Department of Chemistry, Humboldt-Universität zu Berlin, Berlin, Germany.
J Phys Chem B. 2013 May 9;117(18):5441-7. doi: 10.1021/jp312571d. Epub 2013 Apr 30.
Excited-state protonation of riboflavin in the oxidized form is studied in water. In the -1 < pH < 2 range, neutral and N(1)-protonated riboflavin coexist in the electronic ground state. Transient absorption shows that the protonated form converts to the ground state in <40 fs after optical excitation. Broadband fluorescence upconversion is therefore used to monitor solvation and protonation of the neutral species in the excited singlet state exclusively. A weak fluorescence band around 660 nm is assigned to the product of protonation at N(5). Its radiative rate and quantum yield relative to neutral riboflavin are estimated. Protonation rates agree with proton diffusion times for H(+) concentrations below 5 M but increase at higher acidities, where the average proton distance is below the diameter of the riboflavin molecule.
氧化态核黄素的激发态质子化在水中进行研究。在 -1 < pH < 2 的范围内，电子基态中存在中性和 N(1)-质子化核黄素。瞬态吸收表明，光激发后质子化形式在 <40 fs 内转换回基态。因此，宽带荧光上转换用于专门监测中性物质在激发单线态下的溶剂化和质子化。约 660nm 的弱荧光带被分配给 N(5)质子化的产物。估计了其相对中性核黄素的辐射速率和量子产率。质子化速率与 H(+)浓度低于 5 M 时的质子扩散时间一致，但在更高酸度下增加，此时质子的平均距离低于核黄素分子的直径。
