Reactive species accumulation

DCE interface in the presence of TPBCl [43,82]. The accumulation of products of the redox reactions were followed by spectrophotometry in situ, and quantitative relationships were obtained between the accumulation of products and the charge transfer across the interface. These results confirmed the higher stability of this anion in comparison to TPB . It was also reported that the redox potential of TPBCP is 0.51V more positive than (see Fig. 3). However, the redox stability of the chlorinated derivative of tetra-phenylborate is not sufficient in the presence of highly reactive species such as photoex-cited water-soluble porphyrins. Fermin et al. have shown that TPBCP can be oxidized by adsorbed zinc tetrakis-(carboxyphenyl)porphyrin at the water-DCE interface under illumination [50]. Under these conditions, the fully fluorinated derivative TPFB has proved to be extremely stable and consequently ideal for photoinduced ET studies [49,83]. Another anion which exhibits high redox stability is PFg- however, its solubility in the water phase restricts the positive end of the ideally polarizable window to < —0.2V [85]. 

McKee BA, DeMaster DJ, Nittrouer CA (1983) Concepts of sediment deposition and accumulation applied to the continental shelf near the mouth of the Yangtze River. Geol 11 631-633 McKee BA, DeMaster DJ, Nittrouer CA (1986a) Temporal variability in the partitioning of thorium between dissolved and particulate phases on the Amazon shelf - implications for the scavenging of particle-reactive species. Cont Shelf Res 6 87-106... 

In the presence of 10% H20 but no C02, the same orange species accumulated in the solution and electrocatalysis was slow, with H2 being generated with a current efficiency of c. 85% (only a tiny amount of H2 was observed under the same conditions in the absence of the complex). In the presence of C02 the water reduction reaction was completely inhibited, showing that the orange species is less reactive towards water than COz and hence is a highly specific catalyst for the conversion of C02 to CO. 

The application of very low temperatures to detect, to thermally trap, and to characterize intermediates in enzyme-catalyzed reactions. This is made possible by the fact that each individual, elementary step in a reaction pathway has its own activation energy. Lowering the temperature reduces the fraction of molecules that can react in certain steps, thereby permitting otherwise reactive species to accumulate. 

Oxygen plays a major role in radiation-induced processes. If it is either present during irradiation or admitted to the substance after irradiation, another type of reactive species may arise—a peroxide. Peroxides are usually fairly stable at moderate temperatures and accumulate in the system to a certain extent. They are easily decomposed at elevated temperatures moreover, they are selectively decomposed on further irradiation. 

Most photochromic systems are not reversible indefinitely. However, very little careful analytical data have been accumulated to characterize the nature of the degradation products or to specify the degree of quantitative reversibility. The reasons for side reactions are inherent in the high photochemical reactivities of the systems. First of all, there must be an excited state formed by absorption this state is then transformed into other excited states or reactive species. The latter may include triplet states, carbonium ions, carbanions, free radicals, or other highly reactive intermediates. Certain of these are oxygen sensitive so that exclusion of the atmosphere and other potential sources of contaminants during irradiation is necessary. A second major route of degradation involves the excited state of the colored form which may already be... 

The production of superoxide anions is one of the major factors involved in NO toxicity because superoxide anions can react with NO to form the highly toxic free-radical peroxynitrite. A pivotal role for superoxide anions in NO-related insults is emphasized by results showing that transgenic mice overexpressing superoxide dismutase (SOD) are resistant to brain ischemia. Superoxide can protect against SNP-induced toxicity. Thus, the superoxide-scavenging properties of EGb 761 are likely to explain, at least in part, its ability to block cell death and the increase in reactive oxygen species accumulation induced by the two NO donors used here, SNP and SIN-1. 

Chiche et a/.[56] have studied the oligomerization of butene over a series of zeolite (HBeta and HZSM-5), amorphous silica alumina and mesoporous MTS-type aluminosilicates with different pores. The authors found that MTS catalyst converts selectively butenes into a mixture of branched dimers at 423 K and 1.5-2 MPa. Under the same reaction conditions, acid zeolites and amorphous silica alumina are practically inactive due to rapid deactivation caused by the accumulation of hydrocarbon residue on the catalyst surface blocking pores and active sites. The catalytic behaviour observed for the MTS catalyst was attributed to the low density of sites on their surface along with the absence of diffusional limitations due to an open porosity. This would result in a low concentration of reactive species on the surface with short residence times, and favour deprotonation and desorption of the octyl cations, thus preventing secondary reaction of the olefinic products. 

