s c i e n c e / t e c h n o l o g y ^ ;%!k :

vinced. "In my heart of hearts, I think it's a coincidence that the (3 + 2) [transition state] fit the data we obtained," he says.

Houk doesn't discount that a coinci­dence is possible. After all, the theoreti­cal KIEs are based only on a simplified model of the reaction used to obtain ex­perimental KIEs. Besides, there's no guar­antee that all possible transition-state ge­ometries were identified and calculated. But he believes a coincidence is very un­likely. "Thomas and Jan looked hard for every transition state," he says. "We are confident we found them all."

While others may think the case is closed, Sharpless vows to renew his group's efforts on the problem. "It's a fascinating, basic problem affecting a lot of transition-metal-catalyzed reactions of olefins," he tells C&EN. "It has not been solved and it's worthy of much further study."

Sharpless' role in getting and reporting the latest KIE data—even though they ap­pear to support a position contrary to his own—and his continued commitment to getting at the bottom of this mechanism draws praise from others. "As is usual with controversies, the losing side is not 100% convinced," says Singleton. "What is un­usual is that Sharpless has consistently sought the key experiments to determine the truth and is willing to publish the re­sults of those experiments whether he likes them or not."

And from Schowen: "Sharpless is a re­ally beautiful example of a fine scientist who is not engaging in confrontational politics but is just buying the evidence as it comes. Right now, the latest evi­dence goes against his view. But instead of dropping the issue, he is asking him­self: Could we be jumping to a conclu­sion? This open-mindedness is really ad­mirable as against the who's right, who's wrong approach that lots of us like to in­dulge in."^

Pungent odor of ancient seeds yields clues to soil formation

The same chemical reaction that colors toast brown and creates roast chicken's enticing aroma may be going on in your garden compost heap. By following their noses, a team of chemists and archaeolo­gists has found that the Maillard reaction contributes to the decay of buried plant materials.

Chemist Richard P. Evershed, a reader at the University of Bristol's school of chemistry in England, studies ancient plant remains. Evershed, graduate stu­dent Helen A. Bland, and coworkers have been analyzing samples from the ar­chaeological site of Qasr Ibrîm in the southern part of Egypt that used to be Nubia.

"At the time of its original occupation, the settlement was located on a rocky hilltop overlooking the Nile," Evershed says. "However, it is now an island in Lake Nasser and the rising level of the reservoir is threatening the site." Despite the waters rising behind the Aswan Dam, the region is very arid, which is the rea­son the cache of grains, beans, and seeds has survived.

To the researchers' surprise, they smelled a pungent odor when they crushed some of the plant samples-something they'd never noticed in mate­rials from other locations [Science, 278, 432 (1997)].

The researchers trapped the volatile compounds given off by l,500year-old bar­ley and radish seeds. Analysis by gas chro-matography/mass spectrometry showed a mix of alkyl pyrazines, alkyl polysul-fides, and a series of six- to 10-carbon ox­ygenated compounds.

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Ught microscope Images show

cross sections of a modem radish

seed (top) and Its ancient

counterpart. The ancient

specimen has darkened

through the Maillard

reaction.

"As soon as we saw what the volatile com-pounds were, we thought of the Maillard reac­tion," Evershed says. The Maillard reac­tion—also known as the browning reac­tion—produces dark polymers and the characteristic "cooked" odors from the reac­tion of sugars with amino groups in pep­tides and proteins.

The Maillard reaction has been hypoth­esized to play a role in the formation of humus from decaying plant material. That notion has been difficult to prove, howev­er. Researchers have focused on identify­ing the reaction's polymeric products in soil, Evershed notes, because the lower molecular weight products would long ago have wafted away. But that's not what happened at Qasr Ibrim.

"It's quite remarkable that these very volatile compounds were presumably trapped within the beautifully preserved seeds," Evershed says. "It isn't until you crush the seeds that you get odors com­ing off."

Food biochemist William L. Porter of the U.S. Army's Natick Research, Devel­opment & Engineering Center in Natick, Mass., says the evidence for the Maillard reaction taking place within the seeds is convincing. "These pyrazines are not just formed by anything that comes down the pike—they are pretty distinctive," he says. "When we see them, we say 'browning.' " Porter is collaborating with the British scientists on fluorescence spectrophotometer analyses to character­ize the macromolecular products in the archaeological samples.

The next step in pinning down the Maillard reaction's role in soil formation, Evershed says, is to look for the volatile products in compost heaps and leaf lit­ter. "Now that we know what to look for," he says, "we should be able to fish these compounds out."

Pamela Zurer

26 NOVEMBER 3, 1997 C&EN