science & technology agents that can damage them in a num ber of ways, and the breakdown of the cell's ability to defend against this dam age is thought to be involved in condi tions as diverse as cancer and aging. "Iron-sulfur proteins are extraordi narily sensitive to oxidative stress," Dancis points out. "If a cell is exposed to an oxidant, it seems as though the ironsulfur proteins are targeted very specifi cally." In some cells, he says, the ironsulfur cluster-containing protein aconitase appears to be the first protein to react to oxidants such as superoxide. Cells protect themselves against su peroxide, in particular, through a family of enzymes called superoxide dismutases that scavenge superoxide and in activate it. Experiments in Culotta's lab establish a genetic connection between iron-sulfur cluster synthesis, superox ide dismutase, and oxidative stress. In these experiments, oxidative stress in a yeast strain that lacks superoxide dis mutase led to growth defects that were corrected by mutations in genes in volved in iron-sulfur cluster assembly. The mechanism of this genetic interac tion is being examined.
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Reactions of hydrogen on silicon lie at the heart of a variety of semiconduc tor processing steps. In microelectron ics fabrication, for example, manufactur ers anneal products in hydrogen at high temperature. The treatment passivates devices by causing hydrogen to react with surface dangling bonds (unsaturat ed silicon valences) that could other wise alter the material's electronic prop erties and degrade performance. In other procedures, such as chemical vapor deposition (CVD), hydrogen is re moved from semiconductor surfaces. In the CVD process, pure and defect-free layers of silicon, germanium, or other ma terials are grown on top of silicon by de composing Si2H6, GeH4, or related gases. Films of the semiconductor elements are left behind as hydrogen desorbs. "Here you have the simplest mole cule—hydrogen—and the material about which more is known than just about any other material under the sun—silicon. And amazingly, we don't have afirmhan dle on silicon-hydrogen chemistry," says University of North Carolina, Chapel Hill, chemistry professor John J. Boiand, who led the investigation. Boiand points out that, at room tem perature, less than one in 10 trillion hy drogen molecules impinging upon the (100) crystal face of silicon bond to the semiconductor. Studies show that the adsorption process is impeded by a 0.7eV reaction barrier. "If hydrogen molecules have to huff and puff to climb onto the surface, then laying a role as important as hydro unmodified surface [Science, 2 9 0 , 506 you'd expect them to move rapidly gen's in semiconductor process (2000) ]. In addition to offering an answer down the potential energy hill as they ing, you might think that the ba to the decade-old question of energetics, leave," Boiand remarks. But that sics of that element's surface chemistry the study verifies recent theoretical pre doesn't happen. on silicon would be pretty well pinned dictions and provides new insight for un A clue as to what's going on came down by now. Not so. Certain pieces of the derstanding and controlling reaction dy from recent theoretical studies by other reaction-dynamics puzzle have been namics on semiconductor surfaces. research groups that focused on structur worked out But others remain elusive. al subtleties of silicon's (100) surface. Particularly perplexing is a ques On the pristine surface, rows of close tion of reaction barriers. Κ hydrogen ly spaced silicon dimers undergo con needs to be energized to form chem stant seesaw motions that, when ical bonds (adsorb) on silicon, why viewed as a snapshot, leave nearestdoesn't the diatomic molecule carry neighbor dimers in an alternating off much kinetic energy as it under (anticorrelated) tilt pattern. By con goes the reverse process—desorptrast, hydrogen-capped dimers have tion from the surface? elongated Si-Si bonds that are locked parallel to the surface. By studying a silicon surface that was designed to mimic the transitionIn the theoretical treatments, the state structure of the hydrogentransition-state structure of the adsorption reaction, researchers dimers was "drawn" essentially unhave uncovered an important piece of tilted—like the hydrogenated prod the puzzle. On the modified surface, ucts—and that configuration led to the reaction has almost no energy good agreement with available data. barrier and proceeds billions of times University of North Carolina, Chapel Hill, surface At that point, the trick was to come more efficiently than it does on the chemists Boiand (left) and Buehler up with a way to prepare bare, untilt"It's likely, I think, that there will be other human diseases linked to this path way," Dancis says. 'Whenever you screw up fundamental cellular processes, it's likely to cause a human disease, and when there are many proteins involved, there are probably many ways to do that." Discovering those links, of course, will require better understanding of those cel lular processes themselves. As the workshop made clear, the rapid progress in understanding iron-sulfur cluster-containing proteins and their syn thesis comes about in large part because the field has attracted the interest of re searchers from many disciplines. 'This field needs both physical scientists and biologists," Kiley notes. The tools for studying iron-sulfur cluster-containing proteins are all spectroscopic tools, she points out. "So you need spectroscopists to work on that, as well as chemists who can understand the basic inorganic chemistry. On the biology side, biochem ists and geneticists bring knowledge of the proteins that are facilitating these re actions. We're all going to have to work together to figure out what these mecha nisms are."^
Silicon Surface Key To Energetics Puzzle
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Quantum mechanical calculations show that on a clean silicon (100) surface (top), silicon dimers adopt an anticorrelated (alternating) tilt pattern, whereas on a hydrogen-covered crystal (bottom), the dimers line up parallel to the surface. By selectively capping some dimers with hydrogen while leaving others bare (green), researchers can pin uncapped dimers in a highly reactive untilted configuration.
ed dimers and measure their reactivity toward hydrogen experimentally. By exposing a crystal to small quanti ties of hydrogen atoms and then briefly warming the specimen, Boland and graduate student Emily J. Buehler were able to pin some silicon dimers parallel to the surface by carrying out chemical reactions on neighboring dimers. Using scanning tunneling microscopy and spectroscopy, the Chapel Hill chemists showed that Η-capped dimers influence their uncapped neighbors to remain un tilted. Those findings are further sup ported by quantum mechanical calcula tions carried out by Dongxue Chen, a graduate student working with Boland. Once the specially configured sites were prepared, the researchers mea sured reaction probabilities and found that untilted dimers are more than nine orders of magnitude more efficient at hydrogen adsorption than tilted dimers. "Apparently, we've succeeded in ma nipulating the dimer into a configura tion that mimics the transition-state structure for the reaction," Boland notes. The evidence for that claim is that, over a large temperature range (77 to 520 K), reaction rates at untilted dimers remain nearly constant. Based on these findings, the research ers propose that the 0.7-eV barrier to re
action is a measure of the energy re quired or gained from converting a patch of dimers from one configuration to the other—not a barrier that hydrogen mole cules must surmount to bond to the sur face. That explains why hydrogen desorbs from silicon with little kinetic energy. 'The paper makes a very important contribution to our understanding of this system," comments Kenneth D. Jor dan, a chemistry professor at the Uni versity of Pittsburgh. "The results intriguingly suggest that the high barrier to adsorption on the tilted dimer surface is a cooperative effect resulting from do mains of tilted dimers." Jordan adds that the work has "poten tially far-reaching consequences," be cause pinning dimers in untilted posi tions could be used to alter silicon's reac tivity to other chemical species. The technique could lead to a new approach for spatial control of surface chemistry, he says. Indeed, Boland is thinking along the same lines. The Chapel Hill chemist says his group now plans to apply the dimermanipulation method to mechanisms and transition-state structures of other sys tems such as the [2+2] and [4+2] cycload dition reactions on silicon. Mitch Jacoby
