H₂O     H₂O^+. (water radical cation) + e^-

e^- + nH₂O---fast -- e^-_{aq  (electron becomes solvated)}

H₂O------- H₂O^*(excited water)---^.OH (hydroxyl radical) + H^.-atom  Very short-lived and highly oxidizing

H₂O^+. + H₂O---fast--- H₃O + ^.OH

^.OH  +  ^.OH--slower-- H₂O₂---fast----Fe(II) ------OH + OH^-  (Fenton Chemistry (& Haber-Weiss))

^.OH + H^. ----fast---- H₂O considerable re-conversion to water

e^-_aq + O₂ -------limited by [O₂]------- O₂^.-(superoxide: moderately oxidizing and longer lived)

O₂^.-   +    O₂^.---2H⁺---moderate --- H₂O₂ (long-lived) fast----Fe(II) ------^.OH diffusable

O₂^.-   +    O₂^.-+ SOD --2H⁺---faster--- H₂O₂ fast----Fe(II) ------^.OH

The most important source of ROS in vivo in aerobic bacterial cells is the electron transport chain, that can give rise to high levels of O₂^.- which is rapidly converted to H₂O₂ by dismutation.

O₂^.-   +    O₂^.---2H⁺--moderate->//-- + SOD (faster)-- H₂O₂   (long-lived and diffusable)

Electron Transport systems leak electrons directly on to O₂, yielding superoxide

Following return-to-growth, electron leaks may be greater

Normal E. coli generates 5 mM O₂^.- per second in rich medium yielding steady-state 0.1-0.2 mM H₂O₂, & Much higher in minimal medium.