LIGHT-INDUCED DNA BASE EXCITATION, DEACTIVATION AND OXIDATION

光诱导的DMA碱基激发、失活和氧化 DNA碱基是唯一可以被太阳紫外线辐射电子激发的核酸成分。 活性氧物种(ROS)，如由激发碱基产生的单线态氧(1O2)、超氧化物(O2“)和羟基自由基(OH)具有细胞毒性，与多种人类疾病的病因学有关。令人惊讶的是，控制碱基激发和ROS形成的详细机制远未完全了解，部分原因是DNA碱基的荧光和磷光的低量子效率使测量变得困难。定量测定由激发碱产生的1O2和O2~2将能够克服这一障碍。我们的假设是基于这样的观点，即ROS的产生与DNA碱基的激发和失活途径直接相关，这可能受激发光(能量和强度)和微环境(pH， 抗氧化剂、取代基等)。通过定量测定激发碱基产生1O2和Oz~，鉴定光氧化产物，检测DNA损伤和氧化应激，旨在系统地研究DNA碱基的激发和失活机制，并阐明控制ROS形成的关键因素。具体地说，我们试图回答以下问题： 碱基、核苷和核苷酸是否分别通过II型和I型光敏化过程(包括单光子和双光子吸收机制)有效地产生1O2和O2~？选定的碱基、核苷和核苷酸对1O2的物理和化学猝灭速率常数可能是多少？何谓 激发能和强度对1O2和O2“产生的量子产率的影响？微环境(如pH、抗氧化剂、取代基、溶剂等)将如何影响ROS的产生？核苷/核苷酸光敏化的1O2和O2”的靶点和氧化应激可能是什么？ 研究中使用的仪器包括：(1)配备Ge_1O_2探测器的时间分辨的Nd：YAG激光器；(2)波长在200~700 nm之间可调的稳态光解装置；(3)皮秒激光闪光光解；(4)其他分析技术，如核磁共振、GC/MS、LC/MS、荧光、UV/Vis、EPR等。 本课题主要研究生物医学相关系统中的光诱导光氧化反应。紫外线对DNA的氧化损伤被认为是严重的个人和公共健康问题。该项目的完成将为更好地理解DNA损伤的机制，特别是通过光诱导的自光氧化机制提供基础知识。

LIGHT-INDUCED DMA BASE EXCITATION, DEACTIVATION AND OXIDATION DMA bases are the only nucleic acid components that can be electronically excited by solar UV irradiation. Reactive oxygen species (ROS), such as singlet oxygen (1O2), superoxide (O2") and hydroxyl radicals (OH) produced by excited bases are cytotoxic and have been implicated in the etiology of a wide array of human diseases. Surprisingly, the detailed mechanisms governing the base excitation and ROS formation are far from being fully understood, which is partly due to the fact that low quantum efficiencies of fluorescence and phosphorescence from DNA bases make the measurements difficult. Quantitative determination of 1O2 and O2~ production from excited bases will be able to surmount this obstacle. Our hypothesis is based on the ideas that the production of ROS is directly related to the excitation and deactivation pathways of DNA bases, which may be controlled by excitation light (energy and intensity) and microenvironments (pH, antioxidants, substituents, etc.). By quantitatively determining the production of 1O2 and Oz~ from excited bases, identifying photooxidation products, testing DNA damage and oxidative stress, this proposal aims to systematically investigate the excitation and deactivation mechanisms of DNA bases and to clarify the key factors controlling the formation of ROS. Specifically, we seek to answer the following questions: How efficiently do the bases, nucleosides and nucleotides produce 1O2 and O2~ via type II and I photosensitization processes (including one- and two-photon absorption mechanisms), respectively? What may be the physical and chemical quenching rate constants of 1O2 by selected bases, nucleosides and nucleotides? What are the effects of excitation energy and intensity on the quantum yields of 1O2 and O2" production? How will the microenvironments (e.g., pH, antioxidants, substituent, solvents, etc.) affect the production of ROS? What may be the targeting sites and oxidative stress of the nucleoside/nucleotide photosensitized 1O2 and O2"? The instruments employed in this research include (1) time-resolved Nd:YAG laser equipped with germanium 1O2 detector, (2) steady-state photolysis setup with wavelengths tunable from 200 to 700 nm, (3) pico-second laser flash photolysis and (4) other analytical techniques, such as NMR, GC/MS, HPLC/MS, fluorescence, UV/Vis, EPR, etc. This project deals with the reactions of light-induced photooxidation in biomedical relevant system. Oxidative DNA damage by UV light is considered to be of serious personal and public health concern. The accomplishment of this project will provide fundamental knowledge for better understanding the mechanism of DNA damage, especially via light-induced self-photooxidation mechanisms.