**2. Free radicals**

Free radicals are molecular species with unpaired electrons in their atomic orbital capable of independent existence. As such, these radicals are highly reactive and can either extract an electron from molecules or donate an electron to other molecules thus acting as a reductant or an oxidant. Though free radicals have high reactivity, most of them have a very short half-life of less than 10−6 s in biological systems [1]. Some oxygen species known as reactive oxygen species (ROS) are non-reactive in their natural state but are capable of generating free radicals.

The idea of free radicals began in chemistry around the beginning of the twentieth century, where chemists initially described them as intermediate organic and inorganic compounds with several suggested definitions. A clear understand of these radicals was then proposed based on the work of Daniel Gilbert and Rebecca Gersham in 1954 [2] in which these radicals were suggested to play important roles in biological environments but also responsible for certain deleterious processes in the cell. Thereafter by 1956, Herman Denham further suggested that these reactive species may play critical roles in physiological process particularly aging process [3]. This hypothesis on the theory of free-radical on aging, inspired numerous research and studies which significantly contributed to the understanding of radicals and other related species such as ROS, reactive nitrogen species (RNS) and non-radical reactive species [4].
