science/ technology concentrates Narrowest nanotube yet A variation of the commonly used electric-arc technique for producing carbon nanotubes yields tubes with diameters of only 0.5 nm, report researchers at the Center for Condensed Matter Physics of the Chinese Academy of Sciences' Institute of Physics in Beijing [Nature, 4 0 3 , 384 (2000) ]. The narrow nanotubes occur as the innermost shell in multiwall structures, according to professor of physics SiShen Xie and colleagues. The smallest nanotube previously reported had the same 0.7-nm diameter as C60, they note, whereas the 0.5-nm tubes have the same diameter as the smaller strained spherical cage molecule C36. To make the tiny tubes, the researchers modified the usual method of vaporizing a graphite electrode in an electric-arc discharge, substituting a graphite anode filled with a mixture of cobalt metal powder, carbon nanotubes, and amorphous carbonaceous material. They characterized the resulting nanotubes using high-resolution transmission electron microscopy. Because the modified anode produces narrower nanotubes, they propose the tubes are built up from carbon fragments of varying shape rather than from atoms or atomic ions as other scientists have suggested. "An important question that remains," the authors write, "is whether these small-diameter nanotubes could occur as single-wall nanotubes."^
grating viral DNA. Instead, the agents were inhibiting assembly of an integration complex, a step that is difficult to target in the cell. The team identified its compounds by screening more than 250,000 candidate molecules for the ability to bind to a preassembled integration complex. The most potent and specific inhibitors from this screen all contained a distinct diketo moiety, which later work showed to be important but not sufficient for antiviral activity. In further studies, the researchers show that their compounds work by blocking the enzymecatalyzed step that mediates formation of the covalent linkage between the viral and host cell DNA.^
'Plastic' solar cells from doped pentacene Currently, solar cells are made with inorganic compounds and can have efficiencies of more than 15%. Photovoltaic materials based on organics, however, have been much less efficient. But Bertram Batlogg, a physicist at Lucent Technologies' Bell Laboratories in Murray Hill, N.J., and colleagues there and at the University of Konstanz in Germany have created a "plastic" solar cell using the semiconducting organic molecule pentacene [Nature, 403,408 (2000)]. Pentacene already has been studied by researchers as a possible material for organic thin-film transistors. Batlogg and colleagues doped pentacene with iodine or bromine and used the materials to construct thinfilm photovoltaic diodes. The dopants increased the quantum efficiency of the devices by more than five orders of magnitude and imbued some of them with an efficiency of up to 2.4%. The authors write that even higher efficiencies might be obtained with films optimized for thickness, conductivity, and doping ratio, or films layered with other materials.^
bykol. A pheromone-binding protein in the male moth's antennae binds bombykol and then passes it along to its ultimate target, a nerve cell receptor. Professor of chemistry and chemical biology Jon C. Clardy of Cornell University and coworkers have now determined by X-ray diffraction the structure of the complex that bombykol forms with the pheromonebinding protein [Chem. Biol, 7 , 143 (2000)]. The structure shows that bombykol binds in a completely enclosed hydrophobic cavity formed by four of the pheromone-binding protein's six a-helices. According to the researchers, 'This encapsulation is needed both to transport the hydrophobic bombykol through the aqueous medium surrounding the nerve cell and to protect bombykol from destruction by the pheromone-degrading enzymes that assure that chemical signals don't linger."^
ATP sensor pairs rational design, peptide libraries
By inserting a rationally designed molecule with a bias for phosphate groups into roughly 4,000 tripeptides in a combinatorial library, chemists at the University of Texas, Austin, have discovered a tripeptide-based sensor for adenosine triphosphate (ATP) [/. Am. Chem. Soc, 122, 542 (2000)]. Associate chemistry professor Eric V. Anslyn and coworkers Stephen E. Schneider and Shannon N. O'Neil derivatized a benzene-based scaffold containing guanidinium groups that forms a binding site for phosphates by adding tripeptides to increase selectivity for ATP. Potential biosensors (about 15% Compounds block of the library members) were identified integration of DNA by their affinity for ATP by screening them against fluorescence-labeled ATP. from HIV into host cell Several of these tripeptide "hits" were then labeled with a fluorophore and their Researchers at Merck Research Laborafluorescence in the presence of unlabeled tories, West Point, Pa., have identified a ATP was monitored. The peptide that class of HIV-suppressing compounds produced the largest fluorescence— that work by blocking the integration of Binding protein serine-tyrosine-serine—was chosen for the virus' DNA into the DNA of an infectencapsulates further studies with adenosine monoed cell [Science, 287, 646 (2000)]. This phosphate (AMP) and guanosine triphosessential step in the life cycle of the virus pheromone phate (GTP). It was found to be highly sehas long been a target for new drugs to treat patients infected with HIV, but In a study that could lead to a better un- lective for ATP over either AMP or GTP. agents identified earlier in vitro have not derstanding of how a pheromone's chem- The lack of response to AMP indicates proven effective in cells. Daria J. Hazuda, ical message is transduced into a nerve that the biosensor relies on strong bindMichael D. Miller, and their colleagues in signal, researchers have obtained the ing of ATP's three phosphates to the guaMerck's antiviral research department first three-dimensional structure of a nidinium groups, the team suggests, reasoned that this earlier lack of success pheromone bound to a pheromone- whereas the lack of response to GTP indiwas because the agents were not block- binding protein. Female silkworm moths cates that the tripeptide "arms" impart ing the catalytic steps involved in inte- attract males with the pheromone bom- specificity for adenosine.^ 2 4 JANUARY 31, 2000 C&EN
