science/ technology
CONCENTRATES Study quantifies dioxin-cancer link An extended study of chemical industry workers has confirmed previous find ings that high exposures to 2,3,7,8-tetrachlorodibenzo-/)-dioxin (TCDD) result in significant increases in cancer deaths \J. Natl. Cancer Inst, 91,779 (1999)]. Ep idemiologist Kyle Steenland and cowork ers at the National Institute for Occupa tional Safety & Health, Cincinnati, carried out exposure-response analyses on about 3,500 workers from U.S. plants that used to produce chemicals contaminated with TCDD. They found an average increase in cancer mortality of 13% in the group, and an increase of 60% among workers with the highest level of TCDD expo sures. "Our analyses suggest that high TCDD exposure results in an excess of all cancers combined, without any marked specificity," they note. "However, excess cancer was limited to the highest exposed workers," whose exposures like ly were 100 to 1,000 times higher than the general population's and similar to TCDD levels used in animal studies.^
fill with gallium above the metal's melt ing point (30 °C), Barsoum speculates that it may be possible to scribe galli um patterns on a surface as a potential first step in fabricating semiconductor devices.^
First dicarbaporphyrin synthesized The study of hydrocarbon analogs of por phyrins is likely to provide valuable in sights into the nature of porphyrin aromaticity. One of the key aims is the syn thesis of a tetracarbapoφhyrin, in which all four pyrrole units of the porphyrinoid structure are replaced by cyclopentadiene units. In a significant step toward this aim, chemists at Illinois State Univer sity, Normal, have synthesized the first dicarbaporphyrin, which has cyclopentadiene units in place of two pyrrole units /^^v
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microscopy, cell biologist W. T. Godbey and bioengineer Antonios G. Mikos at Rice University and cell biologist Ken neth K. Wu at the University of Texas Health Science Center show that after at taching to the cell, the PEI-DNA com plexes move around the cell membrane to form clumps that are taken up by the cell within two to three hours. The com plexes reach the nucleus in ordered structures about an hour later, and the DNA they deliver is expressed after about another hour. In other experi ments, the group has traced the path of PEI alone and showed that the polycation enters the nucleus—also as an ordered structure. 'This is a new aspect of nonvi ral gene delivery that warrants consider ation," says team leader Mikos, because it suggests that the potential exists for PEI or other polycations used in gene de livery to adversely interact with host genes. It raises concerns about using such nonviral vectors to treat human dis ease with gene therapy, he says.^
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[Chem. Commun., 1999,819]. Professor Mystery of gallium of organic chemistry Timothy D. Lash and coworkers prepare the compound by extrusion from nitride condensing 3,4-diethylpyrrole with 1,3-diResearchers have made the surprising formylindane in the presence of catalytic discovery that samples of the layered ter HBr followed by oxidation with FeCl3. Ac nary nitride Cr2GaN extrudefilamentsof cording to Lash, the dicarbaporphyrin is pure single-crystal gallium from the sam very porphyrin-like, with a strong diaple's pores at room temperature [Science, magnetic ring current in the [18]an284, 937 (1999)]. Postdoc Leon Farber, nulene system (shown in bold). The aro working with materials engineering pro matic nature of the porphyrin system—a fessor Michel W. Barsoum at Drexel Uni subject of much debate—is critical for its versity, Philadelphia, made the discovery chemistry, stability, and potential applica when he noticed that the surface of a re tions, Lash tells C&EN. The dicarbapor cently prepared sample of Cr2GaN had phyrin is relatively unstable in solution, developed a fuzz. The fuzz consisted of however. 'This possibly suggests that we gallium filaments 2 to 100 \im thick and are approaching the outer limits for por up to several centimeters long. Barsoum phyrinoid aromaticity," Lash says.^ thinks the gallium is driven out of the ni tride's gallium layers as the nitride slowly reacts with a constituent of the atmo Route of nonviral gene sphere, possibly 0 2 or N2. Experiments delivery mapped thus far have failed to nail the gaseous culprit. Once the extrusion process is The pathway taken by the nonviral vector understood, the researchers write, it poly(ethylenimine) (PEI) and its DNA may be possible to manipulate and con cargo as the complex traverses the cell trol it to produce micrometer-sized from membrane to nucleus has been wires, springs, and rods for microelec- mapped by scientists in Houston [Proc. tromechanical devices. And since Natl. Acad. Sci. USA, 9 6 , 5177 (1999)]. scratched grooves on Cr2GaN quickly Using fluorescence imaging and confocal
Diels-Alder reaction pins biomolecules to surfaces Chemists at the University of Chicago have devised a method for selectively im mobilizing biologically active molecules on surfaces using Diels-Alder chemistry \J. Am. Chem. Soc, 121, 4286 (1999)]. The ability to pin biomolecules in place is important for diagnostic assays, highthroughput drug discovery, and other ap plications. Demonstrating their proce dure, assistant professor Milan Mrksich and graduate student Muhammad N. Yousaf coat gold surfaces with a compos ite self-assembled monolayer consisting of hydroxy- and hydroquinone-terminated alkanethiolates. After oxidizing the hydroquinone moieties to quinones, the re searchers carry out Diels-Alder reactions between the quinones and a cyclopentadiene-biotin conjugate. In a final step, they show that the protein streptavidin binds selectively to immobilized biotin. In control experiments, hydroxy-terminated molecules and other surface sites that lack immobilized biotin did not bind streptavidin. Douglas L. Gin, an assistant chemistry professor at the University of California, Berkeley, notes that this re search might lead to new ways to carry out mechanistic studies in cell biology and may be directed toward developing biosensors and biocoatings.^ MAY 10,1999 C&EN 3 5
