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On the Efficiency of Hole and Electron Transfer from the Hydration Layer to DNA: An EPR Study of Crystalline DNA X-Irradiated at 4 K

机译：空穴和电子从水化层到DNA转移的效率：X射线4 K照射的结晶DNA的EPR研究

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The aim of this project was to gain an improved understanding of how the efficiency of hole and electron transfer from the solvation layer to DNA decreases as a function of distance from DNA. The packing of DNA in crystals of known structure makes it possible to calculate the degree of DNA hydration with a precision that is significantly greater than that achievable for amorphous samples. Previous work on oligodeoxynucleotide crystals has demonstrated that the efficiency of free radical trapping by DNA exposed to ionizing radiation at 4 K is relatively insensitive to base sequence, conformation, counterion, or base stacking continuity. Having eliminated these confounding variables, it is now possible to ascertain the degree of radical transfer that occurs from ionized water as a function of DNA hydration (Γ, in mol water/mol nucleotide). EPR is used to measure the hydroxyl radical concentration in crystals irradiated at 4 K. From a lack of hydroxyl radicals trapped in the inner hydration mantle, we determine that hole transfer to DNA is complete for water molecules located within 8 Å. This corresponds to Γ = 9–11 and indicates that hole transfer is 100% (as efficient as direct ionization of DNA) for water molecules adjacent to DNA. Beyond ~8 Å (Γ > 10), hydroxyl radicals are observed; thus deprotonation of the water radical cation is seen to compete with hole transfer to DNA as soon as one water intervenes between the ionized water and DNA. The boundary for 0% hole transfer is projected to occur somewhere between 15 and 20 waters per nucleotide. Electron transfer, on the other hand, is 100% efficient across the entire range studied, 4.2 ≤ Γ ≤ 15.6.
机译：该项目的目的是为了更好地理解从溶剂化层到DNA的空穴和电子转移效率如何随与DNA的距离的变化而降低。 DNA在已知结构的晶体中的堆积使计算水合度的精确度大大高于无定形样品所能达到的精确度。先前对寡脱氧核苷酸晶体的研究表明，暴露于4 K电离辐射的DNA捕获自由基的效率对碱基序列​​，构象，抗衡离子或碱基堆积连续性相对不敏感。消除了这些混杂变量后，现在有可能确定离子化水发生的自由基转移程度与DNA水合作用的关系（Γ，以摩尔水/摩尔核苷酸计）。 EPR用于测量在4 K照射的晶体中的羟基自由基浓度。由于内部水合套中没有羟基自由基，我们确定对于8Å以内的水分子，空穴向DNA的转移是完全的。这对应于Γ= 9-11，并且表明与DNA相邻的水分子的空穴转移率为100％（与DNA的直接电离一样有效）。超过〜8Å（Γ> 10），观察到羟基自由基。因此，一旦一种水介于电离水和DNA之间，水自由基阳离子的去质子就会与空穴转移到DNA竞争。空穴转移0％的边界预计会出现在每个核苷酸15至20个水之间的某个位置。另一方面，在整个研究范围（4.2≤Γ≤15.6）中，电子转移的效率为100％。

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