Sagstuen E, Hole E O, Nelson W H, Close D M
Department of Physics, University of Oslo, Norway.
Radiat Res. 1996 Oct;146(4):425-35.
Single crystals of the complex 1-methylthymine.9-methyl-adenine were X -irradiated at 10 and at 65 K and studied in the temperature range 10 to 290 K using K-band EPR, ENDOR and field-swept ENDOR (FSE) techniques. The EPR and ENDOR spectra are dominated by two major and four minor resonances. The two major resonances are: MTMA1, the well-known radical formed by net hydrogen abstraction fr om the CS methyl group of the thymine moiety, and MTMA2, the radical formed by net hydrogen abstraction from the N1 methyl group of the thymine moiety. The latter product has not been observed previously in any 1-methylthymine derivative. The four minor resonances are: MTMA3, the anion of 1-methylthymine, possibly protonated at the O4 position; MTMA4, the well-known species formed by net hydrogen addition to C6 of the thymine moiety; MTMA5, the species formed by net hydrogen addition to C2 of the adenine moiety; and MTMA6, the species formed by net hydrogen addition to C8 of the adenine moiety. Radical MTMA3, the O4-protonated thymine anion, was clearly detected at 10 K, but upon thermal annealing at 40 K the lines began to disappear. In crystals irradiated at 65 K MTMA3 was only weakly present. Radical MTMA2 decayed at about 250 K with no detectable successor, and radical MTMA5 disappeared at about 180 K. It was not possible to learn from the d ata if MTMA5 transformed into MTMA6. The radical distribution in the 1-methylthymine.9-methyladenine crystal system is different from that in crystals of the individual components. Reasons for this behavior are discussed in light of the hydrogen bonding schemes and molecular stacking interactions in each of the crystals. An important feature is the concept of excited-state transfer from the adenine to the thymine moiety, followed by dehydrogenation at the thymine Nl-methyl group, the mechanism resulting in radical MTMA2.
对配合物1 - 甲基胸腺嘧啶·9 - 甲基腺嘌呤的单晶在10 K和65 K下进行X射线辐照，并使用K波段电子顺磁共振（EPR）、电子核双共振（ENDOR）和场扫电子核双共振（FSE）技术在10至290 K的温度范围内进行研究。EPR和ENDOR光谱主要由两个主要共振峰和四个次要共振峰组成。两个主要共振峰分别是：MTMA1，由胸腺嘧啶部分的CS甲基净氢提取形成的著名自由基；以及MTMA2，由胸腺嘧啶部分的N1甲基净氢提取形成的自由基。后一种产物在任何1 - 甲基胸腺嘧啶衍生物中均未被观察到。四个次要共振峰分别是：MTMA3，1 - 甲基胸腺嘧啶的阴离子，可能在O4位置质子化；MTMA4，由向胸腺嘧啶部分的C6净加氢形成的著名物种；MTMA5，由向腺嘌呤部分的C2净加氢形成的物种；以及MTMA6，由向腺嘌呤部分的C8净加氢形成的物种。自由基MTMA3，即O4 - 质子化胸腺嘧啶阴离子，在10 K时被清晰检测到，但在40 K进行热退火时，谱线开始消失。在65 K辐照的晶体中，MTMA3仅微弱存在。自由基MTMA2在约250 K时衰变，没有可检测到的后续产物，自由基MTMA5在约180 K时消失。无法从数据中得知MTMA5是否转化为MTMA6。1 - 甲基胸腺嘧啶·9 - 甲基腺嘌呤晶体系统中的自由基分布与单个组分晶体中的不同。根据每个晶体中的氢键模式和分子堆积相互作用对这种行为的原因进行了讨论。一个重要特征是激发态从腺嘌呤转移到胸腺嘧啶部分，随后在胸腺嘧啶N1 - 甲基处脱氢的概念，该机制产生自由基MTMA2。
