Photosensitized cleavage of plasmidic DNA by norharmane, a naturally occurring beta-carboline

UV-A radiation (320-400 nm) induces damages to the DNA molecule and its components through photosensitized reactions. beta-Carbolines (beta Cs), heterocyclic compounds widespread in biological systems, participate in several biological processes and are able to act as photosensitizers. The photosensitization of plasmidic DNA by norharmane in aqueous solution under UV-A radiation was studied. The effect of pH was evaluated and the participation of reactive oxygen species (ROS), such as hydroxyl radical (HOS), superoxide anion (O-2(center dot-)) and singlet oxygen (O-1(2)) was investigated. A strong dependence of the photosensitized DNA relaxation on the pH was observed. The extent of the reaction was shown to be higher in the experiments performed at pH 4.7 than those performed at pH 10.2. As was expected, an intermediate extent of the reaction was observed at physiological pH (pH 7.4). Kinetic studies using ROS scavengers revealed that the chemical reactions between ROS and DNA are not the main pathways responsible for the damage of DNA. Consequently, the predominant mechanism yielding the DNA strand break takes place most probably via a type I mechanism ( electron transfer) from the single excited state (S-1) of the protonated form of norharmane ((1)[nHoH(+)]*). Additional information about the nature of the norharmane electronic excited states involved in the photocleavage reaction was obtained by using the N-methyl derivative of norharmane (N-methyl-norharmane).