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MARGINALIA

The Story of O

Roald Hoffmann

Burning Water

Part of the immune system is adaptive, an assortment of receptors that cover the surface of white blood cells. Some of these receptors are antibodies, which recognize and bind to pathogens. To actually destroy them, to disrupt bacterial membranes or initiate ingestion, immunologists thought that antibodies had to signal other parts of the system.

In the past three years, Scripps Research Institute chemists and biochemists have dramatically revised this picture. Richard Lerner, Albert Eschenmoser and Paul Wentworth, Jr., collaborating with several groups, first discovered that all antibodies could produce hydrogen peroxide. More recently, they have found that antibodies appear to catalyze the generation of ozone! The overall reaction (unbalanced, and obscuring many intermediate steps) is

H2O + 1O2 H2O3 O3 + H2O2 + O2

This reaction includes many of oxygen's protean molecular manifestations: Aside from the familiar water, hydrogen peroxide, excited singlet and normal ground state oxygen, it contains the unusual HOOOH molecule. Dihydrogen-trioxide was proposed more than 60 years ago, but direct proof for its existence has trickled in over the last 30 years. There is also a possible role for the HOOO radical, not shown in the equation, which in turn may be a masked OH. All are reactive molecules, as shown by their short half-lives in water: around 1 minute for ozone, 20 milliseconds for HOOOH and 1 microsecond for 1O2.

The narrative of discovery that details the unraveling dogma is fascinating. Initial observations of H2O2 production in the presence of antibodies were startling enough. But as the Scripps group thought about what they saw, so much H2O2 was being produced that it became an enigma where all the electrons come from. The problem was resolved when the researchers realized that two molecules of singlet oxygen could react with water to give HOOOH, which could then go on to form ozone and hydrogen peroxide. A simple experiment ran the process in 18O-labeled water and showed that the label wound up in the H2O2; this result immediately supported the hypothesis. Albert Eschenmoser says:

The situation is reminiscent of photosynthesis when it started to use water as an electron donor and then began to poison the environment. It is "burning water," in one case by excitation of a cofactor by light, and in the other case by singlet oxygen mediated by an antibody.

And where does the singlet oxygen, the energy source, come from? Certain white blood cells produce it when stimulated by infection. In the tight economy of the cell (Lavoisier, the banker, would have loved this), everything is utilized.

Figure 2. Shining bright . . .Click to Enlarge Image

Recent work from the same imaginative collective has shown that atherosclerotic plaques also generate ozone. That ozone attacks cholesterol and the consequent oxidation products are all-around bad actors, implicated in cytotoxicity and mutagenesis.

This seemingly unorthodox chemistry has been with us (albeit unknowingly) for years. There is an old, potent oxidation system used to cleanse industrial quantities of soil and water contaminated with PCBs and worse, the peroxone process. In it, the samples are treated with a synergistic mixture of O3 and H2O2. In another cross-disciplinary study, the Scripps group, in collaboration with theoretician William A. Goddard, III, at California Institute of Technology, has shown that the peroxone process involves the same H2O3 molecule.





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