For release: Sept. 22, 2005

Contacts: 
Andreja Bakac, Chemical and Biological Sciences, (515) 294-3544
Saren Johnston, Public Affairs, (515) 294-3474

AMES LAB CHEMISTS RESOLVE CENTURY-OLD CONTROVERSY

Mystery no longer surrounds mechanism of powerful reaction

AMES, Iowa – Researchers at the U.S. Department of Energy’s Ames Laboratory on the Iowa State University campus have resolved the 100-year-old debate over the mechanism that triggers one of the most powerful oxidizing reactions available for breaking apart organic molecules. The Ames team has generated, characterized and ruled out iron(IV), an obscure and short-lived (half-life of seven seconds) intermediate, as the crucial substance in the Fenton reaction, a tremendously important, complex and pervasive reaction in matters associated with biological systems, environmental and atmospheric processes, and catalytic chemistry.

Ames Lab’s basic research effort, done in collaboration with Carnegie Mellon University in Pittsburgh and the University of Minnesota in Minneapolis, establishes hydroxyl radicals (or OH radicals) as the crucial Fenton intermediates, once and for all settling the long-standing controversy that surrounded the Fenton reaction. An article by the collaborating scientists describing their research and its indisputable results will appear in an upcoming issue of Angewandte Chemie, one of the world’s foremost chemistry journals. Angewandte Chemie gave the article a “Very Important Paper,” or VIP, rating, which less than 5 percent of their manuscripts receive.

“The Fenton reaction is that between iron(II) and hydrogen peroxide in aqueous solutions,” said Andreja Bakac, an Ames Lab senior chemist and an ISU adjunct professor of chemistry, who leads the Fenton reaction research. “Hydrogen peroxide oxidizes iron(II) and in the process generates an extremely reactive intermediate which then carries out oxidations of various substrates. It is the nature of this intermediate – OH radicals vs. iron(IV) – that was the focus of the long debate. The fact that we have now eliminated iron(IV) in Fenton reaction, and confirmed it in ozone reaction, may provide a foundation for the development of new and useful catalytic reactions based on Fe(IV).”

As with many scientific achievements, the work done by Bakac and Ames Lab assistant
scientist Oleg Pestovsky that eventually ruled out iron(IV) as the Fenton intermediate did not
initially have that goal. “We started out studying aqueous iron(IV) for several reasons,” said Bakac.

“It is a very unusual species, but in stabilizing biological environments, iron(IV) has been found to
play a role, especially in enzymatic reactions. And as soon as you find something that is considered unusual to actually function in real life, you know it’s not that uncommon – it just hasn’t been recognized before.”

With that thought in mind, Bakac and Pestovsky set out to look at the chemistry of the simplest, but probably most reactive, iron(IV) species, aqueous iron(IV). By allowing iron(II) to react with ozone, a gas that is a variety of oxygen, the two researchers generated an intermediate that exhibited chemistry consistent with that of an iron(IV) species, but still they had no proof.

“We saw some beautiful chemistry, but we definitely needed to identify this species,” said Bakac, “so we got in touch with Eckard Münck at Carnegie Mellon. He is the world expert in Mössbauer spectroscopy.” (Mössbauer spectroscopy is extremely well suited to identify the oxidation state of iron and provides unique insights into the electronic structure of a compound.)

Mössbauer studies of the Ames samples done under Münck’s direction by Carnegie Mellon research associate Emile Bominaar and graduate student Sebastian Stoian proved the intermediate generated by Bakac and Pestovsky was exactly what they were hoping for – the iron(IV) species.

In addition to the Carnegie Mellon work, contributions by Lawrence Que and his postdoctoral associate Xiaopeng Shan at the University of Minnesota further confirmed the iron(IV) species. “They took our sample to Stanford University to get an X-ray absorption spectrum, and that spectrum was consistent with the oxidation state of iron(IV)” said Bakac.

At that point, knowing the nature of the intermediate, Bakac and Pestovsky decided to carry out some very specific experiments. In one set of experiments, they oxidized a substance with iron(IV), and in a parallel series of experiments with an identical substrate and under identical conditions, they oxidized the substrate using the Fenton reaction.

“The products were different,” said Bakac. “And more than that, the products generated from the Fenton reaction were identical to those known to be formed from reactions involving OH radicals. So we both ruled out iron(IV) as the intermediate and confirmed OH radicals,” she said.

Ames Laboratory is operated for the Department of Energy by Iowa State University. The Lab conducts research into various areas of national concern, including energy resources, high-speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials.

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