Krivokapić André, Hole Eli O, Sagstuen Einar
Department of Physics, University of Oslo, N-0316 Oslo, Norway.
Radiat Res. 2003 Sep;160(3):340-54. doi: 10.1667/rr3048.
Radical formation and hole transfer were investigated in crystals of cytosine.HCl (C.HCl) doped with 0-1.1 mol-% 5-methylcytosine x HCl (5MC x HCl). The doping level was determined by NMR spectroscopy. Crystals and polycrystalline samples were X-irradiated at 295 K, 77 K and 12 K and studied with EPR, ENDOR and FSE spectroscopy at these temperatures. At 295 K the dominant radicals were the so-called 3alphaH radical, formed in 5MC by a net H-abstraction from the methyl group, and the cytosine C6 H-addition (5-yl) radical. At 12 K five radicals were identified. These were the 3alphaH radical, cytosine reduction and oxidation products, and the cytosine C6 and C5 H-addition (5-yl and 6-yl, respectively) radicals. The spectroscopic parameters for the 3alphaH radical are very similar to those of a radical observed previously in the crystalline cytosine derivatives cytidine (CR), 2'deoxycytidine hydrochloride (CdR x HCl), 5'dCMP and 3'CMP as well as in the uracil derivative 2-thiouracil (2-TU). It was shown that amounts of the order of tenths of a percent 5MC x HCl doped into crystals of C.HCl give rise to a considerable yield of 3alphaH radicals after exposure to ionizing radiation both at room temperature and at lower temperatures. This supports a previous suggestion that naturally occurring 5-methylated cytosine impurities may be responsible for the formation of 3alphaH radicals in the crystalline cytosine derivatives CR, CdR.HCl, 5'dCMP and 3'CMP and suggests that the 3alphaH radical in these systems is a 5-methylated base-centered radical. The total radical yield in doped C x HCl crystals increased considerably with the doping level, both at low temperatures and at room temperature, implying that the 3alphaH radical is more stable than the primary cytosine radicals. The relative amounts of the 3alphaH radical were obtained by using simulated benchmark spectra to reconstruct experimental EPR spectra of doped polycrystalline samples. Evidence is presented suggesting that the enhanced yield of the 3alphaH radical in doped samples is due to holes originally formed at cytosine bases and transferred to 5-methylcytosine bases in addition to the 3alphaH radical being less exposed to recombination than other cytosine radicals.
在掺杂了0 - 1.1摩尔百分比的5 - 甲基胞嘧啶x盐酸盐（5MC x HCl）的胞嘧啶盐酸盐（C.HCl）晶体中研究了自由基形成和空穴转移。掺杂水平通过核磁共振光谱法测定。晶体和多晶样品在295 K、77 K和12 K下进行X射线辐照，并在这些温度下用电子顺磁共振（EPR）、电子核双共振（ENDOR）和场扫电子自旋共振（FSE）光谱进行研究。在295 K时，主要的自由基是所谓的3αH自由基，它是由5MC中的甲基净氢提取形成的，以及胞嘧啶C6氢加成（5 - 基）自由基。在12 K时鉴定出了五种自由基。它们是3αH自由基、胞嘧啶还原和氧化产物，以及胞嘧啶C6和C5氢加成（分别为5 - 基和6 - 基）自由基。3αH自由基的光谱参数与先前在结晶胞嘧啶衍生物胞苷（CR）、2'-脱氧胞苷盐酸盐（CdR x HCl）、5'-脱氧胞苷一磷酸（5'dCMP）和3'-胞苷一磷酸（3'CMP）以及尿嘧啶衍生物2 - 硫尿嘧啶（2 - TU）中观察到的自由基非常相似。结果表明，向C.HCl晶体中掺杂千分之几数量级的5MC x HCl，在室温及更低温度下暴露于电离辐射后会产生相当数量的3αH自由基。这支持了先前的一个观点，即天然存在的5 - 甲基化胞嘧啶杂质可能是结晶胞嘧啶衍生物CR、CdR.HCl、5'dCMP和3'CMP中3αH自由基形成的原因，并表明这些体系中的3αH自由基是以5 - 甲基化碱基为中心的自由基。掺杂的C x HCl晶体中的总自由基产率在低温和室温下都随掺杂水平显著增加，这意味着3αH自由基比主要的胞嘧啶自由基更稳定。通过使用模拟基准光谱重建掺杂多晶样品的实验EPR光谱，获得了3αH自由基的相对含量。有证据表明，掺杂样品中3αH自由基产率的提高是由于最初在胞嘧啶碱基处形成并转移到5 - 甲基胞嘧啶碱基的空穴，此外3αH自由基比其他胞嘧啶自由基更不易发生复合。
