SCIENCE & TECHNOLOGY

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Fullerenes' golden child Scientists have assumed that collections of a few dozen gold atoms prefer to exist as relatively uninteresting space-filling clusters. But now comes a prediction that a molecule composed of 32 gold atoms should also be able to form an exotic golden fullerene [Angew. Chem. Int. Ed., 43 , 2678 (2004)}. Chemist Mikael Ε Johansson and colleagues at the University of Helsinki, in Finland, performed quantum mechanical calculations that take into account relativis-

tic effects, such as increased inner electron mass, that occur in heavier atoms. These relativistic influences

help make possible a stable Au32 cage structure, as shown. Like its fullerene cousin C60, Au32 would be icosahedral. But unlike C60, electrons on the surface of Au32 would be able to move about freely

The interior of the molecule would also be mag­netically shielded, perhaps more strongly than any other known molecule. The authors envision nu­

merous uses for golden fullerenes, including drug transport.

Icy reactions lead to more toxic products Ice can be more dangerous than water when it comes to the toxicity of environmental contaminants . Chemists at Masaryk University in the Czech Republic compared the fate of 2-chlorophenol and 4-chlorophenol—contaminants from the paper and pesticide industries—when exposed to ultraviolet light in either ice or

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water {Environ. S ci. Techno I., 38 , 2873(2004)}. In water, both chlorophenols were con­verted within 30 minutes to di-hydroxybenzenes, which are themselves not stable in sun­light. In ice, the chlorophenols lasted for up to 10 hours, and the major products were more stable dimers, such as the one shown. In addition, the ice-de­rived products, after prolonged irradiation, were more toxic to marine bacteria and rat liver

cells than either the water pho-toproducts or the parent con­taminants. Arctic ecosystems are a sink for anthropogenic pollutants, says Jana Klânovâ, one of the coauthors. The Masaryk researchers believe that ice catalyzes unique reac­tions, but the mechanisms re­main unclear. They are now studying the photoproducts of other pollutants in ice.

Fishing out rare fullerenes Despite considerable inter­est in engineered nanoparti-cles, the study of fullerenes larg­er than C76 has proven to be a challenge for scientists. That's because these molecules are formed in lowyield as hard-to-isolate by-products during combustion-based industrial production of C6 0 and C70. Us­ing a linked zinc porphyrin dimer as a host molecule (shown), Tàkuzo Aida, Kentaro Tashiro, and Ybshiaki Shoji at the University of Tokyo have devised a way of extracting these rare fullerenes [J. Am. Chem. Soc, published online May 11, http://dx.doi.org/10.10 21/ja04899471. When added to a mixed fullerene solution,

the porphyrin dimer will selec­tively sequester the larger ful­lerenes. This host-guest com­plex can then be isolated via chromatography on alumina, and the two molecules can be separated by treatment with 4,4 /-bipyridine. Using this method, the group was able to start with a mixture containing 10% higher fullerenes and, in a single extraction, isolate a mix­ture consisting of 97% higher fullerenes. Changing the length of the hydrocarbon linkers or adding substituents to the por­phyrins can alter the host's affinity for a particular higher fullerene.

Drug target for tuberculosis The flavin-containing en­zyme uridine 5/-diphosphate (UDP)-galactopyranose mutase could be a target for drugs against tuberculosis, according to a study of the enzyme's mech­anism of action [Nat. Struct. Mol. Biol., 11, 549 (2004)}. The en­zyme catalyzes the conversion of UDP-galactopyranose to UDP-galactofuranose, akeypre-cursor to a cell-wall component of Mycobacterium tuberculosis. Research by chemistry profes­sor Laura L. Kiessling and co­workers at the University of Wisconsin, Madison, indicates that the conversion begins with nucleophilic attack on the pyra-nose by the flavin in the form of a reduced anion. Pyranose-to-furanose isomerization then occurs via a flavin-derived imi-nium ion. This mode of flavo-enzyme reactivity has not been

observed before, Kiessling says. Compounds that can bind the enzyme and alter the flavin's nucleophilicity would have antimycobacterial activity the researchers suggest.

New route to NAD+

A previously unidentified metabolic route to the oxidized form of nicotinamide adenine dinucleotide (NAD4)—a small-molecule coenzyme essential for life—has been discovered by Pawel Bieganowski and Charles Brenner of Dartmouth Medical School {Cell, 117,495 (2004)]. The finding upends the con­ventional wisdom that trypto­phan and niacin are the only pre­cursors to NAD + in yeast,

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humans, and other eukaryotes. The researchers observed that even without agene required for all described NAD+ pathways, yeast still managed to produce the coenzyme. Upon further in­vestigation, Bieganowski and Brenner identified enzymes and genes in bothyeast and humans that could convert a different molecule, the vitamin nicotin­amide riboside (shown), into NAD+ . Previously, scientists thought this NAD+ pathway ex­isted only in certain bacteria. Bieganowski and Brenner also found that milk is a source of the vitamin. The Dartmouth re-searchers think that nicotin­amide riboside could be useful as a vitamin supplement and that the vitamin and genes re­quired for its utilization may be important in certain types of cancer treatment.

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