SCIENCE
Learning from history When Jeffrey Bada wanted a better way to look for signs of extraterrestrial life, he started by searching the literature. Turn-ofthe-century literature. While thinking about new ways to isolate amino acids from rock and soil samples, Bada remembered that researchers back in the 1920s had grappled with a similar problem. Not yet blessed with GC or ion-exchange resins, those scientists used sublimation, he says. They heated dried tissue extracts in a vacuum chamber, and amino acids would condense on the unit's coldfinger. "I'd read those papers and filed them away. But several years ago I dug them out, and we started testing the idea," says Bada, a researcher at the Scripps Institution of Oceanography. "It worked remarkably well." The technique is fast and simple and essentially requires no liquid reagents (Anal. Chem. 1998, 70,3119), BaBadaysa For detection alone, no fluid is used. If the amino acids are going to be characterized, though, some liquid is needed for the vapor hydrolysis step used to break down the proteins before sublimation. But once the sample dries, results come quickly. "You get amino acid extracts in hours," he says. "Conventional methods using hydrolysis desalting used to take us days." Sublimation works, Bada says, because amino acids are stable in a dry, low-pressure
environment. There's little decomposition or racemization, which means the technique doesn't alter the telltale sign of biotic processes: an excess of either D- or L-amino acids. To show just how effective sublimation is, Bada and graduate student Daniel Glavin isolated amino acids from fossilized mollusk shells and deep-sea sediments. They are the kinds of deposits most likely to contain extraterrestrial amino acids, if there are any to be found, Bada says. One reason is that carbonates, which make up the bulk of these samples, are very good at trapping amino acids. In addition, some scientists think that sedimentation DI*0" cesses on Mars might have been similar to those that produce the carbonate-rich deposits in our
Filling in the blank spot
Protein high-performance membrane chromatography (HPMC) is evolving rapidly as an analytical and semipreparative tool, but ultracentrifugation and extraction—the technology to similarly treat DNA—has lagged behind, according to Ruth Freitag of the Cellular Biotechnology Laboratory at the Federal Polytechnic School in Lausanne (Switzerland). Freitag believes chromatography would be the preferred technique for obtaining very pure nucleic acids from biological samples. In the August 15 issue of Analytical Chemistry (p. 3348), shs and her colleague Roberto Giovannini, working with Tatiana Tennikova of the Institute of Macromolecular Compounds at the Russian Academy of Sciences in Stt Petersown fifPflns burg, describe work with HPMC that allows The results so far have been impressive them to analyze DNA plasmids. They hope enough for NASA to award Bada and colthe technique will fill in what they describe laborators at the Jet Propulsion Laboratory as a "blank spot" in the appllcation of mema grant to build a prototype instrument, brane chromatography called the Mars Organic Detector. The The basic difference between HPMC compact device will use sublimation comand generic column chromatography is the bined with a tag that fluoresces in the presstationary phase. Rather than a column ence of amino acids to determine if soil packed with a particulate separating mesamples contain the building blocks of life. dium, HPMC relies on a flat disk an inch or "The neat thing is that we won't need a so across and a few millimeters thick. This vacuum pump to do the sublimation bedisk contains pores of 0.45-um diameter; in cause we can use J^ars's ambient pressure essence, it is a separating membrane, which is only aboutfiveTorr" Bada hence the name. Bada is so optimistic about the device Proteins that vary widely in their interthat he's already planning a second version, actions with stationary phases do not need which contains a microchip-based CE systo be eluted down a long column to be septem for identifying any amino acids found. At arated. All that is needed is a membranous the same time, he and his students material through which the proteins can are evaluating sublimation as a way pass at rates that depend on mass transfer of detecting other biological moleor diffusional effects. "HPMC thus cules, including nucleic acids. Knowing that amino acids aren't emerges as the perfect tool for fast, highresolution protein chromatography," Freharmed by sublimation also means it's possible that they could survive itag says. She and her colleagues suspected that long enough in the upper atmosphere to recondense and rain down HPMC could be similarly used to separate DNA They tested the process with a suon Earth, Bada says. The idea has percoiled DNA plasmidfromE. coli. Under been around for a while, he adds, but seems more plausible since the optimized conditions, using a macroporous polyglycidylmethacrylate-co-ethylene rediscovery of the old sublimation dimethacrylate disk, the original plasmid literature. But if the technique is so useful, peak in the chromatogram could be split why did it lay dormant for 80 years into three maxima. These, Freitag says, correspond to the three isolated bands in or so? Bada says most scientists agarose-gel electrophoresis. "Presumably just didn't seem to know the previthe threefractionswere supercoiled, ous work existed. Amazing what nicked, and open-circular plasmid DNA" picking the right reading can do. Elizabeth Zubritsky she explains. Analytical Chemistry News & Features, September 1, 1998 5 7 3 A
