Bernhard W A, Barnes J, Mercer K R, Mroczka N
Department of Biophysics, University of Rochester, New York 14642.
Radiat Res. 1994 Nov;140(2):199-214.
The total free radical yield has been measured for crystals of five pyrimidine derivatives: thymine (T), 1-methylthymine (1MeT), 1-methyluracil (1MeU), 1-methylcytosine (1MeC) and cytosine monohydrate (C:HOH). Q-band EPR measurements were made on samples X-irradiated between 4 and 12 K. The G values in units of 10(-7) mol/J are 1MeC < 0.01, T < 0.04, 1MeU = 0.15, 1MeT = 0.25, and C:HOH = 0.8. The types of free radicals formed in these crystals are known through previous EPR investigations. A model is presented that attempts to identify the salient variables behind the large range in G values and, simultaneously, explain the variation in radical types. It is concluded that packing is a critical variable. Hydrogen-bonding networks promote the trapping of radicals through reversible proton transfer. In the absence of such a network less probable radical types are observed, such as radicals formed by irreversible protonation/deprotonation, higher-order reactions and homolytic bond cleavage. Crystals with low G values trap radicals formed predominantly by irreversible protonation/deprotonation at carbon positions and by excitation-spawned homolytic bond cleavage. In contrast, crystals with high G values trap radicals formed predominantly by reversible protonation/deprotonation at heteroatom positions. This model is extended to polynucleotides irradiated at low temperatures, where G values are typically 2-6. The high trapping efficiency seen in polynucleotides reflects highly efficient proton transfer. Consistent with this is the predominance of radicals formed by reversible protonation/deprotonation compared to those formed by irreversible protonation/deprotonation.
胸腺嘧啶（T）、1-甲基胸腺嘧啶（1MeT）、1-甲基尿嘧啶（1MeU）、1-甲基胞嘧啶（1MeC）和一水合胞嘧啶（C:HOH）。对在4至12 K之间进行X射线辐照的样品进行了Q波段电子顺磁共振（EPR）测量。以10(-7) mol/J为单位的G值分别为：1MeC < 0.01、T < 0.04、1MeU = 0.15、1MeT = 0.25以及C:HOH = 0.8。通过先前的EPR研究已了解这些晶体中形成的自由基类型。提出了一个模型，该模型试图确定G值大范围变化背后的显著变量，并同时解释自由基类型的变化。得出的结论是堆积是一个关键变量。氢键网络通过可逆质子转移促进自由基的捕获。在没有这种网络的情况下，会观察到不太可能的自由基类型，例如由不可逆质子化/去质子化、高阶反应和均裂键断裂形成的自由基。具有低G值的晶体捕获主要由碳位置的不可逆质子化/去质子化以及激发引发的均裂键断裂形成的自由基。相比之下，具有高G值的晶体捕获主要由杂原子位置的可逆质子化/去质子化形成的自由基。该模型扩展到低温辐照的多核苷酸，其中G值通常为2 - 6。多核苷酸中观察到的高捕获效率反映了高效的质子转移。与此一致的是，与由不可逆质子化/去质子化形成的自由基相比，由可逆质子化/去质子化形成的自由基占主导地位。
