Oxidative stress

Chemists and biologists typically define oxidative stress as an imbalance, particularly in biological cells, between the rate of formation and/or concentration of 'oxygen free radicals' (or 'reactive oxygen species') — potent 'oxidizing' (electron capturing) molecules — and their elimination or 'neutralization' by 'antioxidants' — 'reducing' (electron donating) molecules — the imbalance characterized by an excess of the former or a deficiency of the latter, leading to an alteration of a cell's 'redox' state towards the 'oxidized' state. The concept of oxidative stress occupies central importance importance in biology, as it applies both to physiological phenomena essential for optimal functioning of organisms and to pathophysiological phenomenona, such as cardiovascular diseases, cancer and other clinical disease states, and to considerations of the mechanisms underlying aging.

In the definition of oxidative stress given above, the terms embraced by single quotes will require further discussion before any practical understanding of the concept can emerge. Accordingly, we will try to explicate those sub-concepts in turn, and relate them in a synthesis of the concept of oxidative stress.

Free radicals

Chemists define a 'free radical' as an atom or molecule that has at least one unpaired electron. The qualifier 'free' denotes the independent existence of the species with one or more unpaired electrons, and many chemists drop the term as they consider only those radicals which can exist independently, however briefly.

Consider the most common variety (isotope) of hydrogen atom, which comprises a nucleus consisting of one proton, a positively charged particle, and one electron, the latter occupying a surrounding electron shell, having no sub-shells, a shell that has the intrinsic capacity two hold two electrons. We can symbolize it as H•, or H^•. For quantum mechanical reasons relating to a kind of stability, so to speak, the hydrogen atom tends to behaves as if it wanted two electrons in that shell, specifically a pair of electrons 'spinning' in opposite directions. Accordingly, the hydrogen atom tends to react with another hydrogen atom like itself, one with an oppositely spinning electron in its only electron shell, such that the two hydrogen nuclei share the two electrons in the shell, binding the two atoms together in what chemists refer to as a molecule with a 'covalent bond'. We can symbolize the hydrogen molecule as H:H.

Referring then to our definition of 'free radical', or just 'radical', we can recognize the hydrogen atom, H^•, as a radical.