氯乙基亚硝基脲烷化碱基导致DNA单链断裂的机理研究

Researches on the Mechanism of Single Strand Break Following the Alkylation of DNA Base Induced by Chloroethylnitrosoureas

采用密度泛函理论(DFT)和Mφller-Pleset微扰理论(MP)方法对氯乙基亚硝基脲(CENUs)烷化DNA碱基导致单链断裂的机理进行了研究.对包括CENUs的分解、DNA碱基的烷化、糖苷键的水解、脱嘌呤位点的开环以及最终导致单链断裂的磷酸二酯的消除在内的多步反应过程进行了探讨.在B3LYP/6-31++G**水平上对各驻点(反应物、中间体、过渡态和生成物)进行了全几何结构优化;为了模拟细胞环境,采用自洽场连续极化模型(CPCM)在相同计算水平上对各驻点进行了水相中的单点能计算或全几何结构优化.分别在B3LYP/6-31++G**和MP2/6-311++G**水平上绘出反应势能曲线,结果显示,在整个反应过程中,磷酸二酯的消除反应能垒最高,而氯乙基重氮盐离子烷化DNA碱基的反应能垒最低.理论分析结果表明,CENUs一旦分解便很容易烷化DNA碱基,随后生成的氯乙基化鸟嘌呤会迅速从DNA链上脱去,尽管如此,脱嘌呤位点最终导致DNA单链断裂的一系列反应则是一个缓慢的过程,这与断链反应的动力学实验结果相符合.

Density functional theory（DFT） and Mller-Pleset perturbation（MP） theory methods were carried out to investigate the mechanism of single strand break following the alkylation of DNA base induced by chloroethylnitrosoureas（CENUs）.A multiple-step process,involving the decomposition of CENUs,the alkylation on DNA base,the hydrolysis of N—glycosidic bond,the ring-opening reaction of basic sites and the elimination of phosphodiester finally leading to the DNA strand break,has been examined.The geometries of stationary points（reactions,intermediates,transition states and product） were optimized at B3LYP/6-31 ＋＋G＊＊ level.For simulating the reaction in cellular environment,the optimization or single energy computations were performed with the conductor-like polarizable continuum model（CPCM） model in water.The potential energy curve of the whole reactions was ploted by B3LYP/6-31 ＋＋G＊＊ and MP2/6-311 ＋＋G＊＊ computation respectively,then the results showed that the energy barrier of phosphodiester elimination reaction was the maximal value in the processes,while that of the alkylation reaction was the minimum.So a conclusion from theoretical analysis,which was consisted with the kinetics experimental phenomenon,was drawed that the alkylation reaction would be completed as soon as CENUs decomposed,and then the alkylated base would eliminate from the strand in a short time,however,the final DNA single strand break reaction would be a much more time-consuming process after the generation of the abasic sites.