Wang W, Becker D, Sevilla M D
Department of Chemistry, Oakland University, Rochester, Michigan 48309.
Radiat Res. 1993 Aug;135(2):146-54.
An ESR (electron spin resonance) investigation of the effect of hydration water (D2O) on the absolute yields (G) of ion radicals in gamma-irradiated DNA at 77 K is reported. The total DNA radical yield is found to increase by over fourfold on addition of the primary hydration layer (20 water molecules/nucleotide) to DNA. At hydrations greater than ca. 20 waters/nucleotide the excess water freezes into a separate and apparently radiologically independent bulk ice phase. Ice formation steals ca. 5 waters from the hydration layer which causes a proportionate (by weight) drop in the total yield of DNA radical ions. Hydroxyl radicals (.OD) are not observed even at great signal amplification in the primary hydration layer but are found only in the ice phase, which suggests efficient transfer of holes and electrons from the hydration layer to the DNA; as a consequence, DNA and its associated hydration water form the target mass for radiation damage. These results show that water of hydration is critical to radical formation and stabilization in DNA; however, the ice surrounding DNA does not contribute to direct DNA damage and is found to have the same properties as bulk ice. At 18 waters per nucleotide (gamma = 18) about one-fourth of all ion radicals produced by the radiation are trapped at 77 K. The k value for destruction is found to vary only slightly with hydration. Based on the measured G and k values for hydrated DNA, a trapped radical ion cluster size is estimated to be about 6 nm. For hydrated DNA the cluster size suggests electron migration distances of about 17 bases (for migration along the DNA strand).
报道了一项关于水合水（D2O）对77K下γ射线辐照DNA中离子自由基绝对产率（G）影响的电子自旋共振（ESR）研究。发现向DNA添加初级水合层（20个水分子/核苷酸）后，DNA自由基的总产率增加了四倍多。当水合程度大于约20个水/核苷酸时，过量的水冻结成一个单独的、显然在放射学上独立的块状冰相。冰的形成从水合层中夺取了约5个水分子，这导致DNA自由基离子的总产率按比例（按重量）下降。即使在初级水合层中进行大量信号放大，也未观察到羟基自由基（·OD），但仅在冰相中发现，这表明空穴和电子从水合层有效地转移到了DNA；因此，DNA及其相关的水合水构成了辐射损伤的靶物质。这些结果表明，水合水对于DNA中自由基的形成和稳定至关重要；然而，包围DNA的冰对直接的DNA损伤没有贡献，并且发现其具有与块状冰相同的性质。在每个核苷酸18个水（γ = 18）时，辐射产生所有离子自由基中约四分之一在77K下被捕获。发现破坏的k值仅随水合程度略有变化。根据测量的水合DNA的G和k值，估计捕获的自由基离子簇大小约为6nm。对于水合DNA，簇大小表明电子迁移距离约为17个碱基（沿DNA链迁移）。
