Figure 35.1 Structure of oxygen and reactive oxygen species (ROS). Oxygen is shown at the far left as the incorrect double-bonded diatomic form. This form, known as singlet oxygen, exists to a significant extent only at high temperature or in response to irradiation. The diradical is the natural, ground-state form of O2 at body temperature. ROS are partially reduced, reactive forms of oxygen. The first product is the anion radical, superoxide (O2•), which is in equilibrium with the weak acid, hydroperoxyl radical (pKa ≈ 4.5). Reduction of superoxide yields hydroperoxide O2-2, in the form of H2O2. Reduction of H2O2 causes a hemolytic cleavage reaction that releases hydroxyl radical (OH•) and hydroxide ion (OH-). Water is the end product of complete reduction of O2.

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> 35 Oxygen and Life > THE INERTNESS OF OXYGEN

   In most textbooks, oxygen is shown as a diatomic molecule with two bonds between the oxygen atoms. This is an attractive presentation, from the viewpoint of the electron dot structures and electron pairing to form chemical bonds, but it is incorrect. In fact, at body temperature, O2 is a biradical, a molecule with two unpaired electrons (Fig. 35.1). These electrons have parallel spins and are unpaired. Since most organic oxidation reactions, e.g. the oxidation of an alkane to an alcohol or an aldehyde to an acid, are two-electron oxidation reactions, O2 is generally not very reactive. In fact, O2 is completely stable in the presence of H2, a strong reducing agent, until enough heat is provided to flip an O2 electron and initiate the combustion reaction. Once it is started, the combustion provides the heat needed to propagate the reaction, sometimes explosively.















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