Hildenbrand K, Schulte-Frohlinde D
Max-Planck-Institut für Strahlenchemie, Mülheim a.d. Ruhr, FRG.
Free Radic Res Commun. 1990;11(4-5):195-206. doi: 10.3109/10715769009088916.
In situ photolysis at 20 degrees C (argon plasma light source, lambda approximately greater than 200 mm) of oxygen-free solutions containing 2 mM H2O2 and heat-denatured, single-stranded (ss)DNA from calf-thymus resulted in the ESR spectra of the 6-hydroxy-5,6-dihydro-thymin-5-yl (1) and 5-methyleneuracil (3) radicals linked to the sugar-phosphate backbone. They were generated by reaction of OH radicals with DNA. By comparison of the decay characteristics of the ESR signals with rate constants from pulse-conductivity measurements [E. Bothe, G.A. Qureshi and D. Schulte-Frohlinde, Z. Naturforsch., 38c, 1030, (1983)] the thymine-derived radicals (1) and (3) can be excluded as precursors of the fast, dominating component of strand breakage of ssDNA. In the absence of H2O2 from native, double-stranded (ds)DNA an ESR signal was obtained (singlet, g approximately 2.004, delta v1/2 approximately 0.8 mT) which was assigned to the deprotonated guanine radical cation, [G.(-H)] of a DNA subunit. It is assumed that by the UV irradiation the guanine radical cation, (G+.), is generated, either by monophotonic photoionization or by electron transfer to pyrimidine bases. By rapid transfer of the bridging proton from (G+.) to the hydrogen bonded cytosine [G.(-H)] is formed. When photolysis of dsDNA was carried out in the presence of H2O2, reaction of photolytically generated .OH resulted in peroxyl radicals and purine radicals. The oxygen for formation of the peroxyl radicals is probably produced by reaction of [G.(-H)] with H2O2. Photolysis of N2O-saturated solutions containing dsDNA or ssDNA provided another possibility of generation of OH radicals. Under those conditions the .OH-induced radicals (1) and (3) were obtained not only from ssDNA but also from dsDNA.
在20摄氏度下对含有2 mM过氧化氢和来自小牛胸腺的热变性单链（ss）DNA的无氧溶液进行原位光解（氩等离子体光源，波长约大于200 nm），得到了与糖 - 磷酸主链相连的6 - 羟基 - 5,6 - 二氢胸腺嘧啶 - 5 - 基（1）和5 - 亚甲基尿嘧啶（3）自由基的电子自旋共振（ESR）谱。它们是由羟基自由基与DNA反应生成的。通过将ESR信号的衰减特性与脉冲电导率测量得到的速率常数进行比较[E. 博特，G.A. 库雷希和D. 舒尔特 - 弗罗林德，《自然科学杂志》，38c，1030，（1983）]，胸腺嘧啶衍生的自由基（1）和（3）可以被排除作为ssDNA链断裂快速主导成分的前体。在天然双链（ds）DNA不存在过氧化氢的情况下，获得了一个ESR信号（单重态，g约为2.004，半高宽约为0.8 mT），该信号被归属于DNA亚基的去质子化鸟嘌呤自由基阳离子[G.(-H)]。据推测，通过紫外线照射，鸟嘌呤自由基阳离子（G+·）通过单光子光电离或电子转移到嘧啶碱基而产生。通过桥连质子从（G+·）快速转移到氢键结合的胞嘧啶上，形成了[G.(-H)]。当在过氧化氢存在下对dsDNA进行光解时，光解产生的·OH反应生成过氧自由基和嘌呤自由基。过氧自由基形成所需的氧可能是由[G.(-H)]与过氧化氢反应产生的。对含有dsDNA或ssDNA的一氧化二氮饱和溶液进行光解提供了另一种产生羟基自由基的可能性。在这些条件下，·OH诱导的自由基（1）和（3）不仅从ssDNA中获得，也从dsDNA中获得。
