MEETING BRIEFS FROM ST LOUIS
Vibrational circular dichroism of sugars Chemists for the first time have used Fourier transform infrared (FTIR) spectroscopy to study circular dichroism in vibrational transitions (VCD) of simple sugars. Historically, circular dichroism has been used to probe electronic transitions in chiral molecules using visible light. Such measurements shed light on a molecule's stereochemistry. But because most carbohydrates do not absorb in the visible region, they cannot be studied in this way. They do absorb in the infrared region, and thus their vibrational transitions can be probed, albeit with much greater difficulty. Assistant professor Prasad L. Polavarapu and graduate student Darlene M. Back of Vanderbilt University have found that the FTIR technique allows one to make VCD measurements of carbohydrates "with confidence," despite the feebleness of the VCD effect. Moreover, says Polavarapu, a VCD band arising from carbon-oxygen and carbon-carbon stretching motions "reflects the overall arrangement of hydroxy I groups in carbohydrates." Hence, VCD could be used as a stereochemical probe for carbohydrates. Metal-containing polymer traps aldehydes, ketones Boulder, Colo., chemists Eric J. Williams of the Cooperative Institute for Research in Environmental Sciences and Robert E. Sievers of the University of Colorado have solved a problem in air sampling by developing a porous polymer that selectively and quantitatively adsorbs oxygenated compounds.The polymer is fashioned by bonding a fluorinated /3-diketone moiety to a styrene-divinylbenzene copolymer and then incorporating a europium(lll) ion. Eu-Sorb, as the material is called, retains many aldehydes and ketones from air samples by complexing the compounds to the metal ion. If the polymer is then heated to about 200 °C in a helium flow, the sorbed molecules are released and can be directed into a capillary gas chromatograph for determination. EuSorb exhibits very high retentivities for even highly volatile compounds like acetaldehyde. Monomer crosslinks at room temperature Methyl a-acrylamino-cv-methoxyacetate is an activated monomer that can be copolymerized with acrylates and/or styrene to produce resins for one-package or high-solids, solvent-borne coatings curable at ambient temperatures, according to organic chemist David A. Ley of American Cyanamid Co., Stamford, Conn. Ley described a two-component system cured with 3,3'-ethylenedioxybis(propylamine); a moisture-cured coating that uses the methyl isobutyl ketone-blocked adducts of melamine and hexamethylenediamine as crosslinker; and additionally a one-component formulation in which the activated monomer group couples to another of its own kind, catalyzed by p-toluenesulfonic acid. Uranium-uranium bonding seems unlikely Although the group VI elements molybdenum and tungsten in some of their oxidation states have a strong tendency to form multiple metal-metal bonds, the next element in the group, uranium, does not, says Willi Schwotzer, a staff scientist at Texas A&M University. Along with colleagues F. Albert Cotton and David O. Marler, Schwotzer has made a systematic attempt to synthesize and structurally char30
April 23, 1984 C&EN
acterize the uranium analogs of molybdenum and tungsten species that have strong metal-to-metal interactions. They prepared two likely candidates, U3O[OCH2C(CH3)]10 and U2[OCH(CH3)2]io. Although both clusters have the same general structure as their molybdenum analogs, the U—U distance in each is greater than 3.5 A—too far for even a single bond to be present. Two examples do not prove that U—U bonds never form, Schwotzer says, but "we feel that the hopes of observing U—U bonds are rather dim." The reason, he suggests, is that electrons in these metals reside in 5f orbitals, which do not have adequate overlap with similar orbitals on adjacent metal atoms to cause bonding. Quinine binds directly to heme iron High-field nuclear magnetic resonance studies by postdoctoral fellow Digambar Behere and chemistry associate professor Harold M. Goff of the University of Iowa are clearing up the picture of how the antimalarial drug quinine binds to the iron-containing blood pigment, heme. The malarial parasite attacks the protein portion of the blood oxygen carrier, hemoglobin, leaving the heme portion behind as a breakdown product. Many effective antimalarial drugs, including chloroquine and quinine, zero in on affected red blood cells by binding to these heme residues. The Iowa chemists find that this binding occurs directly to the heme iron through a deprotonated alcohol residue on the drug. It thus represents a novel example of metalloporphyrin coordination chemistry in which a potentially ambidentate ligand is converted to its zwitterionic form as a consequence of the Lewis acidity of the metal center. The finding suggests that other potential antimalarial drugs also might be targeted to the heme by including alcohol or hindered base moieties in their structures. Polysilanes act as photoinitiators Polysilanes, versatile polymers in their own right, also can act as efficient photoinitiators of addition polymerization reactions, according to Robert West and Andrew Wolff of the University of Wisconsin, Madison. West explains that ultraviolet energy breaks bonds between silicon atoms forming the polymer backbone. That reaction creates radicals that can then lead to the formation of such polymers as polystyrene, polyvinyl chloride, or acrylates. The approach likely will be useful in the manufacture of films and tapes, West says. No solvents are needed. Products can be purer, since only minute quantities of the photoinitiator are used. Also, the polysilanes can be protected against oxygen degradation by the addition of amine side groups (a refinement devised by 3M chemist Donald Peterson), thus eliminating the need for expensive inert gas blanketing during polymerization. The Wisconsin chemists also point out that polysilanes can be tailored (through appropriate choice of side groups) to be sensitive only to specific UV wavelengths; that characteristic may prove useful in the future, as more is learned about controlling chemical reactions with lasers. Olive pesticides characterized The bitter olive (Olea europaea) is known to contain natural pesticides that render it resistant to insect and fungal attack. Natural products chemist Isao Kubo and associates at the
MEETING BRIEFS FROM ST. LOUIS
