The Haber-Weiss cycle--70 years later.

The chain reactions HO* + H2O2 --> H2O + O2*- + H+ and O2*- + H+ + H2O2 --> O2 + HO* + H2O, commonly known as the Haber-Weiss cycle, were first mentioned by Haber and Willstätter in 1931. George showed in 1947 that the second reaction is insignificant in comparison to the fast dismutation of superoxide, and this finding appears to have been accepted by Weiss in 1949. In 1970, the Haber-Weiss reaction was revived by Beauchamp and Fridovich to explain the toxicity of superoxide. During the 1970s various groups determined that the rate constant for this reaction is of the order of 1 M(-1) s(-1) or less, which confirmed George's conclusion. The reaction of superoxide with hydrogen peroxide was dropped from the scheme of oxygen toxicity, and superoxide became the source of hydrogen peroxide, which yields hydroxyl radicals via the Fenton reaction, Fe2+ + H2O2 --> Fe3+ + HO- + HO*. In 1994, Kahn and Kasha resurrected the Haber-Weiss reaction again, but this time the oxygen was believed to be in the singlet (1delta(g)) state. As toxicity arises not from a Fenton-catalysed Haber-Weiss reaction, but from the Fenton reaction, the Haber-Weiss reaction should not be mentioned anymore.