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IRON-SUL FUR CORE ASS EMBLED 1 sis of an inorganic complex 1 very similar to the catalyt ic iron-sulfur core of bacterial hydrogenase suggests that sim ilar synthetic systems could be developed for reversible hydro gen fuel cells that would not re quire expensive and rare plat inum catalysts. Hydrogenases are enzymes that enable bacteria to evolve or take up hydrogen and use it as a meta bolic oxidizing or reducing agent. The new complex accelerates the same reaction as hydrogenase, al though less efficiendy It was syn thesized by biological chemistryr professor Christopher J. Pickett ofJohn Innes Centre, Norwich England, and collaborators there at Pacific Northwest Nationa Laboratory and at the University ofMilan-Bicocca, in Italy [Nature, 433,610(2005)]. This goal "has been a long standing challenge for inorganic synthetic chemists," writes chem istry professor Marcetta Y Darensbourg ofTexas A&M Univer sity in a Nature commentary The synthetic structure "points to ward a next generation of bio-in spired catalysts." "It's a significant pièce oi work," comments associate professor of biochemistry John W Peters of Montana State University Bozeman. "This type ofasymmetric bridged-metal assembly is a really difficult synthetic target. Getting something that so closely mimics the hydrogenase core will be invaluable in dissecting some intricacies that are difficult to probe in the enzyme." The new molecule "is a real tour de force and very nice chemistry HTTP://WWW.CEN-ONLINE.ORG
indeed," adds crystallographer Juan Carlos Fontecilla-Camps of the Institute ofStructural Biology Grenoble, France. In hydrogenase, the core (Hcluster) consists of two components — an Fe-Fe unit and a larger 4Fe4S cluster— that are joined through a sulfur atom on one of the enzyme's cysteine residues. The synthesis involved the creation and joining of the two components, which is very difficult. "Chemists have never before tried this build-up approach with such precise mimics of the two subsites," Darensbourg tells C&EN. Pickett and coworkers achieved a configuration similar to that in the enzyme by activating the Fe-Fe unit with a thioacetyl group and protecting the 4Fe4S cluster with a large bowl-shaped ligand. The ligandleft only one ofthe cluster's Fe atoms free to react, enabling formation of a correctly joined enzymelike assembly The researchers then showed that the synthetic cluster catalyzes reduction of H + to H 2 —currendy with poor efficiency but they hope this can eventually be improved. "In addition to advancing our understanding of the natural biological system, the availability of an active, free-standing analog of the Η-cluster may enable us to de velop useful electrocatalytic ma terials for application in, for ex ample, reversible hydrogen fuel cells," the researchers write. "Plat inum is currently the preferred electrocatalyst for such applica tions but is expensive, limited in availability and, in the long term, unsustainable." Reports on the hy drogen economy and fuel cells published in 2003 by the U.S. De-
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Η-CLUSTER Framework synth
