Oxygen free-radicals A free-radical may be defined as any species that has one or more unpaired electrons. This broad definition includes the hydrogen atom (one un-paired electron), most transition metals and the oxygen molecule itself. 02 has two unpaired electrons, eachlocated in a different,* anti-bonding orbital (for a detailed explanation see Halliwell, 1981b). These two electrons have the same spin quantumnumber and so if 02 attempts to oxidize another atom or molecule by accepting a Abbreviations used: SOD, superoxide dismutase; DETAPAC, diethylenetriamine penta-acetic acid. pair of electrons from it, both new electrons must be of parallel spin so as to fit into the vacant spaces in the 7r orbitals. A pair of electrons in an atomic or molecular orbital would however have anti-parallel spins (of+ 1 and-i), in accordance with Pauli's principle. This imposes a restriction on oxidations by 02 which tends to make02 accept its electrons one at a time and slows its reaction with non-radical species (Fig: 1). Transition metals are found at the active site of many oxidases and oxygenases because theirability to acceptand donate single electronscan overcome this spin restriction (Hill, 1981).

Singlet oxygen Another way of increasing the reactivity of 02 iS to move one of the unpaired electrons in a way that alleviates the spin restriction. Two singlet states of 02 exist (Fig. 1). Singlet 021Ag, the most important in biological systems, has no unpaired electrons and is not a radical. Singlet 021Eg+ usually decays to the 1Ag state before it has time to react with anything. Excitation of 02 to the 1Ag state can be achieved when several biological pigments, such as chlorophylls, retinal, flavins or por-phyrins, are illuminated in the presence of 02 (Foote, 1982). Formation of singlet 2 occurs in vivo in illuminated chloroplasts (Halliwell, 1981a) and in both the lens and retina of the mammalian eye (Zigler & Goosey, 1981; Katz et al., 1982; Kirschfeld, 1982).