Department of Chemistry, University of Gdańsk, Gdańsk, Poland.
J Phys Chem B. 2011 Apr 21;115(15):4532-7. doi: 10.1021/jp201028a. Epub 2011 Mar 28.
The UV electronic transition energies and their oscillator strengths for two stacked dimers having B-DNA geometries and consisting of 5-bromouracil ((Br)U) and a purine base were studied at the MS-CASPT2/6-311G(d) level with an active space of 12 orbitals and 12 electrons. The calculated energy of the first vertical (π,π*) transitions for the studied dimers remain in fair agreement with the maxima in the difference spectra measured for duplexes with the 5'-A(Br)U-3' or 5'-G(Br)U-3' sequences. Our MS-CASPT2 results show that the charge transfer (CT) states in which an electron is transferred from A/G to (Br)U are located at much higher energies than the first (π,π*) transitions, which involve local excitation (LE) of (Br)U. Moreover, CT transitions are characterized by small oscillator strengths, which implies that they could not be excited directly. The results of the current studies suggest that the formation of the reactive uracil-5-yl radical in DNA is preceded by the formation of the highly oxidative LE state of (Br)U, which is followed by electron transfer, presumably from guanine.
采用 MS-CASPT2/6-311G(d) 方法，在包含 12 个轨道和 12 个电子的活性空间中，研究了具有 B-DNA 构象的两个堆叠二聚体的 UV 电子跃迁能及其振子强度，这两个二聚体由 5-溴尿嘧啶（(Br)U）和嘌呤碱基组成。研究二聚体的第一个垂直（π，π*）跃迁的计算能量与为具有 5'-A(Br)U-3'或 5'-G(Br)U-3'序列的双链体测量的差谱中的最大值吻合良好。我们的 MS-CASPT2 结果表明，电子从 A/G 转移到(Br)U 的电荷转移（CT）态位于比涉及（Br）U 局部激发（LE）的第一个（π，π*）跃迁高得多的能量。此外，CT 跃迁的振子强度很小，这意味着它们不能直接被激发。当前研究的结果表明，在 DNA 中形成反应性尿嘧啶-5-基自由基之前，（Br）U 的高度氧化 LE 态先形成，随后可能是从鸟嘌呤发生电子转移。
