Hole E O, Nelson W H, Sagstuen E, Close D M
Department of Physics, University of Oslo, Norway.
Radiat Res. 1992 May;130(2):148-59.
Single crystals of anhydrous inosine were studied subsequent to exposure to high and low doses of X radiation at 10 K using K-band, EPR, ENDOR, and field-swept-ENDOR (FSE) techniques. Immediately following high radiation doses at 10 K at least eight different radicals, RI-RVIII, were observed. All radicals, except for RVIII, were also observed at low doses, but the relative yields varied with the radiation doses. RI, which decayed with no observable successor at about 65 K, has magnetic characteristics similar to those expected for the hypoxanthine base cation. RII, the dominating radical at low radiation doses, exhibits only one hyperfine coupling amenable for ENDOR analysis. From the nature of this coupling and the EPR and FSE characteristics of the resonance, it is suggested that RII is formed by addition of a neighbor sugar fragment to the C2 position of a hypoxanthine base, forming a C2-O5'-C5' ester bond. RII is unstable and decayed at about 60 K without any detectable successor. RIII and RIV are the C2 and C8 H-addition radicals, respectively. These species are formed in minor amounts after irradiation at low temperatures, and they are the only observable radicals left at room temperature. Two sugar-centered radicals, RV and RVI, are formed by net H-abstraction from the C4' and C5' positions, respectively. These radicals dominate the EPR spectra after high radiation doses at low temperatures. A transformation from RV into RIII, the C2 H-adduct, started at about 80 K. Similarly, a transformation of RVI into RIV started at about 210 K. Several minor species were analyzed. RVII is characterized by an alpha-coupling due to 26% spin density at C8, and RVIII is characterized by 12% pi-spin density at N1. Possible structures for these radicals are discussed.
利用K波段、电子顺磁共振（EPR）、电子核双共振（ENDOR）和场扫描电子核双共振（FSE）技术，在10K温度下，对无水肌苷单晶在高剂量和低剂量X射线辐射后进行了研究。在10K温度下接受高剂量辐射后，立即观察到至少八种不同的自由基，RI - RVIII。除R VIII外，所有自由基在低剂量下也能观察到，但相对产率随辐射剂量而变化。RI在约65K时衰变，没有可观察到的后续产物，其磁特性与次黄嘌呤碱阳离子预期的磁特性相似。RII是低辐射剂量下的主要自由基，仅表现出一个适合ENDOR分析的超精细耦合。根据这种耦合的性质以及共振的EPR和FSE特性，推测RII是由相邻糖片段加成到次黄嘌呤碱的C2位置形成的，形成了C2 - O5'- C5'酯键。RII不稳定，在约60K时衰变，没有任何可检测到的后续产物。RIII和RIV分别是C2和C8氢加成自由基。这些物种在低温辐照后少量形成，并且是室温下唯一可观察到的自由基。两个以糖为中心的自由基RV和RVI分别通过从C4'和C5'位置净氢提取形成。在低温下高辐射剂量后，这些自由基主导了EPR谱。从约80K开始，RV向C2氢加合物RIII发生转变。同样，从约210K开始，RVI向RIV发生转变。对几种次要物种进行了分析。RVII的特征是由于C8处26%的自旋密度引起的α耦合，R VIII的特征是N1处12%的π自旋密度。讨论了这些自由基的可能结构。
