Science Concentrates

Any shape you want, you get

Rods, cubes, stars, and hexagons: These are just a few of the shapes into which gold nanoparticles can be coaxed into forming uniformly and in high yield. University of South Carolina, Columbia, chemists Tapan K. Sau and Catherine J. Murphy control nanoparticle shape by systematically varying the parameters of a solution-based seed-mediated procedure they developed for producing nanocrystals. The procedure involves preparing gold seed particles and then adding an amount of the seed solution to a solution containing cetyltrimethylammonium bromide, chloroauric acid (HAuCl₄), ascorbic acid, and sometimes a small amount of silver nitrate [J. Am. Chem. Soc., 126, 8648 (2004)]. The procedure is noteworthy not only for its high yield but also for its simplicity, requiring only aqueous solutions, room temperature, and one surfactant instead of various additives to control particle size and shape.

Taking the heat off PCR

A new method of DNA amplification promises to be just as simple as the polymerase chain reaction (PCR) but far more portable, making it ideal for handheld DNA diagnostic devices that could be used to detect pathogens in the field or in the doctor's office. PCR has largely been confined to the laboratory because it requires a power-hungry thermocycling instrument to perform multiple rounds of heating (to separate the DNA strands in a DNA sample) and cooling (to enzymatically copy the strands to make new DNA duplexes). So Huimin Kong and colleagues at New England Biolabs in Beverly, Mass., took a cue from nature and used a helicase enzyme to separate the DNA strands instead of heat [EMBO Reports, published online July 9,
http://dx.doi.org/10.1038/sj.embor.7400200].
Their helicase-dependent amplification (HDA) method can thus be performed at one temperature that is optimized for synthesis. HDA is simpler and more efficient than previous isothermal DNA amplification methods, Kong says, and should be useful for making simple portable DNA diagnostic devices.

Location defines enzyme's activity

Location, location, location. The real estate rule seems to hold for enzymes, too, according to a team led by John Shanklin of Brookhaven National Laboratory. While characterizing plant 16:0 desaturases--enzymes that create a double bond in fatty acids with 16-carbon chains--Shanklin's team discovered that these enzymes produce slightly different products when they are moved to different cellular addresses [Proc. Nat. Acad. Sci. USA, published online July 6,
http://www.pnas.org/cgi/doi/10.1073/
pnas.0402200101]. The researchers coaxed a 16:0 desaturase normally found in chloroplasts into moving to the endoplasmic reticulum (ER). To their surprise, the relocated enzyme inserted a double bond in a different place in the carbon chain than it does in the chloroplast, acting instead like ER desaturases. Conversely, when they moved several different ER desaturases to the chloroplast, the displaced enzymes performed like chloroplast desaturases. They find that the unique head groups that top off fatty acids with 16-carbon tails in the ER and chloroplasts drive the location-specific change in enzyme regiospecificity. To change an enzyme's specificity, scientists normally turn to mutations--but in some cases just a change in scenery could do the trick, Shanklin says.

Binary catalyst aids synthesis of polycarbonates

One of the most promising green polymerization processes is the alternating copolymerization of CO₂ and epoxides to make polycarbonates. Several research groups have been exploring ongoing problems in polycarbonate synthesis related to catalyst efficiency, reaction conditions, and control of polymer structure and molecular-weight distribution. In the latest effort, Xiao-Bing Lu and Yi Wang at Dalian University of Technology, in China, report a novel binary catalyst system that affords an efficient conversion of CO₂ and propylene oxide to poly(propylene carbonate) with high selectivity and stereoregular control under mild reaction conditions [Angew. Chem. Int. Ed., 43, 3574 (2004)]. The researchers used a chiral cobalt(III) salen complex in conjunction with a quaternary ammonium salt, optimizing the catalyst activity by changing a substituent group on the cobalt catalyst and changing the quaternary ammonium anion. They achieved two- to three-times higher catalyst turnover frequency at significantly lower CO₂ pressure than previously reported by others using a cobalt catalyst alone.

Third type of HIV treatment progresses

Current HIV treatments tackle two of the three viral enzymes necessary for the virus to replicate in a host's infected cells: reverse transcriptase and protease. Daria J. Hazuda, Steven D. Young, and their colleagues at Merck Research Laboratories in West Point, Pa., have been working for several years to develop a compound that targets the third enzyme, integrase. They now report results from animal trials showing that their compound, L-870812 (shown), can prevent integrase from inserting viral DNA into the animals' cellular genome [Science, published online July 8, http://www.sciencemag.org/<br > cgi/content/abstract/1098632v1]. Rhesus macaques that are treated early during their infection with simian-human immunodeficiency virus respond best to the treatment. The animals tolerated the drug well and showed no signs of toxicity. The researchers note that L-870812 also exhibits potent antiviral activity in vitro against HIV-1, the most virulent and widespread of the types of HIV that cause AIDS.