ABSTRACT
The term reactive oxygen species (ROS) refers to highly reactive molecules which are derived from the partial reduction of molecular oxygen. Oxygen itself is a reactive molecule since it has two unpaired electrons in its outmost π orbital. These two electrons have the same spin quantum number (or, in other words, they have parallel spins). This makes molecular oxygen a strong oxidant. The reduction of oxygen by non radical species requires the simultaneous transfer of two electrons having parallel spins to oxygen, in order to fi t with the parallel spins of its two unpaired electrons. This spin restriction for the putative electron donor is probably the main reason why O2 slowly reacts with non radical species (Halliwell and Gutteridge 2006). On the other hand, oxygen can be converted to ROS either by energy transfer or by univalent electron transfer reactions (Apel and Hirt 2004).
