LIGHT-INDUCED DNA BASE EXCITATION, DEACTIVATION AND OXIDATION

光诱导DMA碱基的激发、失活和氧化 DNA碱基是唯一可以通过太阳紫外线照射电子激发的核酸组分。 活性氧物质（ROS），例如由激发的碱基产生的单线态氧（1 O2）、超氧化物（O2-）和羟基自由基（OH）是细胞毒性的，并且已经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被完全理解，这部分是由于DNA碱基的荧光和磷光的低量子效率使得测量困难。定量测定受激基产生的~ 1O_2和~（2-）O_2将克服这一障碍。我们的假设是基于这样的想法，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、取代物等）。本研究通过定量测定受激碱基产生的1 O2和Oz~-，鉴定光氧化产物，检测DNA损伤和氧化应激，系统研究DNA碱基的激发和失活机制，阐明控制ROS形成的关键因素。具体而言，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1 O2和O2~？1 O2被选定的碱基、核苷和核苷酸的物理和化学猝灭速率常数可能是多少？是什么 激发能和强度对1 O2和O2 ~+量子产率的影响？微环境（例如，pH、抗氧化剂、取代基、溶剂等）影响ROS的产生光敏化的核苷/核苷酸1 O2和O2”的靶点和氧化应激可能是什么？ 本研究所用的仪器包括（1）配备锗1 O2检测器的时间分辨Nd：YAG激光器，（2）波长在200 - 700 nm之间可调的稳态光解装置，（3）皮秒激光闪光光解和（4）其他分析技术，如NMR，GC/MS，HPLC/MS，荧光，UV/维斯，EPR等。 本项目主要研究生物医学相关体系中的光诱导光氧化反应。紫外线引起的DNA氧化损伤被认为是严重的个人和公共健康问题。本课题的完成将为深入了解DNA损伤机制，特别是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.