Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
University of Chinese Academy of Sciences, Beijing 100049, China.
Sci Adv. 2017 Jun 2;3(6):e1700171. doi: 10.1126/sciadv.1700171. eCollection 2017 Jun.
Although the radical ion pair has been frequently invoked as a key intermediate in DNA oxidative damage reactions and photoinduced electron transfer processes, the unambiguous detection and characterization of this species remain formidable and unresolved due to its extremely unstable nature and low concentration. We use the strategy that, at cryogenic temperatures, the transient species could be sufficiently stabilized to be detectable spectroscopically. By coupling the two techniques (the cryogenic stabilization and the time-resolved laser flash photolysis spectroscopy) together, we are able to capture the ion-pair transient G⋯Cl in the chlorine radical-initiated DNA guanine (G) oxidation reaction, and provide direct evidence to ascertain the intricate type of addition/charge separation mechanism underlying guanine oxidation. The unique spectral signature of the radical ion-pair G⋯Cl is identified, revealing a markedly intense absorption feature peaking at 570 nm that is distinctive from G alone. Moreover, the ion-pair spectrum is found to be highly sensitive to the protonation equilibria within guanine-cytosine base pair (G:C), which splits into two resolved bands at 480 and 610 nm as the acidic proton transfers along the central hydrogen bond from G to C. We thus use this exquisite sensitivity to track the intrabase-pair proton transfer dynamics in the double-stranded DNA oligonucleotides, which is of critical importance for the description of the proton-coupled charge transfer mechanisms in DNA.
虽然自由基离子对作为 DNA 氧化损伤反应和光诱导电子转移过程中的关键中间体被频繁提及，但由于其极不稳定的性质和低浓度，该物种的明确检测和特征描述仍然具有挑战性且尚未解决。我们使用低温策略，使瞬态物种能够充分稳定以进行光谱检测。通过将两种技术（低温稳定和时间分辨激光闪光光解光谱学）结合使用，我们能够捕获氯自由基引发的 DNA 鸟嘌呤（G）氧化反应中的离子对瞬态 G⋯Cl，并提供直接证据确定鸟嘌呤氧化所涉及的复杂加成/电荷分离机制的类型。鉴定出自由基离子对 G⋯Cl 的独特光谱特征，显示出在 570nm 处显著增强的吸收特征，与单独的 G 不同。此外，发现离子对光谱对鸟嘌呤-胞嘧啶碱基对（G:C）内的质子平衡高度敏感，当酸性质子沿中央氢键从 G 转移到 C 时，它会分裂成两个分辨率的带，分别在 480nm 和 610nm 处。因此，我们利用这种极高的灵敏度来跟踪双链 DNA 寡核苷酸中的碱基内质子转移动力学，这对于描述 DNA 中的质子耦合电荷转移机制至关重要。
