Department of Chemistry, University of York, Heslington, York YO10 5DD, UK.
Molecules. 2018 Aug 14;23(8):2036. doi: 10.3390/molecules23082036.
Flavin chromophores play key roles in a wide range of photoactive proteins, but key questions exist in relation to their fundamental spectroscopic and photochemical properties. In this work, we report the first gas-phase spectroscopy study of protonated alloxazine (AL∙H⁺), a model flavin chromophore. Laser photodissociation is employed across a wide range (2.34⁻5.64 eV) to obtain the electronic spectrum and characterize the photofragmentation pathways. By comparison to TDDFT quantum chemical calculations, the spectrum is assigned to two AL∙H⁺ protomers; an N5 (dominant) and O4 (minor) form. The protomers have distinctly different spectral profiles in the region above 4.8 eV due to the presence of a strong electronic transition for the O4 protomer corresponding to an electron-density shift from the benzene to uracil moiety. AL∙H⁺ photoexcitation leads to fragmentation via loss of HCN and HNCO (along with small molecules such as CO₂ and H₂O), but the photofragmentation patterns differ dramatically from those observed upon collision excitation of the ground electronic state. This reveals that fragmentation is occurring during the excited state lifetime. Finally, our results show that the N5 protomer is associated primarily with HNCO loss while the O4 protomer is associated with HCN loss, indicating that the ring-opening dynamics are dependent on the location of protonation in the ground-state molecule.
黄素发色团在广泛的光活性蛋白质中发挥着关键作用，但在它们的基本光谱和光化学性质方面仍存在关键问题。在这项工作中，我们报告了质子化蝶呤（AL·H⁺）的第一个气相光谱研究，这是一种模型黄素发色团。我们使用激光光解在很宽的范围内（2.34⁻5.64 eV）进行了研究，以获得电子光谱并表征光解途径。通过与 TDDFT 量子化学计算的比较，该光谱被分配给两个 AL·H⁺二聚体；N5（主要）和 O4（次要）形式。由于 O4 二聚体存在一个对应于从苯到尿嘧啶部分的电子密度转移的强电子跃迁，因此在 4.8 eV 以上的区域中，二聚体具有明显不同的光谱轮廓。AL·H⁺光激发导致通过 HCN 和 HNCO 的损失而发生碎片化（以及 CO₂和 H₂O 等小分子），但光解碎片模式与通过基态电子激发碰撞所观察到的显著不同。这表明碎片化是在激发态寿命期间发生的。最后，我们的结果表明，N5 二聚体主要与 HNCO 的损失有关，而 O4 二聚体与 HCN 的损失有关，这表明开环动力学取决于基态分子中质子化的位置。
