science/
technology
concentrates Iron-molybdenum complex 'fixes' nitrogen
Though almost empty, porous H-bonded network remains intact
When guest molecules flee the cavities of zeolites, the order and porosity of these Chemists at Massachusetts Institute of robust inorganic structures often are pre Technology have made a molybdenum served. But can this structural integrity compound that reacts with nitrogen gas to be mimicked in hydrogen-bonded organ yield a salt of a (RMo-N=N)" ion, where R ic analogs? Yes, say chemistry professor is a triamidoamine group. Inorganic chem James D. Wuest and coworkers Philippe istry professor Richard R. Schrock, gradu Brunei and Michel Simard of the Uni ate student Myra B. O'Donoghue, postdoc versity of Montreal. Initially, Wuest tells toral fellow Nadia C. Zanetti, and staff crys- C&EN, they had some doubts because tallographer William M. Davis go on to "hydrogen bonds are individually much convert that salt to the (RMo-N=N)3Fe weaker than the bonds that hold zeolites complex \J. Am. Chem. Soc., 119, 2753 together." But (1997)]. The iron-molybdenum dinitrogen by using a crysNH2 complex—the only one of its kind ever re tallization tech/p^\ / ^ ported, the authors believe—may shed nique, the re- c ~ ( \ 0 ) — ( Q N L light on the iron-molybdenum species searchers were — Ν—( NH found in nitrogenase, the enzyme that fix able to create a 2 es nitrogen. Additional facts that may have three-dimen a bearing on eventually discovering the sional network in which the tetrahedral mechanism of nitrogenase are that the building block (shown here) is held in po complex has trigonally coordinated iron, sition by 16 hydrogen bonds to identi which is rare, and that all three ligands cal units [/. Am. Chem. Soc, 119, 2737 about iron are derived from dinitrogen. (1997)]. The network contains promi Further research at MIT will focus on pos nent channels large enough to hold up sible reactions at either end of the -N=N- to 10 molecules of formic acid or five of group, as well as reactions that will con dioxane per building block unit. Exper vert the group to nitrates or ammonia.^ iments have established that the porous network remains intact during the ex change of guests. And even when most of the guests (63% in one experiment) Stereochemistry are removed under vacuum, the network of antitumor drug remains ordered, although it does shrink systematically. The unprecedented stmcfostriecin determined tural integrity of these organic networks The complete stereochemistry of fostrie is due to the large numbers of hydrogen cin, an antitumor antibiotic currently in Na bonds holding them together, Wuest tional Cancer Institute-sponsored Phase I believes.^ clinical trials, has been determined by chemistry professor Dale L. Boger and co workers at Scripps Research Institute, La Receptor cloned for Jolla, Calif. [/. Org. Chem., 62, 1748 neurotransmitter GABA (1997)]. Fostriecin has a unique mecha nism of action for an antitumor agent—it The last of the major receptors for a neu obstructs a cellular checkpoint for entry rotransmitter has been cloned by a group into mitosis, possibly via inliibition of pro of researchers at Novartis Pharma, Basel, tein phosphatases. However, the stereo Switzerland. The GABAB receptor is one chemistry of the compound has been un of two types used by the brain's main in known. Boger and coworkers determined hibitory signaling molecule, γ-aminobuthe relative and absolute stereochemistry of tyric acid (GABA). Klemens Kaupmann, fostriecin's four chiral centers by a strategy Bernhard Bettler, and their colleagues de that combined nuclear magnetic resonance signed a new liigh-affinity GABAB recep spectroscopy, synthesis of a degradation tor antagonist and used this ligand to product, and chiral-phase high-perfor identify complementary DNA sequences mance liquid cliromatography. According that encode two GABAB receptor pro to Boger, the work "opens the way for teins [Nature, 386, 239 (1997)]. The two [fostriecin's] total synthesis and the prepa proteins differ only in the length of their ration of structural analogs."^ amino-terminal sequences and, the au 42 MARCH 24, 1997 C&EN
thors suggest, may represent two forms of the GABAB receptor that have been found in different parts of the brain. Like most neurotransmitters, including L-glutamate, serotonin, and acetylcholine, GABA can activate two types of receptors. GABAA receptors are ion channels that, when ac tivated by binding of the GABA mole cule, allow ions such as CI" to flow into the nerve cell to rapidly stop a nerve im pulse. GABAB receptors work differently to trigger a series of slower—but longer lasting—responses involving the G-protein family of intracellular signals. Be cause the receptor seems to play a role in conditions such as epilepsy, anxiety, depression, and some kinds of memory loss, it is an important target for drug de sign. Cloning it should also help neuroscientists pin down questions such as how many subtypes of GABAB receptors there are and how their distributions and functions vary.^
Physicists define current carried by one electron One hundred years after Sir Joseph John Thomson's 1897 discovery of the elec tron at Cambridge University's Cavendish Laboratory, physicists from the same lab oratory have revealed a new standard for electric current, which, they claim, is a "major breakthrough" in the application of experimental quantum physics to electronics. Physics professor Michael Pepper and coworkers developed a way of capturing a single electron, moving it through a semiconductor chip, and mea suring its charge precisely. The group used high-frequency sound waves to trap and push electrons through a one dimen sional structure, known as a "splitgate," in a gallium arsenide chip. "The minima of the sound wave fill up with electrons and are dragged through the device in a process known as surfing," Pepper tells C&EN. "We then squeeze the potential of the sound wave and gradually expel electrons until we get down to the oneelectron limit." He explains that the cur rent (Τ) carried by the one electron is the electron charge (e) times the frequency of the sound wave (/). The equation / = ef provides a quantum measure of elec tric current. This new standard for elec tric current is a more reliable method of measuring electricity than Ohm's law (voltage = current χ resistance), according to the team.^
