Free dioxygen

It has already been stressed that the discovery of superoxide as the enzymatically produced diffusion-free dioxygen radical anion [1-3] was a pivotal event in the study of free radical processes in biology. It is not of course that the McCord and Fridovich works were the first ones in free radical biology, but the previous works were more of hypothetical character, and only after the identification of superoxide by physicochemical, spectral, and biochemical analytical methods the enzymatic superoxide production became a proven fact. 

The singlet (S = 0) state lies about 1000 cm above the ground state triplet (S = 1) in the EPR spectrum of free dioxygen. Transitions associated with triplet oxygen in solution are detectable by EPR at low temperatures, but dioxygen complexes with even electron metal centers (e.g., ferroheme) are not generally observable by this method. Usually, only odd electron systems (Kramers systems) are detectable by magnetic resonance. 

Hydroperoxides represent a reduced and easier to control form of dioxygen. As is clear from Figure 1, in peroxides the 0-0 bond is longer and its energy lower than in free dioxygen. Therefore it is possible to deliver one of the oxygens of hydroperoxides under mild conditions and in a controlled manner. 

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