We synthesized a dinucleoside monophosphate of the 15N-labeled (6â4) photoproduct, which is one of the major UV-induced lesions in DNA, to investigate the (6â4) photolyase repair mechanism, and characterized its protonation state by measuring 15N NMR spectra as a function of pH. We expected that chemical-shift changes of the pyrimidone15N3, due to protonation, would be observed at pH 3, as observed for the 15N-labeled 5-methylpyrimidin-2-one nucleoside. Interestingly, however, the changes were observed only in alkaline solutions. In UV absorption spectroscopy and HPLC analyses under acidic conditions, a change in the maximum absorption wavelength, due to the protonation-induced hydrolysis, was observed at and below pH 1, but not at pH 2, whereas the protonation of 5-methylpyrimidin-2-one occurred at pH values between 2 and 3. These results indicated that the pKa value for this N3 is remarkably lower than that of a normal pyrimidone ring, and strongly suggest that an intramolecular hydrogen bond is formed between the N3 of the 3â² base and the 5-OH of the 5â² base under physiological conditions. The results of this study have implications not only for the recognition and reaction mechanisms of (6â4) photolyase, but also for the chemical nature of the (6â4) photoproduct.
我们合成了 15N 标记的 (6-4) 光产物的二核苷一
磷酸,这是 DNA 中紫外线诱导的主要损伤之一,以研究 (6-4) 光裂合酶修复机制,并通过以下方式表征其质子化状态:测量 15N NMR 谱作为 pH 的函数。我们预计在 pH 3 时会观察到由于质子化而引起的
嘧啶酮 15N3 的
化学位移变化,正如在 15N 标记的 5-
甲基嘧啶-2-一核苷中所观察到的那样。然而有趣的是,这些变化仅在碱性溶液中观察到。在酸性条件下的紫外吸收光谱和 HPLC 分析中,在 pH 1 及以下时观察到由于质子化诱导的
水解而导致的最大吸收波长的变化,但在 pH 2 时未观察到,而 5-methylpyrimidin-2 的质子化-一个发生在pH值2和3之间。这些结果表明该N3的pKa值明显低于正常
嘧啶酮环的pKa值,强烈表明3-2的N3之间形成了分子内氢键碱基和生理条件下5-碱基的5-OH。这项研究的结果不仅对(6-4)光裂合酶的识别和反应机制有影响,而且对(6-4)光产物的
化学性质也有影响。