Active role

A catalyst may play an active role in a different sense. There are interesting temporal oscillations in the rate of the Pt-catalyzed oxidation of CO. Ertl and coworkers have related the effect to back-and-forth transitions between Pt surface structures [220] (note Fig. XVI-8). See also Ref. 221 and citations therein. More recently Ertl and co-workers have produced spiral as well as plane waves of surface reconstruction in this system [222] as well as reconstruction waves on the Pt tip of a field emission microscope as the reaction of H2 with O2 to form water occurred [223]. Theoretical simulations of these types of effects have been reviewed [224]. 

Interaction of vitamin D and its metaboUtes with sex hormones has been demonstrated, particularly ia birds ia which the egg-laying functions combine calcium needs and reproductive activity. The metaboUtes of vitamin D behave as hormones. As such, they play an active role ia the endocrine system, along with other hormones, to maintain the various body functions. Several biological influences of metaboUtes of vitamin D have been studied, including effects related to cancer (193—197), skin diseases (198—201), immunomodulatory effects (202,203), and Alzheimer s disease (204—206) (Fig. 9). 

Vitamin D metaboUtes may therefore play an active role ia diseases related to these functions, ie, leukemia, cancer (breast, colon, prostate), and autoimmune diseases (AIDS, immune encephaUtis, and diabetes) (51, 141,193—197, 202, 203). 

Atoms and free radicals are highly reactive intermediates in the reaction mechanism and therefore play active roles. They are highly reactive because of their incomplete electron shells and are often able to react with stable molecules at ordinary temperatures. They produce new atoms and radicals that result in other reactions. As a consequence of their high reactivity, atoms and free radicals are present in reaction systems only at very low concentrations. They are often involved in reactions known as chain reactions. The reaction mechanisms involving the conversion of reactants to products can be a sequence of elementary steps. The intermediate steps disappear and only stable product molecules remain once these sequences are completed. These types of reactions are refeiTcd to as open sequence reactions because an active center is not reproduced in any other step of the sequence. There are no closed reaction cycles where a product of one elementary reaction is fed back to react with another species. Reversible reactions of the type A -i- B C -i- D are known as open sequence mechanisms. The chain reactions are classified as a closed sequence in which an active center is reproduced so that a cyclic reaction pattern is set up. In chain reaction mechanisms, one of the reaction intermediates is regenerated during one step of the reaction. This is then fed back to an earlier stage to react with other species so that a closed loop or... 

However, the two ions NOf (ag) and Na+(agj do not play an active role in the reaction, nor do they influence the reaction that does occur [reaction (70)]. Consequently, they are not included in the equation for the reaction. The balanced chemical equation should show only species which actually participate in the reaction. These species are called the predominant reacting species. 

The concept that different ligands play an active role in ER function is apparent at the biochemical level. In addition to competitive inhibition of estrogen binding, antiestrogens induce unique conformations/stiuctures of both ERa and ER 3. This provides a structural basis for the unique biological activities displayed by the different compounds [1]. 

The learning process involves many students"activities Students interest increased because they liked playing an active role in the learning process (e.g. hands on experiments, constraction of representations of chemical reactions with the use of models). Those who were not used to such an approach from earlier teaching also improved their experimental skills and gained experiences through the work with models. In the reflective diaries of all the teachers, statements to support this point could be found as follows ... 

The approach puts students in an active role and increases their motivation for learning. Higher cognitive processes are also emphasized, because students have to make the analyses and draw the conclusions. [Teacher from School N° 4, general remarks to the approach]... 

The spatial arrangement of the Fe-S clusters in D. gigas NiFe-hydrogenase (see Fig. 1) suggests an active role for the [Fe3S4] ° cluster in mediating electron transfer from the NiFe active site to the... 

Another problem that required solving was the moderate yield obtained in the cyclopropanation reaction when only one equivalent of styrene was used. By increasing the amount of styrene up to its use as the reaction solvent, a noticeable effect on the selectivities was observed when laponite was used as the support [58]. The active role of the clay support was definitely estabhshed when the results in homogeneous and heterogeneous phases were compared (Table 9). These effects involved the reversal of the trans preference in solution to the cis preference with the laponite-supported catalyst in styrene, and also a reversal in the absolute configuration of the major cis enantiomer ob-... 

Our search procedures represent a departure from the above type of paradigm. Rather than simply accepting and implementing a decision policy found by DUg, that optimizes an overall measure of performance, the infimal subsystems and corresponding plant personnel play an active role in the construction and validation of solutions. One tries to build a consensus decision policy, Xpp, validated by all subsystems, DU , k = I,..., K, as well as by the whole plant, DUg, and only when that consensus has been reached does one move toward implementation. Within this context, the upper-level decision unit, DUg, assumes a eoordination role. 

A. Synthetic Methods.—Electrophilic addition of P compounds to olefinic compounds is a well-established route to phosphonic acids, although yields are often disappointing. With phosphorus pentachloride it has been found that yields are greatly improved when phosphorus trichloride is added to the reaction mixture. Since the orientation of the addition implies that electrophilic addition to phosphorus rather than chlorine is the initial step, it seems likely that the trihalide participates by decreasing the free concentration of chlorine rather than by a more active role. This... 

The consumer movement, particularly the Consumers Health Fomm, also has considerable political influence. As a result of political pressure exerted by HIV/AIDS pressure groups and industry, the DRA was reviewed, new legislation introduced and new committees established to ensure that the regulatory authority responded more efficiently to public and political needs. Professional associations, especially the Pharmacy Guild of Australia, have traditionally played an active role in decisions related to pharmaceutical regulation. The Australian Pharmaceutical Advisory Council was established by the Australian Government to advise the Commonwealth Minister for Health on pharmaceutical policy. 

This surprising results implies that water is not just a medium for transporting products on the catalytic sites but has an active role in determining the peroxo/hydroperoxo species present on the working catalyst. Scheme 3 summarizes the model hypothesized by Prestipino et al. [50] on the basis of the whole set of experiments reported by Bonino et al. [49] and by them selves [50], and here reviewed in Fig. 10. 

The synthesis of glycoconjugates opens the route to one of the most important class of biomolecules which play an active role in relevant biological reactions [26]. One way to do so is to use enzymes, which, however, suffer from instability and slow reaction rates. 

Stannyl enolates give good addition yields in the presence of a catalytic amount of m-(C4II9)4N 1 lir, 315 The bromide ion plays an active role in this reaction by forming a more reactive species via coordination at the tin atom. 

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