Department of Chemistry, Oakland University, Rochester, MI 48309, USA.
Phys Chem Chem Phys. 2010;12(20):5353-68. doi: 10.1039/b925496j.
By use of ESR and UV-vis spectral studies, this work identifies the protonation states of one-electron oxidized G:C (viz. G˙+:C, G(N1–H)˙:C(+H+), G(N1–H)˙:C, and G(N2-H)˙:C) in a DNA oligomer d[TGCGCGCA]2. Benchmark ESR and UV-vis spectra from one electron oxidized 1-Me-dGuo are employed to analyze the spectral data obtained in one-electron oxidized d[TGCGCGCA]2 at various pHs. At pH ≥7, the initial site of deprotonation of one-electron oxidized d[TGCGCGCA]2 to the surrounding solvent is found to be at N1 forming G(N1–H)˙:C at 155 K. However, upon annealing to 175 K, the site of deprotonation to the solvent shifts to an equilibrium mixture of G(N1–H)˙:C and G(N2–H)˙:C. For the first time, the presence of G(N2–H)˙:C in a ds DNA-oligomer is shown to be easily distinguished from the other prototropic forms, owing to its readily observable nitrogen hyperfine coupling (Azz(N2) = 16 G). In addition, for the oligomer in H2O, an additional 8 G N2–H proton HFCC is found. This ESR identification is supported by a UV-vis absorption at 630 nm which is characteristic for G(N2–H)˙ in model compounds and oligomers. We find that the extent of photo-conversion to the C1′ sugar radical (C1′˙) in the one-electron oxidized d[TGCGCGCA]2 allows for a clear distinction among the various G:C protonation states which can not be easily distinguished by ESR or UV-vis spectroscopies with this order for the extent of photo-conversion: G˙+:C > G(N1–H)˙:C(+H+) ≫ G(N1–H)˙:C. We propose that it is the G˙+:C form that undergoes deprotonation at the sugar and this requires reprotonation of G within the lifetime of exited state
利用 ESR 和紫外可见光谱研究，本工作确定了 DNA 寡聚物 d[TGCGCGCA]2 中单电子氧化的 G:C（即 G˙+:C、G(N1–H)˙:C(+H+)、G(N1–H)˙:C 和 G(N2-H)˙:C）的质子化状态。使用单电子氧化 1-Me-dGuo 的基准 ESR 和紫外可见光谱来分析在各种 pH 值下单电子氧化 d[TGCGCGCA]2 获得的光谱数据。在 pH≥7 时，发现单电子氧化 d[TGCGCGCA]2 向周围溶剂的初始去质子化位点位于 N1 上，在 155 K 时形成 G(N1–H)˙:C。然而，在退火至 175 K 时，向溶剂的去质子化位点转移到 G(N1–H)˙:C 和 G(N2–H)˙:C 的平衡混合物。首次表明，在 ds DNA-寡聚物中 G(N2–H)˙的存在很容易与其他质子化形式区分开来，这归因于其易于观察到的氮超精细耦合（Azz(N2) = 16 G）。此外，对于 H2O 中的寡聚物，还发现了另外 8 G 的 N2–H 质子 HFCC。这种 ESR 鉴定得到了 630 nm 处紫外可见吸收的支持，这对于模型化合物和寡聚物中的 G(N2–H)˙是特征性的。我们发现，单电子氧化 d[TGCGCGCA]2 中 C1′糖自由基（C1′˙）的光转化程度允许在各种 G:C 质子化状态之间进行清晰区分，这些状态不能通过 ESR 或紫外可见光谱很容易区分，其光转化程度的顺序为：G˙+:C＞G(N1–H)˙:C(+H+)≫G(N1–H)˙:C。我们提出，正是 G˙+:C 形式在糖上发生去质子化，这需要 G 在激发态寿命内重新质子化。
