Comparing the ultraviolet photostability of azole chromophores
Comparing the ultraviolet photostability of azole chromophores
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DOI：
10.1039/c2sc01000c
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发表时间：
2012-01-01
期刊：
影响因子：
8.4
通讯作者：
Stavros, Vasilios G.
中科院分区：
文献类型：
--
作者：
Roberts, Gareth M.;Williams, Craig A.;Stavros, Vasilios G.
超快时间分辨速度图成像方法用于询问唑异构体咪唑和吡唑中H原子消除的时间尺度，其中前者是生物分子中的普遍部分，其在吸收紫外线（UV）辐射后表现出高度的光稳定性（例如，G. DNA碱基和芳香族氨基酸）。这里呈现的结果，第一次，提请重点在这两个杂芳族化合物的统计H-原子消除动力学，这是由振动热基态（S-0）分子，形成超快内部转换从最初填充的激发电子态（（1）π π * 或（1）π σ *）在200 nm。对咪唑的测量表明，在这些实验的时间窗口（延伸到600 ps）上，统计H原子消除是最小的，并且发生在>270 ps的时间尺度上。相反，吡唑显示出显著的统计H-原子产率为600 ps，时间常数为165 +/- 30 ps。这突出了统计单分子解离动力学，在这些时间尺度上，不能用传统的RRKM理论解释。对这两种物质的氘代同位素异构体的额外实验也表明，在咪唑中，统计H-原子产生局限于N-H键裂变，而在吡唑中，统计C-H和N-H裂解之间的比例约为1：1，并且这两个过程的相关时间常数分别为151 +/- 20 ps和193 +/- 35 ps。我们假设，相对于咪唑，在吡唑中观察到的高比例的快速不可逆C-H裂变可能导致在特定的生物环境中形成有毒的自由基，而仅限于N-H坐标的统计解离可能假设通过氢键网络中的H原子“笼住”和“重组”动力学淬灭UV光损伤产率（例如，G.蛋白质二级结构）。
Ultrafast time-resolved velocity map imaging methods are used to interrogate the timescales for H-atom elimination in the azole isomers imidazole and pyrazole, the former of which is a prevalent moiety in biomolecules that exhibit a high degree of photostability following the absorption of ultraviolet (UV) radiation (e. g. DNA bases and aromatic amino acids). The results presented here, for the first time, draw focus on the statistical H-atom elimination dynamics in these two heteroaromatics, which result from vibrationally hot ground state (S-0) molecules that are formed following ultrafast internal conversion from an initially populated excited electronic state ((1)pi pi* or (1)pi sigma*) at 200 nm. Measurements on imidazole suggest that statistical H-atom elimination is minimal over the temporal window of these experiments (which extends to 600 ps) and occurs on a timescale of >270 ps. Conversely, pyrazole shows a significant statistical H-atom yield by 600 ps with a time constant of 165 +/- 30 ps. This highlights statistical unimolecular dissociation dynamics which, on these timescales, cannot be interpreted with traditional RRKM theory. Additional experiments on deuterated isotopomers of the two species also reveal that in imidazole statistical H-atom generation is localized to N-H bond fission, while in pyrazole there is approximately a 1 : 1 ratio between statistical C-H and N-H cleavage, and the two processes have associated time constants of 151 +/- 20 ps and 193 +/- 35 ps, respectively. We postulate that the observed high fraction of rapid irreversible C-H fission in pyrazole, relative to imidazole, may lead to the formation of toxic free radicals within specific biological environments, whereas statistical dissociation, restricted to only the N-H coordinate, may hypothetically quench UV photodamage yields via H-atom 'caging' and 'recombination' dynamics in hydrogen bonded networks (e. g. secondary protein structures).