Gabelica Valérie, Rosu Frédéric, Tabarin Thibault, Kinet Catherine, Antoine Rodolphe, Broyer Michel, De Pauw Edwin, Dugourd Philippe
Laboratoire de Spectrométrie de Masse, Université de Liège, Institut de Chimie Bat B6c, B-4000 Liège, Belgium.
J Am Chem Soc. 2007 Apr 18;129(15):4706-13. doi: 10.1021/ja068440z. Epub 2007 Mar 23.
DNA multiply charged anions stored in a quadrupole ion trap undergo one-photon electron ejection (oxidation) when subjected to laser irradiation at 260 nm (4.77 eV). Electron photodetachment is likely a fast process, given that photodetachment is able to compete with internal conversion or radiative relaxation to the ground state. The DNA [6-mer]3- ions studied here show a marked sequence dependence of electron photodetachment yield. Remarkably, the photodetachment yield (dG6 > dA6 > dC6 > dT6) is inversely correlated with the base ionization potentials (G < A < C < T). Sequences with guanine runs show increased photodetachment yield as the number of guanine increases, in line with the fact that positive holes are the most stable in guanine runs. This correlation between photodetachment yield and the stability of the base radical may be explained by tunneling of the electron through the repulsive Coulomb barrier. Theoretical calculations on dinucleotide monophosphates show that the HOMO and HOMO-1 orbitals are localized on the bases. The wavelength dependence of electron detachment yield was studied for dG63-. Maximum electron photodetachment is observed in the wavelength range corresponding to base absorption (260-270 nm). This demonstrates the feasibility of gas-phase UV spectroscopy on large DNA anions. The calculations and the wavelength dependence suggest that the electron photodetachment is initiated at the bases and not at the phosphates. This also indicates that, although direct photodetachment could also occur, autodetachment from excited states, presumably corresponding to base excitation, is the dominant process at 260 nm. Excited-state dynamics of large DNA strands still remains largely unexplored, and photo-oxidation studies on trapped DNA multiply charged anions can help in bridging the gap between gas-phase studies on isolated bases or base pairs and solution-phase studies on full DNA strands.
存储在四极离子阱中的DNA多电荷阴离子在260 nm（4.77 eV）的激光照射下会发生单光子电子发射（氧化）。鉴于光电子发射能够与内部转换或向基态的辐射弛豫竞争，电子光剥离可能是一个快速过程。此处研究的DNA [6聚体]3-离子显示出电子光剥离产率具有明显的序列依赖性。值得注意的是，光剥离产率（dG6 > dA6 > dC6 > dT6）与碱基电离势（G < A < C < T）呈负相关。含有鸟嘌呤序列的光剥离产率会随着鸟嘌呤数量的增加而提高，这与正电荷空穴在鸟嘌呤序列中最稳定这一事实相符。光剥离产率与碱基自由基稳定性之间的这种相关性可能是由电子隧穿通过排斥性库仑势垒来解释的。对二核苷酸单磷酸的理论计算表明，最高占据分子轨道（HOMO）和HOMO - 1轨道定域在碱基上。研究了dG63-的电子剥离产率的波长依赖性。在对应于碱基吸收的波长范围（260 - 270 nm）内观察到最大电子光剥离。这证明了对大型DNA阴离子进行气相紫外光谱分析的可行性。计算结果和波长依赖性表明，电子光剥离是在碱基处而非磷酸基团处引发的。这也表明，尽管也可能发生直接光剥离，但从激发态的自剥离（大概对应于碱基激发）是260 nm处的主要过程。大型DNA链的激发态动力学在很大程度上仍未得到充分探索，对捕获的DNA多电荷阴离子进行光氧化研究有助于弥合对孤立碱基或碱基对的气相研究与对完整DNA链的溶液相研究之间的差距。
