N1-甲基鸟嘌呤阳离子脱质子动力学的研究
收稿日期: 2014-09-28
网络出版日期: 2014-10-22
基金资助
项目受国家自然科学基金(No. 21333012)、"973"基金(No. 2013CB834602)及中国科学院先导专项(No. XDB12020200)资助
Deprotonation Kinetics of 1-Methylguanine After One-Electron Oxidation
Received date: 2014-09-28
Online published: 2014-10-22
Supported by
Project supported by the National Natural Science Foundation of China (No. 21333012), the National Basic Research Program of China (No. 2013CB834602), and the Chinese Academy of Sciences (No. XDB12020200).
在所有DNA碱基中, 鸟嘌呤碱基G具有最低的氧化电位, 导致其最容易被氧化. G碱基被单电子氧化成为G正离子自由基(G+·), G+·存在两个脱质子位点, 其中脱嘧啶环上亚氨基质子N1-H比脱环外氨基质子N2-H更有利, 因而在普通G碱基中研究脱N2-H的过程无法排除脱N1-H过程的干扰, 使得其脱N2-H的动力学迄今尚不明确. 在本文中, 通过将G碱基上的N1-H用CH3取代(即mG), 采用纳秒时间分辨瞬态紫外可见吸收光谱方法研究了mG碱基单电子氧化后脱质子N2-H的动力学. 根据瞬态紫外可见吸收光谱, 确定了mG+·脱质子的产物是mG(N2-H)·, 即脱质子的位点是N2-H. 进一步通过测量mG(N2-H)·的生成速率常数与mG的浓度依赖关系, 得到室温下SO4-·单电子氧化mG生成 mG+·的速率常数为(3.7±0.1)×109 L·mol-1·s-1以及 mG+·脱N2-H的速率常数为(7.1±0.2)×106 s-1. 并通过检测不同温度下mG+·脱N2-H的速率常数, 利用阿仑尼乌斯方程得出脱质子N2-H的活化能为19.9±1.0 kJ·mol-1. 这些结果可为DNA碱基的氧化损伤过程提供更为丰富的动力学信息.
吴丽丹 , 节家龙 , 刘坤辉 , 苏红梅 . N1-甲基鸟嘌呤阳离子脱质子动力学的研究[J]. 化学学报, 2014 , 72(11) : 1182 -1186 . DOI: 10.6023/A14090666
Among the four natural DNA bases, guanine (G) is the most sensitive to oxidation due to its lowest oxidation potential. When G base is oxidized to guanine cation radical (G+·), it will deprotonate from both the imino proton N1-H and the amino proton N2-H. According to the pKa values for N1-H and N2-H deprotonation, the main deprotonation site in G base is N1-H which would interfere with the N2-H deprotonation, making the kinetics of N2-H deprotonation difficult to be measured. Herein, the N2-H deprotonation kinetics is investigated using 1-methylguanosine (mG), where N1-H is substituted by methyl group to avoid the N1-H deprotonation and N9-H is substituted by ribose to ensure enough solubility of methylguanine in water, by nanosecond transient absorption (ns-TA) spectroscopy. By 355 nm photolysis of Na2S2O8, the highly oxidizing radical SO4-· is generated, which will oxidize mG to mG+· instantaneously. The time-resolved absorption spectra obtained for reaction of mG with SO4-· exhibits transient absorptions for mG(N2-H)· featured by absorption band at 600 nm, indicating that the mG+· deprotonation product is mG(N2-H)· and the deprotonation site is therefore validated to be N2-H. The mG concentration dependence of mG(N2-H)· formation rate constant is assessed through changing the mG concentration from 0.25 mmol·L-1 to 5 mmol·L-1. The concentration dependence experiment reveals that the rate-limiting step to form mG(N2-H)· is the bimolecular reaction of mG with SO4-· when mG concentration is lower than 2 mmol·L-1 and the bimolecular reaction rate constant to form mG+· is (3.7±0.1)×109 L·mol-1·s-1; when mG concentration is above 2 mmol·L-1, the rate-limiting step to form mG(N2-H)· is the first-order mG+· deprotonation and the N2-H deprotonation rate constant is (7.1±0.2)×106 s-1. Furthermore, the N2-H deprotonation rate constant is measured at different temperatures varying from 278 K to 298 K. According to Arrhenius equation, the activation energy barrier for the N2-H deprotonation is determined to be 19.9±1.0 kJ· mol-1. These results can provide valuable kinetic information on the oxidative damage of DNA.
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