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Molecular Oxygen O2 is Free Radical and Causes Moderate Oxidative Stress

The oxygen molecule (O2 or O-O) itself is free radicals or a bi-radical, with two unpaired electrons. In theoretical chemistry, this ground state bi-radical is called “triplet” oxygen. That is, the electron configuration of the molecule has two unpaired electrons occupying two degenerate molecular orbitals with the same spin orientation (up-up or down-down). “Triplet” means 3 different quantum states of the total magnetic property projected along the direction of an external magnetic field.

A few people argue that oxygen molecule is not a free radical as it is not reactive enough to be radical, with which our PhD scientists don't agree. It is generally accepted that oxygen molecule is a biradical at the level of theoretical chemistry. Any one may search Google or Yahoo, using quoted phrases "oxygen molecule is a free radical" or "oxygen molecule is a biradical".

The unpaired electrons cause radicals to be highly reactive chemically in general. But the reactivity of radicals varies widely.  Organic free radicals are almost always very reactive and short-lived, although organic chemists have made a few examples of special organic free radicals that are stable at room temperature. Therefore, the reactivity is not a criterion for the classification of free radical.

Why an oxygen molecule (O2) has 2 unpaired electrons?

Non-Free radical molecules of higher stability have even number of electrons to form electron pairs, - closed-shell system as called in theoretical chemistry. In the language of classical physics, each electron pair means that the 2 electrons spin in 2 opposite directions (anti-parallel) so that the magnetic forces due to the spins of charged particles (electrons) are complementary to each other. In the language of non-classical quantum physics, there are actually no electron pairs at all. All electrons exist as electron cloude and the net magnetic properties of spins are cancelled out.

But some molecules of even number of electrons may be free radicals, as caused by the interplay of complex electromagnetic forces under specific circumstances. An oxygen molecule of even number of electrons is a biradical with 2 unpaired electrons. This was first established by experimental observation that the ground state of O2 exhibited magnetic properties of 2 electrons spinning in the same direction in the presence of a magnetic field. The classical theories of electronic structure could not explain this. But modern quantum mechanical calculations can rationalize this easily, showing the 2 outer shell electrons occupy 2 degenerate orbitals with the same spin direction. The "quantum effect" is a very tricky term in science, so that even some greatest physicists such as Plank, Gell-Mann and Feynman (3 Nobel Laureates in physics) expressed the "frustration" in some ways. Murray Gell-Mann described quantum mechanics as "that mysterious, confusing discipline which none of us really understands but which we know how to use". Richard Feynman said, “I think I can safely say that no-one understands quantum mechanics… Do not keep asking yourself, if you can possibly avoid it, …”

Do oxygen molecules cause oxidative stress?

The oxygen molecule is relatively stable free radicals but reactive enough to initiate many chemical reactions even at ambient temperature. It is equivalent to 2 free radicals. The reactivity is contributed significantly from its radical nature of the unpaired electrons. Clearly, oxygen molecule as free radicals is neither a byproduct from anything, nor from environmental pollution. It is an essential substance for life. In human body, oxygen is carried within vehicles - oxygen-carrying proteins such as myoglobin, hemoglobin, hemerythrin, and hemocyanin. Normally, these vehicles protect oxygen molecules from directly contacting with any reducing agent and from being harmful during transportation, and send oxygen to pre-defined destination for energy-producing metabolism activities. The di-atomic oxygen molecule (O2) as a biradical causes neglectable oxidative stress inside the body. It is energy-producing event generates free radical byproducts such as oxygen radical or peroxide, which cause oxidative stress and have to be dealt with antioxidants such as OPCs.

Oxidative stress of O2 in the air may lead to skin damage due to its oxidizing ability and contributes the skin ageing and wrinkling in a very slow and chronical way. The oxidative stress on skin may be minimized if effective antioxidants are taken in the long-term.