Dandliker P J, Núñez M E, Barton J K
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA.
Biochemistry. 1998 May 5;37(18):6491-502. doi: 10.1021/bi980041w.
Potent oxidants which intercalate in DNA serve as tools to probe DNA-mediated electron-transfer reactions. A photoexcited rhodium intercalator, Rh(phi)2DMB3+ (phi = 9,10-phenanthrenequinone diimine and DMB = 4,4'-dimethyl-2,2'-bipyridine), tethered to DNA, promotes both oxidative damage to 5'-GG-3' doublets in DNA and the repair of thymine dimers from a remote site on the DNA duplex. DNA-mediated repair of a thymine dimer lesion by charge transfer from the tethered rhodium intercalator is quantitative, albeit with low photoefficiency, occurs in an intraduplex reaction over long range (36 A), and requires that the intervening bases be paired. When both oxidative reactions, repair and oxidative damage, are monitored on the same duplex, competition is evident; the presence of both a 5'-GG-3' site and the thymine dimer diminished the dimer repair efficiency by 20-40% and decreased damage at the 5'-GG-3' sites 2-fold compared to similar sequences lacking either the guanine doublet or thymine dimer, respectively. In addition to damage at the 5'-G of 5'-GG-3' sites, we also observe oxidation at the 3'-G of the 5'-GT<>TG-3' tetrad only in the presence of thymine dimer. Overall, the yield of repaired thymine strand was at least 10 times higher than the yield of oxidized guanine in the same sequences. While the 5-GG-3' may represent the thermodynamically favored site for oxidative reaction, repair of the thymine dimer appears to be kinetically more favorable. Dipyridophenanzine (dppz) complexes of ruthenium(III), less potent oxidants which intercalate in DNA, oxidize 5'-GG-3' doublets efficiently but cannot trigger the repair of the thymine dimer lesion. Oxidative damage to DNA from a distance, mediated by the DNA base pair stack, can, however, be utilized to probe the disruption in the base stack generated by the thymine dimer. The presence of the dimer does not diminish oxidation by a Ru(III) intercalator at a distal guanine doublet, suggesting that the disruption caused by the dimer does not block charge transfer through the DNA duplex. DNA-mediated electron-transfer reactions of metallointercalators therefore serve to illustrate important aspects of radical migration and its consequence with respect to reactions at a distance through the DNA base pair stack.
嵌入DNA的强氧化剂可作为探测DNA介导的电子转移反应的工具。一种与DNA相连的光激发铑嵌入剂Rh(phi)2DMB3+（phi = 9,10-菲醌二亚胺，DMB = 4,4'-二甲基-2,2'-联吡啶），既能促进对DNA中5'-GG-3'双链体的氧化损伤，又能从DNA双链体上的远端位点修复胸腺嘧啶二聚体。通过连接的铑嵌入剂进行电荷转移实现DNA介导的胸腺嘧啶二聚体损伤修复是定量的，尽管光效率较低，发生在长距离（36 Å）的双链内反应中，并且要求中间碱基配对。当在同一双链体上监测修复和氧化损伤这两种氧化反应时，竞争很明显；与分别缺少鸟嘌呤双链体或胸腺嘧啶二聚体的类似序列相比，5'-GG-3'位点和胸腺嘧啶二聚体的同时存在使二聚体修复效率降低了20 - 40%，并使5'-GG-3'位点的损伤减少了2倍。除了5'-GG-3'位点5'-G处的损伤外，我们还仅在胸腺嘧啶二聚体存在的情况下观察到5'-GT<>TG-3'四联体3'-G处的氧化。总体而言，修复后的胸腺嘧啶链的产率比相同序列中氧化鸟嘌呤的产率至少高10倍。虽然5'-GG-3'可能代表氧化反应的热力学有利位点，但胸腺嘧啶二聚体的修复在动力学上似乎更有利。钌(III)的二吡啶菲咯嗪（dppz）配合物是嵌入DNA的较弱氧化剂，能有效氧化5'-GG-3'双链体，但不能触发胸腺嘧啶二聚体损伤的修复。然而，由DNA碱基对堆叠介导的远距离对DNA的氧化损伤可用于探测胸腺嘧啶二聚体产生的碱基堆叠破坏。二聚体的存在不会减少远端鸟嘌呤双链体处钌(III)嵌入剂的氧化，这表明二聚体引起的破坏不会阻止电荷通过DNA双链体转移。因此，金属嵌入剂的DNA介导的电子转移反应有助于说明自由基迁移的重要方面及其通过DNA碱基对堆叠在远距离反应中的后果。
