairs that persist beyond 2.5 ns, which is the upper time limit accessible by our instrument. Furthermore, the hydrogen atom transfer reaction was found to occur 1000 times faster via the singlet channel. Excited state pathways such as these may be important in eumelanin, where similar hydrogen-bonded interfaces are believed to exist between catechol and o-quinone functional groups.
真黑素是一种具有光保护和自由基清除特性的天然色素，对多种生物至关重要。然而，这些功能背后的分子机制仍不清楚，部分原因是真黑素是一种由复杂的结构和化学结构域组成的不均匀聚合物。尽管其确切结构尚不确定，但真黑素的功能单元被认为包括各种氧化态的醌。在这里，我们研究了儿茶酚：邻醌杂二聚体作为揭示真黑素光保护根源的模型系统的光化学。使用 395nm 光选择性激发杂二聚体中的邻醌，在紫外到近红外光谱区域进行超快瞬态吸收测量，以确定随后的光化学反应。我们发现，无论是单重态还是三重态邻醌激发态都会诱导儿茶酚中的氢原子转移，形成半醌自由基对，其持续时间超过 2.5ns，这是我们仪器可达到的上限时间。此外，通过单重态通道，氢原子转移反应的速度快了 1000 倍。在真黑素中，可能存在类似的氢键界面，类似的激发态途径可能很重要，人们认为儿茶酚和邻醌官能团之间存在类似的氢键界面。
