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BIG MOLECULES IN SPACE Prebiotic molecules in interstellar space are hard to detect and study; but astrochemists are closing in ELIZABETH K. WILSON, C&EN WEST COAST NEWS BUREAU
sion of Physical Chemistry at the American Chemical Society national meeting in Atlanta in March, astronomers and chemists reported that they're starting to make headway in the study of large interstellar molecules. For example, models of interstellar molecular abundances are often based on the assumption that molecules are produced in the gas phase. Research scientist Gregory A. Grieves and chemistry professor Thomas Orlando at Georgia Institute ofTechnology have found evidence in support of the theory that surfaces of ice-covered grains, or interstellar dust, may be important reaction mediums. Dust grains tend to be covered first with water ice, which is very sticky. Then, less polar molecules like methane will stick to the ice. The water ice on these grains is also very porous, creating little nanovessels in which organic molecules can pool. Add a cosmic ray or energetic ultraviolet photon, and the organics can polymerize to form more complex molecules. These grain-mediated mechanisms should help modelers devise more accurate predictions, Orlando and Grieves said. Huge, dense clouds of matter known as hot molecular cores, which signal the formation of stars, are another spawning ground for relatively large molecules such as formic acid and aceSTARGAZER The Atacama Large Millimeter Array, under construction in Chile, promises to unveil more tic acid. Anthony J. Remijan, an astronomer at the interstellar secrets. National Radio Astronomy Prebiotic molecules—in particular, simple Observatory, has found some curious patsugars or precursors to proteins or nucleo- terns in their distributions. Formic acid, bases—hold great interest for astronomers. for example, tends to exist in areas known If these molecules were found in space, it as "shocked" regions, where bursts of hot would imply that the chemistry to make the gas hit colder gas. Acetic acid, however, building blocks of life can occur through- appears to reside in calmer, warm gas just out the universe. Although the detection of past the shocked region. Acetic acid and glycine in the interstellar medium doesn't formic acid are found in areas rich in nimean that life originated there (the prebi- trogen, suggesting that nitrogen may play otic molecules that preceded life on Earth a role in their formation. Acetic acid and its were almost certainly produced within our isomers-CH 3 C0 2 H, CH2OHCHO, and solar system), it would still say much about HC02CH3—are seen in different environthe complex chemistry of space. ments, implying that they are formed via In a symposium sponsored by the Divi- different reaction mechanisms. "This puts
S
PACE IS FILLED WITH MOLECULES,
even in the yawning gaps between stars. The simple ones are generally pretty easy to spot: CO, for example, or H 2 0 or HCN. Their spectra, whetherfrommolecular emissions, absorptions, rotations, or vibrations, are relatively simple and well-studied. Big molecules are another story entirely. As the number of atoms in a molecule increases, generally, so does the complexity of its spectrum. By the time you get to molecules as large as amino acids, the spectra have become an indecipherable mush of lines. Identifying such molecules has proven extremely difficult, and the recent announcements of the detection of two large prebiotic molecules, dihydroxyacetone and glycine, continue to be controversial (C&EN, Feb. 14,2005, page 44).
Use Best Practices to Manage Your Lab Chemical Inventory Leveraging Best Practices When Implementing a Chemical Inventory System Helps Ensure System Adoption and SuccessM Incorporation into Lab Processes Optimize Safety and Quality Chemical Data Management Companies that utilize chemicals in their labs and their manufacturing processes must manage those chemicals in a safe environment in accordance with government regulations. At a minimum, to ensure that this is accomplished, a system for managing information about the chemical safety and quality data should be established and maintained. Best practices, on the other hand, take this minimum and leverage the management of the chemical inventory by leveraging the abilities of the people, processes, and technology involved to best effect.
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SCIENCE & TECHNOLOGY
C-C and C-H in-plane
C-H out-of-plane
C-C-C out-of-plane
GOOD VIBRATIONS Polycyclic aromatic hydrocarbons in the interstellar medium can be detected by their characteristic vibrations, in and out of plane.
constraints on formation models people are trying to generate to account for the abundances of big molecules in the interstellar medium," Remijan said. In the future, he said, expect even more detailed pictures from much bigger, more sensitive telescopes, such as the Atacama Large Millimeter Array, now being constructed in Chile. In addition to their spectroscopic complexity, prebiotic molecules occur only sparsely, because they tend to be readily destroyed by the radiation that suffuses space. Much is made of the implications the discovery of their existence in space could
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hold for theories of the origin of life. But that connection tends to get overplayed, according to Pascale Ehrenfreund, an astrobiology professor at Leiden University, in the Netherlands. The chance that an amino acid produced in interstellar space could arrive on Earth intact and seed life is almost nil, she believes. Meteorites, on the other hand, can be loaded with prebiotic molecules such as amino acids and nucleobases. Ehrenfreund's studies of the molecules found in the Murchison meteorite indicate that while the building blocks of amino acids—HCN, ammonia, ketones,
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and aldehydes—could have some interstellar heritage, the prebiotic molecules themselves "were not formed in the interstellar medium, they were formed in the parent body where the meteorite comes from," she said. Polycyclic aromatic hydrocarbons, or PAHs, aren't nearly so delicate. These ubiquitous molecules are much more easily able to withstand the rigors of space. "They are the toughest of the bunch," remarked Louis J. Allamandola, head of the astrochemistry lab at the National Aeronautics & Space Administration's Ames Research Center in Moffett Field, Calif. PAHs are commonly found on Earth as products of combustion. A few decades ago, a pervasive infrared spectral signature was discovered in space, and it's now widely believed to be due to PAHs. Recently, Allamandola, Andrew L. Mattioda, principal investigator at the SETI Institute in Mountain View, Calif, and colleagues discovered cosmic PAH-like molecules that contain nitrogen. In Atlanta, they reported that these molecules, known as PANHs, have a number of interesting properties that make them promising new probes for interstellar chemistry. For example, they possess dipole moments, and they can be traced with radio telescopes. MUCH LESS well-characterized are the diffuse interstellar bands (DIBs), a collection of well-defined lines that don't fit any known spectrum. DIBs were first spotted in 1919 at Lick Observatory in the mountains around Santa Cruz, Calif. They're believed to be due to molecular gases, but which molecules produce them remains unknown. Benjamin McCall, an astronomy and chemistry professor at the University of Illinois, Urbana-Champaign, has found that a subset of DIB lines is strongly correlated with lines from C2—that is, molecules responsible for some of the DIBs exist in the same regions as C2. Since C2is easily photodissociated, it tends to form in denser regions that are more shielded from UV radiation. Perhaps the unknown molecules thrive in a dense, protected environment, McCall said, or perhaps there is a real chemical association, in which they're either formed from C2, or C2 is a building block. "We have no idea at this point," McCall said. His group has also found two spectroscopic bands that have the same intensity ratio wherever in the sky they're measured. That implies the bands may be caused by the same molecule. Eventually, this might allow the scientists to identify a molecular characteristic, such as an aromatic C-H stretch. Until then, DIBs may continue to be "the longest standing mystery in all of spectroscopy," McCall said. •
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