Theoretical investigation of the photosensitization mechanisms of urocanic acid

The photosensitization mechanisms of urocanic acid (UA), the main skin chromophores of ultraviolet (UV) light, are investigated by means of theoretical calculations. The results indicate that the direct photooxidative damage to DNA bases by triplet state UA through electron transfer reaction is not favorable on thermodynamic grounds. However, UA can photogenerate various reactive oxygen species (ROS, e.g., (1)O(2), O(2)(-)) theoretically and the ROS-generating mechanisms are illustrated as follows. Firstly, the (1)O(2)-generating pathway involves direct energy transfer between triplet state UA and (3)O(2). Secondly, UA gives birth to O(2)(-) through two pathways: (i) direct electron transfer between triplet state UA and (3)O(2); (ii) electron transfer between anion radical of UA (generated through autoionization reactions) and (3)O(2).