Schuchmann M N, Steenken S, Wroblewski J, von Sonntag C
Int J Radiat Biol Relat Stud Phys Chem Med. 1984 Sep;46(3):225-32. doi: 10.1080/09553008414551341.
The site of attack of OH radicals on dihydrouracil and five of its methylated derivatives was determined by pulse radiolysis using N,N,N',N'-tetramethylphenylenediamine (TMPD) to detect oxidizing radicals and tetranitromethane (TNM) as well as K3Fe(CN)6 to detect reducing radicals. In the case of dihydrouracil OH radicals abstract preferentially an H atom at C(6) to give the 6-yl radical (greater than or equal to 90 per cent) which at pH approximately 6.5 reduces TNM and K3Fe(CN)6 at almost diffusion-controlled rates. Only a small fraction of OH radicals abstract the H atom at C(5) (less than or equal to 10 per cent). The resulting 5-yl radical oxidizes TMPD to TMPD+ at pH 7-8. With the methylated derivatives of dihydrouracil, OH radicals react less selectively, especially in the case of N(1)-methyl derivatives. This methyl group is activated to a similar degree as the methylene group at C(6). In 1-Medihydrouracil the yield of N(1)-CH2 radicals is about 29 per cent, which has been deduced from the yield of formaldehyde formed after oxidation of this radical by TNM at pH approximately 6.5 and the subsequent hydrolysis. Radicals at the other methyl substituents are generated to a lesser extent (less than or equal to 10 per cent) and are relatively unreactive towards oxidizing agents such as TNM and K3Fe(CN)6 as well as towards the reducing agent TMPD. Although methyl substitution opens new routes for OH attack the preferred site of H abstraction remains C(6) (greater than 60 per cent).
利用脉冲辐解，通过使用N，N，N'，N'-四甲基对苯二胺（TMPD）检测氧化自由基、四硝基甲烷（TNM）以及铁氰化钾（K3Fe(CN)6）检测还原自由基，确定了羟基自由基对二氢尿嘧啶及其五种甲基化衍生物的攻击位点。在二氢尿嘧啶的情况下，羟基自由基优先在C(6)处夺取一个氢原子生成6-基自由基（大于或等于90%），在pH约为6.5时，该自由基以几乎扩散控制的速率还原TNM和K3Fe(CN)6。只有一小部分羟基自由基在C(5)处夺取氢原子（小于或等于10%）。生成的5-基自由基在pH 7 - 8时将TMPD氧化为TMPD+。对于二氢尿嘧啶的甲基化衍生物，羟基自由基的反应选择性较低，尤其是在N(1)-甲基衍生物的情况下。该甲基的活性与C(6)处的亚甲基相似。在1-甲基二氢尿嘧啶中，N(1)-CH2自由基的产率约为29%，这是根据该自由基在pH约为6.5时被TNM氧化并随后水解后生成甲醛的产率推断出来的。其他甲基取代基处产生的自由基较少（小于或等于10%），并且对诸如TNM和K3Fe(CN)6等氧化剂以及对还原剂TMPD的反应性相对较低。尽管甲基取代为羟基攻击开辟了新途径，但夺取氢原子的优先位点仍然是C(6)（大于60%）。
