Effect on clusters

Gokhman, F., Bergner, A., and Ulbricht, U. (2005) Iron matrix effects on cluster evolution in neutron irradiated reactor steels, 9th Research Workshop Nucleation Theory and Applications, Joint Institute for Nuclear Research, 25 June - 3 July, Dubna, in Schmelzer, J. W., Roepke P., Priezzhev, V. B. (eds.) (2006) Nucleation Theory and Applications, J/VRPwM, 408-419... 

Ab initio calculations of cooperativity effects on clusters of methanol, ethanol, 1-propanol and methanethioP ... 

The catalysed reaction was considered to arise from the heterolysis of dinitrogen pentoxide induced by aggregates of molecules of nitric acid, to yield nitronium ions and nitrate ions. The reaction is autocatalytic because water produced in the nitration reacts with the pentoxide to form nitric acid. This explanation of the mechanism is supported by the fact that carbon tetrachloride is not a polar solvent, and in it molecules of nitric acid may form clusters rather than be solvated by the solvent ( 2.2). The observation that increasing the temperature, which will tend to break up the clusters, diminishes the importance of the catalysed reaction relative to that of the uncatalysed one is also consistent with this explanation. The effect of temperature is reminiscent of the corresponding effect on nitration in solutions of nitric acid in carbon tetrachloride ( 3.2) in which, for the same reason, an increase in the temperature decreases the rate. 

X-ray diffraction work (11,15) shows that there is an ionomer peak at 4°C which is absent in the acid precursor. This low, broad peak is not affected by annealing or ion type and persists up to 300°C. Since the 4°C peak corresponds to a spacing of about 2.5 nm, it is reasonable to propose a stmctural feature of this dimension in the ionomer. The concept of ionic clusters was initially suggested to explain the large effects on properties of relatively sparse ionic species (1). The exact size of the clusters has been the subject of much debate and has been discussed in a substantial body of Hterature (3,4,18—20). A theoretical treatment has shown that various models can give rise to supramoleculat stmctures containing ionic multiplets which ate about 10 nm in diameter (19). 

P/M Tool Steels. In conventionally produced high alloy tool steels (slowly cooled cast ingots), carbide tends to segregate (48). Segregated clusters of carbide persist even after hot working, and cause undesirable effects on tool fabrication and tool performance. P/M tool steels, on the other hand, provide very fine and uniform carbides in the compact, the final bar stock, and the tools. Several tool steel suppHers consoHdate gas-atomized tool steel powder by HIP to intermediate shapes, which are then hot-worked to final mill shapes. Water-atomized tool steel powder is also available (see also T OOL materials). ... 

Illuminati and Marino reported an interesting example of the dependence of solvent effects on the position of the reacting center relative to the aza group. The rate constants for the reaction of 2- and 4-chloroquinoline with piperidine were compared in three different solvents, methanol, piperidine, and toluene. These data are reported in Table III. Three main points are apparent from these data (a) the different response of the two substrates to the action of the solvent, (b) the rates for 2-chloroquinoline in the three solvents tend to cluster around the highest reactivity level shown by 4-chloroquinoline in... 

When the second-site revertants were segregated from the original mutations, the bci complexes carrying a single mutation in the linker region of the Rieske protein had steady-state activities of 70-100% of wild-type levels and cytochrome b reduction rates that were approximately half that of the wild type. In all these mutants, the redox potential of the Rieske cluster was increased by about 70 mV compared to the wild type (51). Since the mutations are in residues that are in the flexible linker, at least 27 A away from the cluster, it is extremely unlikely that any of the mutations would have a direct effect on the redox potential of the cluster that would be observed in the water-soluble fragments. However, the mutations in the flexible linker will affect the mobility of the Rieske protein. Therefore, the effect of the mutations described is due to the interaction between the positional state of the Rieske protein and its electrochemical properties (i.e., the redox potential of the cluster). 

Treatment of bovine heart bci complex with ethoxyformic anhydride (EFA), which is known to modify amino acid residues covalently (preferentially histidine residues), inhibits electron transfer and has an effect on the EPR spectra of the Rieske cluster comparable to that observed upon addition of stigmatellin, that is, a decrease in rhombicity (80). This further supports the suggestion that quinones as well as quinonoid inhibitors interact directly with the histidine ligands of the Rieske cluster. 

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