The decrease of the rate of ethene oxide formation with respect to time observed at temperatures >470 K can be explained by the accumulation of oxygen species embedded in the silver surface that decrease the surface area available for the formation of the reactive species. Nucleophilic and electrophilic oxygen, which are the major surface species at 420 K, are still present on the silver surface at 470 K however, they are rapidly removed in the absence of oxygen in the gas phase (see the difference spectrum in Figure 18). 

Potentiodynamic gas sensors have a schematic structure that is practically equal to that of amperometric gas sensors. They are -> electrochemical cells that measure a -> current signal directly related to the concentration of the analyte, but are not necessary operated in a region where -> mass transport is limiting. They are typically employed to detect less reactive species such as benzene and halogenated hydrocarbons that require a previous accumulation step at a suitable -> adsorption potential to be then reduced or oxidized according to a given potential scan [iii]. The adsorption time can be automat-... 

During the last 50 years a great deal of experimental data has been accumulated on the yields of Ps, H2 and e-q in various aqueous solutions. Here, we apply the blob model to calculate these yields in solutions of NOJ, H202, HCIO4, Cl-, Br-, I- and F-. They were selected to embrace the greatest possible variety of solute properties with respect to intratrack reactive species. 

The accumulation of data needed to predict the course of chemical reactions is often hindered by the presence of small concentrations of very unstable intermediates. It is just these highly reactive species, difficult to determine analytically, that are the indispensable link in the chain of causes and consequences in all processes where the substrate is transformed to the product by a chemical reaction. 

In the stationary state, the actual current values of chemical potentials of the intermediate reactive species must decrease in the stepwise fashion toward the final products, the values being only those between the exter nal initial reactants and the final products. Idence, any chemical transfor mations of the intermediates are described only by the transformations between the said stationary values of chemical potentials. Figuratively speaking, the intermediates in their stationary state accumulate the Gibbs... 

The related Mannich reaction is not common. Under the usual acidic reaction conditions TV-substitution occurs, but this is a reversible reaction in the presence of base. Therefore, in basic medium, C- substituted products accumulate, and all positions can be substituted. 2-Methylimidazole gives 1,4,5-tri, 4,5-di- and 4-mono-substituted products. The observation that with formaldehyde and hydrochloric acid histamine gave (98) is at variance with the apparent requirement for basic medium. Since 1-substituted imidazoles do not react it is likely that the imidazole conjugate base is the reactive species. Unless imidazoles contain activating substituents they are not very susceptible to reaction with aldehydes (except HCHO) and ketones. An exception appears to be the product (99) of interaction between imidazole and hexane-2,4-dione. An activated compound such as 4-methylimidazoline-2-thione gives the 5-dimethylamino compound (100) imidazoline-2-thione gave only the (V-hydroxymethyl product under the same reaction conditions. Imidazolin-4-ones with a free 5-position readily form benzylidene derivatives (B-76MI40701). 

No release of the activated species before enzyme inactivation. For a mechanism-based inactivator to retain its high specificity during inactivation, release of the reactive species from the active site must not be part of the normal mechanism of inactivation. A time-dependent increase in the rate of inactivation points to the release of an activated species before inactivation. This increase in the rate of inactivation is brought about by the accumulation of free reactive species in solution. Inhibitors generated in this manner have been termed metabolically activated affinity labels... 

Superoxide dismutase converts many other types of reactive oxygen species to oxygen gas, hydrogen peroxide, or water. This enzyme binds to these various reactive oxygen species by a molybdenum ion (Mo+2) at its active center. Again, fluoride will bind to the molybdenum ion and inhibit this enzyme s activity. Reactive oxygen species accumulate in the tissues and mediate rapid, spontaneous reactions that interfere with many biological processes. 

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