CONCENTRATES
1
âpTei 1 1 pf zX ti r#?i «ΞΤ Europa images further ocean hypothesis The highest resolution images yet of Jupi- ter's moon Europa appear to substantiate the hypothesis that a liquid or slushy wa- ter ocean may lie beneath its icy surface. The images, taken when the spacecraft Galileo flew within about 360 miles of Eu- ropa on Feb. 20, show blocky ice rafts that look strikingly similar to floating icebergs seen on Earth's polar seas during springtime thaws, says Ronald Gree- ley, geologist at Arizona State Uni- versity, Tempe, and Galileo imag- ing team mem- ber. The blocks' size and geometry suggest that "there was a thin, icy layer covering water or slushy ice, and that some motion caused these crustal plates to break up," Greeley says. Other areas of ice-covered Europa appear to be remarkably crater- free, which some scientists believe indi- cates the surface is very young—the sur- face having been perhaps smoothed over by liquid water welling up from cracks in the surface and refrozen. If liquid water does exist on Europa, it may have a signif- icant heat source, and therefore might be able to support microbial life, some scien- tists theorize.^ Record low ozone observed over Arctic The lowest springtime values of strato- spheric ozone ever observed over the North Pole were measured last month by instruments on NASA and National Ocean- ic & Atmospheric Administration satellites. Average ozone amounts in March 1997 were 40% lower than the 1979-82 base- line, with total column ozone over the Arctic falling to a minimum of 219 Dobson units on March 24. (For comparison, ozone minima in the Antarctic ozone hole drop below 110 Dobson units.) The re- gion of exceptionally low ozone was found within the Arctic polar vortex, a stream of stratospheric winds that circles the pole in winter. The 1996-97 polar vor- tex has been unusually strong, with tem- peratures frigid enough for polar strato- spheric clouds to form and persist into late March. Polar stratospheric clouds help convert chlorine compounds into species that can catalyze ozone destruction in sun- light. "The persistence of such cold tem- peratures within the Arctic vortex well into the sunlit period is an essential ingre- dient for driving many of the chemical cy- cles for ozone destruction," notes Michael Kurylo, manager of NASA's Upper Atmo- sphere Research Program.^ Arming yeast with cell-surface catalysts A yeast that expresses the starch-hydrolyz- ing enzyme glucoamylase on its cell sur- face has been engineered by researchers in Japan. The feat paves the way for novel biocatalysts made from enzyme-coated yeast cells [Appl Environ. Microbiol, 63, 1362 (1997)]. Led by professor Atsuo Ta- naka in the department of synthetic chem- istry and biological chemistry at Kyoto University, the team introduced into Sac- charomyces cerevisiae a plasmid contain- ing the gene for glucoamylase fused to a segment of the gene for the yeast cell-wall protein α-agglutinin. The fusion protein containing the extracellular enzyme is co- valently bound to the cell wall. The re- combinant yeast, unlike the wild type, can grow on starch, but this ability "has no in- dustrial purpose," comments Peter J. Reil- ly, professor of chemical engineering at Iowa State University of Science & Tech- nology, Ames—noting that glucoamylase is available in drum quantities. Rather, the work is a predecessor to future industrial techniques. "Now you can attach other enzymes to α-agglutinin and use them in cells that are so easy to grow and so nice to work with," he says.^ Theory matches enzyme behavior A theoretical study of methane monooxy- genases (MMO)—a class of enzymes used by some bacteria to oxidize methane— predicts an intermediate structure of the metal-oxo core exactly like that observed by experiment, and describes a catalytic reaction sequence for MMO's activation of methane and conversion to ethanol. Using quantum chemical methods, theoretical chemist Per Ε. Μ. Siegbahn of Stockholm University in Sweden and Yale University chemistry professor Robert H. Crabtree developed a model based on MMO crystal structure and other biophysical data \J. Am. Chem. Soc, 119, 3103 (1997)]. MMO's active site, a Fe 2 (II,II) dinuclear core, first