NEWS O F TH E W EE K
MAKING WATER STEP BY STEP SURFACE SCIENCE: Atomic resolution
study reveals sequence of events
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M I TC H JACO BY/C& E N
University of Aarhus’ Matthiesen prepares vacuum equipment used to probe surface reaction mechanisms.
ROVIDING A VIEW of the inner workings of a molecular transformation with unprecedented detail, researchers in Denmark have recorded all of the intermediate steps of a surface chemical reaction with atomic-scale resolution (ACS Nano, DOI: 10.1021/nn8008245). Studies that reveal the subtle molecular events that comprise reaction mechanisms provide insights that can aid scientists in improving a reaction’s yield or selectivity or help block undesirable reactions. Such studies generally elucidate a single bonding event or a key reaction intermediate. Using scanning tunneling microscopy and quantum calculations to aid image interpretation, physicists Jesper Matthiesen, Stefan Wendt, Flemming Besenbacher, and coworkers at Aar-
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Follow surface-catalyzed formation of water in video by clicking on this article at C&EN Online, www.cen-online.org.
SELF-ASSEMBLED NUCLEOTIDE HELIX NMR: Structure may have
implications for prebiotic chemistry
HELICAL STACK
J. AM . C HE M . SOC.
5′-GMP quadruple helix, with sodium ions (spheres) and four hydrogen-bonded “strands” (different colors).
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CHEMICAL STRUCTURE that has been sought for nearly 50 years—that of selfassembled 5ʹ-guanosine monophosphate (5ʹ-GMP)—has just been determined. The structure suggests how nucleotide monomers might line up to form covalently bonded nucleic acid oligomers and thus has potential implications for prebiotic chemistry. In 1962, David R. Davies and coworkers at the National Institutes of Health studied GMP assemblies. Their X-ray diffraction data showed that the nucleotides form G-quartets—planar sets of four hydrogen-bonded nucleotides—and suggested that the overall assemblies are helical. But the exact structure of these assemblies has remained unknown, despite efforts over the years by several groups to obtain it. Now, chemistry professor Gang Wu and grad W W W.C EN - ON L INE .ORG
hus University’s Interdisciplinary Nanoscience Center “watched” step by step as hydrogen reacted with oxygen to form water on a titania crystal surface. That deceptively simple-sounding reaction—oxidation on a hydrated form of TiO2, which is a high-volume commercial catalyst—lies at the heart of numerous catalytic processes in wide-ranging applications including self-cleaning surfaces and TiO2-based dye-sensitized solar cells. With a time-lapse series of STM images, the team strung together videos that zoom in on a complex dance of surface species. The videos depict adsorption, dissociation, diffusion, and reaction of oxygen with hydrogen on TiO2. They also capture formation of HO2, H2O2, and H3O2 intermediates and the final reaction product—water molecule dimers—and subsequent desorption of water from the catalyst surface. The study reveals that trace quantities of coadsorbed water help facilitate the reaction by opening low-energy pathways for diffusion of hydrogen atoms. “Mapping out the entire sequence of elementary steps—not just some of them—in a surface-catalyzed reaction is a remarkable accomplishment,” says Manos Mavrikakis, a professor of chemical engineering at the University of Wisconsin, Madison. He adds that TiO2’s widespread use in catalysis and the presence of oxygen and moisture in so many applications makes the chemistry studied here highly relevant.—MITCH JACOBY
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student Irene C. M. Kwan of Queen’s University, in Kingston, Ontario, have determined the nuclear magnetic resonance structure of the assemblies in a solution containing sodium ions (J. Am. Chem. Soc., DOI: 10.1021/ja809258y). The structure shows that G-quartets stack to form a four-stranded helix, with each quartet offset from the next by a twist angle of 30º. Sodium ions run along the helix’s central axis. The quartets are linked to one another not by phosphodiester bonds, as in RNA and DNA oligomers, but by weak molecular interactions, such as hydrogen bonds and ion-dipole forces. Wu speculates that the structure couldn’t be determined earlier because the assembly is hard to crystallize, its NMR spectrum is highly overlapped and hard to interpret, and its noncovalent linkages make it difficult to model theoretically. His group used sophisticated NMR techniques to overcome these problems. The work provides a “convincing and probably definitive structure elucidation,” says chemistry professor Christian Detellier of the University of Ottawa, who has pursued the structure. “This finding opens the way to a variety of studies, including the role of cations in the self-assembly process and potential functions associated with this assembly. For example, would such a supramolecular structure have played a role in prebiotic chemistry?” he asks.—STU BORMAN
F E B RUA RY 16, 20 09