University of California, Berkeley, recently isolated and characterized two of these seco-iridoid compounds— oleuropein and ligstroside—which repel such pests as bollworms and armyworms and also stop growth of bacteria and fungi. The two compounds also kill the snails that are the vector for schistosomiasis. Kubo notes that copper sulfate, poisonous to plants as well as pests, is now used for snail control. The olive compounds, which normally are removed during processing and discarded, could provide effective natural alternatives, he says. Still no sign of proton decay Probably the most sensitive of a number of experiments currently studying proton decay has yet to provide any evidence that such decay occurs, according to Geoffrey Blewitt, a postdoctoral fellow in the laboratory of John LoSecco, physics professor at California Institute of Technology. Caltech is one of seven institutions participating in what is known as the 1MB Collaboration, which has set up a Cerenkov detector in a salt mine in Ohio. In essence, such a detector monitors a quantity of water containing enough protons that the rare decay event would be expected to occur in a reasonable time. If a proton decayed, its products would be traveling faster than the speed of light in water and would produce the Cerenkov effect, a shock wave phenomenon in which such particles emit light in a very characteristic way. The results thus far spell trouble for what is known as the minimal SU(5) theory, the currently favored grand unification theory that links the electroweak and -strong forces. In SU(5) theory, the most likely proton decay process produces a positron and a neutral pion; the proton lifetime is predicted to be about 5 X 1029 years. The current results indicate a minimum proton lifetime of about 2 X 1033 years, Blewitt says. Chemists get warning about CI-MS spectra Chemical ionization-mass spectra may have peaks for combinations of fragment ions with neutral parent molecules, warns process researcher Cynthia A. Maryanoff of McNeil Pharmaceutical, Spring House, Pa. Working with mass spectroscopist Sai Y. Chang, she found the artifact arises from Lewis bases containing oxygen or nitrogen. Such peaks could be mistaken for impurities or parent molecular ions. For example, A/,A/-methylenedipyrrolidine gave a peak corresponding to the neutral molecule combined with an A/-methylenepyrrolidinium ion, whereas 4-diphenylmethylpiperidine spectra had a peak the researchers ascribed to a parent molecule—diphenylmethyl radical-proton aggregate. The phenomenon does not occur with hydrocarbons or during electron ionization, and the McNeil workers ruled out dimer fragmentation as a cause. Nonwovens with high activated carbon content Several novel nonwoven adsorptive feltlike structures have been prepared with high (up to 95%) loadings of activated carbon fibers and/or carbon powders by Robert D. Giglia, Charles P. Beetz Jr., and Robert L. Potter of American Cyanamid. Use of a highly fibrillated acrylic fiber as binder in a wet papermaking process permits the high loadings and helps maintain good wet strength and chemical resistance. Granular and powdered active carbons have long been used to adsorb toxic vapors from air streams, but activated carbon
fibers have much larger adsorption capacities and adsorption rates because of the large concentration of micropores on their surface, and they permit thinner and lighter filter structures. The adsorptive papers prepared at Cyanamid show outstanding dynamic carbon tetrachloride vapor adsorption characteristics, with carbon fiber-powder hybrids yielding the best results. Potential military and civilian applications include gas mask cartridges and other filtering equipment and chemical protective clothing. Boron reagents yield chiral 1,2-diols Reaction of c/s-3-methoxyallyldiisopinocampheylboranes with acetaldehyde yields 3-methoxy-4-penten-2-ols in 95% enantiomeric excesses and 98.5% threo selectivity, according to Purdue University postdoctoral student Prabhakar K. Jadhav. Such chiral 1,2-diols are important in natural product synthesis. Working with organic chemistry professor Herbert C. Brown, Jadhav made methyl diisopinocampheylborinates from diborane, (+)- or (—)-a-pinene, and methanol, reacted them with the lithium salt of allyl methyl ether to get lithium diisopinocampheylmethoxymethoxyallylborates, and converted these to c/s-3-methoxyallyldiisopinocampheylboranes with boron trifluoride etherate. The Purdue chemists held temperatures to —78 °C throughout to avoid cis-trans isomerization. In the final product, the (2R, 3R)-isomer resulted from use of (+)-«-pinene and the (2S, 3S)-isomer from the (—)-terpene. Oxygen scavenger aids spectrofluorometry A chemical method for deoxygenating samples (to minimize the "quenching" that can interfere with fluorescence and phosphorescence spectrometry) is being developed by Isiah M. Warner and coworkers at Emory University. In their method, sample solution is pumped through oxygen-permeable tubing. The tubing is surrounded by another solution containing Cr 2+ and H + ions that immediately reduce the oxygen to water. With the concentration gradient thus maintained, oxygen continues to diffuse through the tubing until the analyte is almost oxygen-free. This method is faster and less tedious than currently used nitrogen purge, freeze-thaw, and vacuum techniques, the Emory chemists say, adding that the approach is expected to be especially useful when incorporated into flow injection analysis systems. Substance protects against tumors in rats Experimentally induced tumors in rats regressed when the rats were fed /3-carotene in conjunction with local x-ray therapy, according to researchers at Albert Einstein College of Medicine, Bronx, N.Y. Eli Seifter and coworkers discontinued the supplements after one year and found that tumors reappeared only after a long lag period and only in an occasional rat, suggesting that stored /3-carotene exerted a therapeutic effect. The researchers also fed rats a basal diet containing 6.4 mg of /3-carotene per kg of diet and separated them into five groups, each of which received /^-carotene supplements ranging from 0 to 270 mg per kg of diet. After one month, the supplements were discontinued and the rats were administered the carcinogen 7,12-dimethylbenz(a)anthracene (DMBA). The incidence of tumors in the five groups suggests that /3-carotene protects against DMBA carcinogenesis in a dose-dependent way. April 23, 1984 C&EN
31
