Squeeze Play the helix. The base pairs are located close to the molecule's axis. Thus, the intercalating metal complex must interact closely with the B-DNA for binding to take place. In contrast, Z-DNA sports a rather shallow groove and the base pairs are ar ranged closer to the outer edge of the molecule. Because the metal complex doesn't need to interact as closely with Z-DNA to bind to it, the steric repulsions are less severe and so both enantiomers manage to hang on to similar extents. Recently some researchers have suggested that left-handed B-like structures (DNA helices contain ing a deep groove) also might exist
in nature. If they do, Barton says, her group's chemical probe ought to be able to detect them. Now that Barton and her collabo rators, graduate student Avis Danishefsky and undergraduates Lena Basile and John Goldberg, have a chiral reagent in hand, they're "very excited," she says, about using it "to examine particular DNAs pro posed to be in the Z-form, particu larly DNA-drug complexes, and spe cific DNA sequences." Their work, supported by the National Institutes of Health, also may result in new stereoselective drugs that bind to and specifically modify either rightor left-handed DNA structures. D
Tem-Pres Pressure Intensifiers
Experiments show cyclopropenone is aromatic basicities, photoelectron spectra, and quantum mechanical calculations. But nuclear magnetic resonance fails to show an expected current of elec trons around the three-membered ring. Most damaging, determination "Almost 25 years of cyclopropenone of the heat of combustion of diphechemistry have failed to decide the nylcyclopropenone a few years ago issue of whether this molecule is, gave a value that seemed too high in fact, the smallest neutral aromat for the compound to be aromatic in ic species," Arthur Greenberg told the three-membered ring. the Division of Organic Chemistry. Working with physical chemistry Greenberg, w h o is professor of professor Reginald P. T. Tomkins physical chemistry at New Jersey and graduate student Mark DobInstitute of Technology, presented rovolny at NJIT, Greenberg redeter evidence based on calculations and mined the heat of combustion of calorimetry indicating that cyclo diphenylcyclopropenone and found propenone is indeed aromatic. His it to be much lower than the previ results give chemists confidence that ously published value. Needing their thinking about aromatic char independent, and more accurate acter is correct. confirmation, Greenberg turned to Aromaticity is a condition of mole physical chemist Norris Smith at the cules containing consecutive unsat Department of Energy's Bartlesville urated atoms in which the mole Energy Technology Center in Okla cules are stabler than expected homa. Smith got results in close on the basis of isolated multiple agreement with those of the NJIT bonds. To predict aromaticity in ring team and calculated a heat of forma compounds, chemists have long re tion of 46.4 kcal per mole for the lied on the Huckel principle that if crystalline compound. the number of 7r-electrons is 4n + Because Greenberg's calculations 2, where η is a whole number, the for the compound were good only compound is aromatic. If the cyclo in the gas phase, he solicited physi propenone carbonyl group is polar cal organic chemistry professor ized with both 7r-electrons on oxy James Chickos at the University of gen, then the three-membered ring Missouri, St. Louis, to determine its has two 7r-electrons and η is zero. heat of vaporization. Chickos found There are indications that cyclo it to be 28.5 kcal per mole, which propenone and its derivatives are yielded a gas-phase heat of forma aromatic, based on thermal stability, tion for diphenylcyclopropenone of dipole moments, infrared spectra, 74.9 kcal per mole.
Washington, D.C
• • • • * • • • • *
The compact Tem-Pres intensifier, originally designed for laboratory use. occupies minimum floor space. The self-contained unit automatically accepts low-pressure gas from an external source and compresses it to elevated pressures. Gauges indicating hydraulic drive pressure as well as inlet and intensified gas pressures are mounted on the cabinet panel. Tem-Pres constructs the units of high-yield, vacuum-melt steel forging, heattreated to exacting specifications.
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LECO CIRCLE 56 ON READER SERVICE CARD September 19, 1983 C&EN
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CIRCLE 31 ON READER SERVICE CARD September 19, 1983 C&EN
Greenberg's next problem was that the compound might be stabilized by aromaticity, but it certainly would be destabilized by the strain of three sp 2 -hybridized carbon atoms in a ring. His approach to the problem was to calculate a hypothetical heat of formation for unstrained, nonaromatic diphenylcyclopropenone, add an estimate for ring strain, and come up with an aromatic stabilization energy (or lack of it) by comparison with the experimental gas-phase heat of formation. He turned to methods for such calculations worked out by physical chemistry professor Sidney W. Benson at the University of Southern California, and extended by physical chemistry professor Joel F. Liebman at the University of Maryland, Baltimore County. These meth-
ods involved adding up atomic group contributions to heat of formation, and resulted in a value of 30.2 kcal per mole. To arrive at a strain energy to add to this hypothetical unstrained, nonaromatic diphenylcyclopropenone, Greenberg considered known strain energies of cyclopropane, cyclopropene, and methylenecyclopropane. This led him to an estimate of 67 kcal per mole. Adding this to the 30.2-kcal-per-mole value calculated earlier gave a heat of formation of 97.2 kcal per mole for a molecule with strain but no aromatic stabilization yet. Subtracting the experimental 74.9 kcal per mole from this led Greenberg to the conclusion that real cyclopropenone has an aromatic stabilization of 22 kcal per mole. D
Lemon odor helps identify male moth pheromone
Washington, D.C. * * * ***••*• "Courting with Cologne—a Male Moth Lures his Love" was the whimsical title at a press conference. "Moths in Love? It's Just Chemistry!" responded the headline over a story in the Washington Post the next day. Both refer to an intriguing research project described to the Division of Agricultural & Food Chemistry. Biochemist Wendell A. Roelofs of New York State Agricultural Experiment Station, Cornell University, Geneva, notes that the story "involves insect pheromones, plant hormones, fruit volatiles, and human perfumes"—and an exhibition of serendipity by a Japanese natural product chemist who likes tea with lemon. And the research indicates that an important commercial perfume ingredient in fact lacks any odor and receives its fragrance from an impurity. The work was carried out by Roelofs, flavor chemist Terry E. Acree, also of the Geneva laboratory, natural product chemist Ritsuo Nishida of Kyoto University in Japan, and entomologist Thomas C.
Baker, now at the University of California, Riverside. Roelofs points out that scientists studying insect pheromones have generally worked with female sex attractants—the chemicals that lure male insects from long distances to bring about mating. However, courtship in a major international insect pest, the Oriental fruit moth— Grapholitha molesta (Busck)—is unusual among Lepidoptera (moths and butterflies): After males are attracted to the vicinity of females by the female pheromone, the courtship behavior is not over. The male moths then propel their own short-
Compounds found in male fruit moth scent organs
Ethyl frans-cinnamate
COOChL Methyl jasmonate
(R)-(-)mellein
COOCH'3 Methyl 2-epijasmonate
