Dioxygen (O2) serves two essential functions in aerobic life. It is both a terminal electron acceptor and a biosynthetic reagent. It is this latter role for dioxygen that will be the focus of this review: the incorporation of one or both of the oxygen atoms of dioxygen into organic substrates by heme enzymes. These dioxygen-utilizing reactions involve the cleavage of the oxygen-oxygen bond and are very energetically favorable, ie, exothermic. Despite this strong thermodynamic driving force, the chemical reactivity of dioxygen with organic molecules at ambient temperatures is intrinsically low. If this were not the case, dioxygen would spontaneously oxidize organic compounds and would therefore be harmful or even fatal rather than useful for living things. The low kinetic reactivity of dioxygen is due to its triplet ground state; ie, it is a diradical having two unpaired electrons. The molecular orbital energy level configuration of triplet dioxygen is (1sσ) 2 (1sσ*) 2-(2sσ) 2 (2sσ*) 2 (2pσ) 2 (2pπ) 4 (2pπx*) 1 (2pπy*). 1 The two oxygen atoms share six electrons in their 2p orbitals and the two unpaired electrons reside in the two degenerate antibonding 2pπx* and 2pπy* orbitals, leaving O2 with a formal bond order of two. The lowest orbital available to accept an electron is an antibonding orbital.

Although dioxygen has a triplet ground state, essentially all stable organic compounds are singlets; ie, all of their electrons are paired. Direct reactions between triplet and singlet molecules to yield a singlet product is a spin-forbidden process because chemical combination reaction rates are much faster than spin inversion rates. Such reactions can proceed via the spin-allowed, but highly energy-requiring formation of an unstable triplet intermediate, followed by slow spin inversion to form a singlet product. Thus, only easily oxidizable singlet organic molecules are able to react with triplet dioxygen by first forming resonance-stabilized (one-electron oxidized) radicals. Reduced flavins (RH2), for example, are thought to react with triplet dioxygen via initial formation of a caged radical pair (a triplet complex), followed by spin inversion to singlet products (eq 1). 4