reacts with 0 2 to form two different Fe 2 (m,ni) peroxo species. Cleavage of the O-O bond follows, forming the high- valent Fe 2 (IV,IV) bis-u-oxo species—an in- termediate with enough oxidizing power to strip a tightly bound hydrogen off meth- ane. The shape of this intermediate, their theory predicts, is a diamond composed of the two iron(IV) atoms bound to two oxy- gen atoms with unequal Fe-O bond lengths. A carboxylate bridge connects the active site to the enzyme. This intermedi- ate structure recently was observed exper- imentally by Lawrence Que Jr. and col- leagues at the University of Minnesota, Minneapolis (C&EN, Jan. 27, page 9). The theory of Crabtree and Siegbahn also fol- lows the entire reaction sequence of methane's conversion to methanol, which provides a jumping-off point for further experiments, the authors write.^ High-yield synthesis of chiral porphyrin analog A straightforward, high-yield synthesis of a chiral porphyrin analog has been devel- oped by chemists at Northwestern Univer- sity, Evanston, Π1., and Imperial College of Science, Technology & Medicine, London [Angew. Chem. Int. Ed. Engl, 36, 761 (1997)]. The enantiomerically pure "winged" spirane porphyrazine (shown with carbon in gray, nitrogen in blue, and oxygen in red) produced by the chemists "opens up the prospect of using such mac- rocycles for enantioselec- tive oxida- tions and ep- oxidations analogous to those carried out by en- zymes,'' says Northwest- ern chemistry professor Brian M. Hofl&nan, who collaborated on the work with Imperi- al College chemistry professor Anthony G. M. Barrett. The six-step synthesis developed by Barrett, Hoflman, and coworkers builds on several years of work on such com- pounds by the groups. The chirality of the "wings" of the porphyrazine is established in the first step of the reaction and main- tained throughout the subsequent steps. The synthesis opens the way to a new fam- ily of ροφηντίη analogs in which the redox properties of the ροφηντίη core can be fine-tuned and the chiral chemistry around the core established precisely.^ 32 APRIL 14, 1997 C&EN science/ technology
Transcript of CONCENTRATES
âpTei 11 pf zX t i r # ? i «ΞΤ
Europa images further ocean hypothesis The highest resolution images yet of Jupiter's moon Europa appear to substantiate the hypothesis that a liquid or slushy water ocean may lie beneath its icy surface. The images, taken when the spacecraft Galileo flew within about 360 miles of Europa on Feb. 20, show blocky ice rafts that look strikingly similar to floating icebergs seen on Earth's polar seas during springtime thaws, says Ronald Greeley, geologist at Arizona State University, Tempe, and Galileo imaging team member. The blocks' size and geometry suggest that "there was a thin, icy layer covering water or slushy ice, and that some motion caused these crustal plates to break up," Greeley says. Other areas of ice-covered Europa appear to be remarkably crater-free, which some scientists believe indicates the surface is very young—the surface having been perhaps smoothed over by liquid water welling up from cracks in the surface and refrozen. If liquid water does exist on Europa, it may have a significant heat source, and therefore might be able to support microbial life, some scientists theorize.^
Record low ozone observed over Arctic The lowest springtime values of stratospheric ozone ever observed over the North Pole were measured last month by instruments on NASA and National Oceanic & Atmospheric Administration satellites. Average ozone amounts in March 1997 were 40% lower than the 1979-82 baseline, with total column ozone over the Arctic falling to a minimum of 219 Dobson units on March 24. (For comparison, ozone minima in the Antarctic ozone hole drop below 110 Dobson units.) The region of exceptionally low ozone was found within the Arctic polar vortex, a stream of stratospheric winds that circles the pole in winter. The 1996-97 polar vortex has been unusually strong, with temperatures frigid enough for polar stratospheric clouds to form and persist into late
