On Oct. 15, 1997, the seven-year voyage of the Cassini spacecraft to Saturn and its largest moon, Titan, began. The orbiter is expected to reach Saturn in mid-2004 and to release the Titan probe Huygens in November 2004. Huygens will acquire data about Titan's atmospheric chemistry as it descends and about Titan's surface chemistry when it lands.

	RENDEZVOUS Computer-rendered image shows Cassini having just crossed the plane of Saturn's rings. JET PROPULSION LAB/CALTECH IMAGE

Even now, scientists are considering missions to Titan beyond Cassini-Huygens. One proposal is to send a so-called Titan Organics Explorer, a robotic analyzer that will perform detailed studies of Titan's atmosphere, surface, and oceans. Among analytes of interest are enantioenriched organic compounds. Although not a signature of life, enantioenrichment is widely believed to indicate chemical complexity that leads to life. The resemblance of Titan's environment to that of prebiotic Earth's leads researchers to believe that the organic-rich world of Titan bears clues to how life on Earth began.

Last year, the challenges of detecting chirality on Titan were considered in a special issue of Enantiomer, edited by Christopher J. Welch, a process research fellow at Merck Research Laboratories, and Jonathan I. Lunine, a professor of planetary sciences and of physics at the University of Arizona, Tucson. The goal was "to get our heads together thinking about what could work for a mission that, if approved and funded, might launch in 2011," Welch says.

Suggestions include Earth-tested methods such as gas chromatography and chemical sensors, with modifications to meet the requirements of a mission to Titan. Others are still conceptual, such as circular dichroism and asymmetric catalysis.

A chiral molecule's differential absorbance of left- and right-circularly polarized light over a range of wavelengths is called circular dichroism (CD). A plot of differential absorbance versus wavelength constitutes a CD spectrum. Host-guest interactions between receptors and chiral analytes generate complexes that may produce amplified CD spectra. In some cases, the absolute configuration of the analyte may be assigned from the CD spectra.

James W. Canary, an associate professor of chemistry at New York University, New York City, says the method can be used to detect enantioenrichment because the spectra of nonracemic mixtures are shaped similarly to that of the enantiopure compound but have different amplitudes, which are linearly related to enantiomeric excess (ee). The method has not been tested for measurement of ee, but Canary and coworkers have established the linear relationship between ee and amplitude for chiral copper(II) complexes with derivatized amino acids (Chirality, in press).

To determine the absolute configurations of chiral compounds with very low ee, chemists Kenso Soai and Itaru Sato, at the Science University of Tokyo, propose to apply the asymmetric amplification of the autocatalytic reaction between 2-methylpyrimidine-5-carbaldehyde and diisopropylzinc to form (S)- or (R)-2-methyl-1-(2-methyl-5-pyrimidyl)propan-1-ol.

In the presence of an amino acid, a slight excess of the S or R product is formed, depending on the absolute configuration of the amino acid. Because the product catalyzes the reaction with amplification of ee, the product with the same absolute configuration as the amino acid initiator will be formed in great excess. Initiation is possible with other enantioenriched materials, including powder from a crushed quartz crystal.

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CHIRAL ROUNDUP As pharmaceutical companies struggle, their suppliers diligently tend the fertile fields of chiral chemistry STEADY RISE Worldwide sales of single-enantiomer drugs approach $150 billion TAKING A MEASURE OF CHIRAL RICHES Researchers respond to high demand for ways to measure enantioenrichment quickly EARTHBOUND A Chemist's Eye For Chirality EXTRATERRESTRIAL How To Detect Enantioenrichment On Saturn's Moon Titan CALENDAR Events Highlight Chiral Chemistry

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