Roberts Gareth M, Marroux Hugo J B, Grubb Michael P, Ashfold Michael N R, Orr-Ewing Andrew J
School of Chemistry, University of Bristol , Cantock's Close, Bristol BS8 1TS, United Kingdom.
J Phys Chem A. 2014 Nov 26;118(47):11211-25. doi: 10.1021/jp508501w. Epub 2014 Oct 24.
A combination of ultrafast transient electronic absorption spectroscopy (TEAS) and transient vibrational absorption spectroscopy (TVAS) is used to investigate whether photoinduced N–H bond fission, mediated by a dissociative 1πσ(*) state, is active in aqueous adenine (Ade) at 266 and 220 nm. In order to isolate UV/visible and IR spectral signatures of the adeninyl radical (Ade[-H]), formed as a result of N–H bond fission, TEAS and TVAS are performed on Ade in D2O under basic conditions (pD = 12.5), which forms Ade-H anions via deprotonation at the N7 or N9 sites of Ade's 7H and 9H tautomers. At 220 nm we observe one-photon detachment of an electron from Ade-H, which generates solvated electrons (eaq(-)) together with Ade[-H] radicals, with clear signatures in both TEAS and TVAS. Additional wavelength dependent TEAS measurements between 240–260 nm identify a threshold of 4.9 ± 0.1 eV (∼250 nm) for this photodetachment process in D2O. Analogous TEAS experiments on aqueous Ade at pD = 7.4 generate a similar photoproduct signal together with eaq(-) after excitation at 266 and 220 nm. These eaq(-) are born from ionization of Ade, together with Ade(+) cations, which are indistinguishable from Ade[-H] radicals in TEAS. Ade(+) and Ade[-H] are found to have different signatures in TVAS and we verify that the pD = 7.4 photoproduct signal observed in TEAS following 220 nm excitation is solely due to Ade(+) cations. Based on these observations, we conclude that: (i) N–H bond fission in aqueous Ade is inactive at wavelengths ≥220 nm; and (ii) if such a channel exists in aqueous solution, its threshold is strongly blue-shifted relative to the onset of the same process in gas phase 9H-Ade (≤233 nm). In addition, we extract excited state lifetimes and vibrational cooling dynamics for 9H-Ade and Ade-H. In both cases, excited state lifetimes of <500 fs are identified, while vibrational cooling occurs within a time frame of 4–5 ps. In contrast, 7H-Ade is confirmed to have a longer excited state lifetime of ∼5–10 ps through both TEAS and TVAS.
结合超快瞬态电子吸收光谱（TEAS）和瞬态振动吸收光谱（TVAS）来研究由离解的1πσ(*)态介导的光诱导N-H键裂变在266和220nm的水相腺嘌呤（Ade）中是否活跃。为了分离由于N-H键裂变形成的腺嘌呤自由基（Ade[-H]）的紫外/可见和红外光谱特征，在碱性条件（pD = 12.5）下于重水中对Ade进行TEAS和TVAS，这通过Ade的7H和9H互变异构体在N7或N9位点去质子化形成Ade-H阴离子。在220nm处，我们观察到电子从Ade-H的单光子脱离，这产生了溶剂化电子（eaq(-)）以及Ade[-H]自由基，在TEAS和TVAS中都有清晰的特征。在240-260nm之间进行的额外波长相关TEAS测量确定了在重水中该光脱离过程的阈值为4.9±0.1eV（~250nm）。在pD = 7.4的水相Ade上进行的类似TEAS实验在266和220nm激发后产生了类似的光产物信号以及eaq(-)。这些eaq(-)源于Ade的电离以及Ade(+)阳离子，它们在TEAS中与Ade[-H]自由基无法区分。发现Ade(+)和Ade[-H]在TVAS中有不同的特征，并且我们验证了在220nm激发后TEAS中观察到的pD = 7.4光产物信号仅归因于Ade(+)阳离子。基于这些观察结果，我们得出结论：（i）水相Ade中的N-H键裂变在波长≥220nm时不活跃；（ii）如果这种通道存在于水溶液中，其阈值相对于气相9H-Ade中相同过程的起始（≤233nm）有强烈的蓝移。此外，我们提取了9H-Ade和Ade-H的激发态寿命和振动冷却动力学。在这两种情况下，都确定了激发态寿命<500fs，而振动冷却发生在4-5ps的时间范围内。相比之下，通过TEAS和TVAS都证实7H-Ade具有约5-10ps的更长激发态寿命。
