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The hydroxyl radical initiated oxidation of diisopropyl ether has been studied in the large-volume outdoor European Photoreactor (EUPHORE) and in a small, laboratory-based reactor system. The product distributions determined from the experiments were found to be significantly dependent on the reaction conditions and provide strong evidence for the existence of three distinct regimes within the reaction system. In the presence of NO x , the peroxy radicals react with NO to produce chemically activated (CH 3 ) 2 CHOC(O)(CH 3 ) 2 alkoxy radicals which undergo decomposition by C C bond scission to yield isopropyl acetate and formaldehyde as the major products. Under conditions where the self-reaction of peroxy radicals dominates, thermoneutral (CH 3 ) 2 CHOC(O)(CH 3 ) 2 radicals are produced, which appear to undergo two reaction pathways; C C bond scission to yield isopropyl acetate and formaldehyde and isomerisation to form acetone, acetic acid and formaldehyde. Under conditions where the reaction between peroxy and hydroperoxy radicals dominates, unstable hydroperoxides are produced which decompose to yield acetone as the only major reaction product. The results of our study are used to construct chemical mechanisms for the gas-phase photooxidation of diisopropyl ether under various tropospheric conditions.

The X-ray structure of nucleoside diphosphate kinase (NDP kinase) from the slime mold Dictyostelium discoideum has been determined to 2.2-A resolution and refined to an R-factor of 0.19 with and without bound ADP-Mg2+. The nucleotide binds near His 122, a residue which becomes phosphorylated during the catalytic cycle. The mode of binding is different from that observed in other phosphokinases, and it involves no glycine-rich sequence. The adenine base makes only nonpolar contacts with the protein. It points outside, explaining the lack of specificity of NDP kinase toward the base. The ribose 2'- and 3'-hydroxyls and the pyrophosphate moiety are H-bonded to polar side chains. A Mg2+ ion bridges the alpha- to the beta-phosphate which approaches the imidazole group of His 122 from the N delta side. The geometry at the active site in the ADP-Mg2+ complex suggests a mechanism for catalysis whereby the gamma-phosphate of a nucleoside triphosphate can be transferred onto His 122 with a minimum of atomic motion.