March. Polar stratospheric clouds help convert chlorine compounds into species that can catalyze ozone destruction in sunlight. "The persistence of such cold temperatures within the Arctic vortex well into the sunlit period is an essential ingredient for driving many of the chemical cycles for ozone destruction," notes Michael Kurylo, manager of NASA's Upper Atmosphere Research Program.^
Arming yeast with cell-surface catalysts A yeast that expresses the starch-hydrolyz-ing enzyme glucoamylase on its cell surface has been engineered by researchers in Japan. The feat paves the way for novel biocatalysts made from enzyme-coated yeast cells [Appl Environ. Microbiol, 63, 1362 (1997)]. Led by professor Atsuo Ta-naka in the department of synthetic chemistry and biological chemistry at Kyoto University, the team introduced into Sac-charomyces cerevisiae a plasmid containing the gene for glucoamylase fused to a segment of the gene for the yeast cell-wall protein α-agglutinin. The fusion protein containing the extracellular enzyme is co-valently bound to the cell wall. The recombinant yeast, unlike the wild type, can grow on starch, but this ability "has no industrial purpose," comments Peter J. Reil-ly, professor of chemical engineering at Iowa State University of Science & Technology, Ames—noting that glucoamylase is available in drum quantities. Rather, the work is a predecessor to future industrial techniques. "Now you can attach other enzymes to α-agglutinin and use them in cells that are so easy to grow and so nice to work with," he says.^
Theory matches enzyme behavior A theoretical study of methane monooxy-genases (MMO)—a class of enzymes used by some bacteria to oxidize methane— predicts an intermediate structure of the metal-oxo core exactly like that observed by experiment, and describes a catalytic reaction sequence for MMO's activation of methane and conversion to ethanol. Using quantum chemical methods, theoretical chemist Per Ε. Μ. Siegbahn of Stockholm University in Sweden and Yale University chemistry professor Robert H. Crabtree developed a model based on MMO crystal structure and other biophysical data \J. Am. Chem. Soc, 119, 3103 (1997)]. MMO's
active site, a Fe2(II,II) dinuclear core, first reacts with 0 2 to form two different Fe2(m,ni) peroxo species. Cleavage of the O-O bond follows, forming the high-valent Fe2(IV,IV) bis-u-oxo species—an intermediate with enough oxidizing power to strip a tightly bound hydrogen off methane. The shape of this intermediate, their theory predicts, is a diamond composed of the two iron(IV) atoms bound to two oxygen atoms with unequal Fe-O bond lengths. A carboxylate bridge connects the active site to the enzyme. This intermediate structure recently was observed experimentally by Lawrence Que Jr. and colleagues at the University of Minnesota, Minneapolis (C&EN, Jan. 27, page 9). The theory of Crabtree and Siegbahn also follows the entire reaction sequence of methane's conversion to methanol, which provides a jumping-off point for further experiments, the authors write.^
High-yield synthesis of chiral porphyrin analog A straightforward, high-yield synthesis of a chiral porphyrin analog has been developed by chemists at Northwestern University, Evanston, Π1., and Imperial College of Science, Technology & Medicine, London [Angew. Chem. Int. Ed. Engl, 36, 761 (1997)] . The enantiomerically pure "winged" spirane porphyrazine (shown with carbon in gray, nitrogen in blue, and oxygen in red) produced by the chemists "opens up the prospect of using such macrocycles for enantioselec-tive oxidations and ep-oxidations analogous to those carried out by enzymes,'' says Northwestern chemistry professor Brian M. Hofl&nan, who collaborated on the work with Imperial College chemistry professor Anthony G. M. Barrett. The six-step synthesis developed by Barrett, Hoflman, and coworkers builds on several years of work on such compounds by the groups. The chirality of the "wings" of the porphyrazine is established in the first step of the reaction and maintained throughout the subsequent steps. The synthesis opens the way to a new family of ροφηντίη analogs in which the redox properties of the ροφηντίη core can be fine-tuned and the chiral chemistry around the core established precisely.^
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science/ technology
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