Accumulation of Reactive Species

In flash chemistry, we have to activate molecules to generate highly reactive species. However, it is important to note that the concentration of the thus-generated reactive species should be high enough to drive the reaction at high speed. Otherwise, the reaction is slow even if the reactivity of the species is very high. 

In contrast, carbocations are usually generated in the presence of nucleophiles. This is probably because carbocations are considered to be highly unstable and transient in conventional reaction media and should 

Therefore, development of a new method that enables the generation and accumulation of carbocations or organic cations in the absence of nucleophiles is highly desirable to achieve extremely fast chemical cationic reactions. In the next section, we will discuss the realization of this method in detail. 

---

The first two processes listed above involve accumulation of reactive species during some periods of operation, followed by reaction of these species during subsequent periods of operation. I will call these "accumulation-reaction" processes. The presence of this type of transient process can be identified by the presence of a transient discrepancy in the mass balance of one or more chemical elements across the converter. For example, more oxygen atoms might be coming out of the converter in the exhaust at a particular instant in time than are entering in the exhaust (when correction is made for the residence time of exhaust in the plug-flow converter). 

In order to determine whether the response of a catalyst is complex, we only need to compare the measured response to that predicted for a model catalyst that has a "simple" response. Such a model catalyst with a simple response would have (a) the same steady-state performance as the real catalyst, (b) no accumulation of reactive species or reaction of accumulated species during transients, and (c) only instantaneous changes in catalyst activity during transients. We call the response of a model catalyst with this behavior "instantaneous response." Essentially, the model catalyst exhibits instantaneous response to steady-state conditions. 

Increased accumulation of reactive oxygen species (ROS) in biological systems. 

The mitochondrial dysfunctionality seen in manganese neurotoxicity might be related to the accumulation of reactive oxygen species (Verity, 1999). Mitochondrial Mn superoxide dismutase (MnSOD) is found to be low or absent in tumour cells and may act as a tumour suppressor. It is induced by inflammatory cytokines like TNF, presumably to protect host cells. In a rat model, iron-rich diets were found to decrease MnSOD activity, although a recent study reported that in rat epithelial cell cultures iron supplementation increased MnSOD protein levels and activity, but did not compromise the ability of inflammatory mediators like TNF to further increase the enzyme activity (Kuratko, 1999). 

Chemical capacitance. When the mechanism involves significant involvement of the bulk, accumulation of reactive intermediates not only involves surface species but oxidation and reduction of the bulk. This can be detected as an anomalously high effective capacitance, often referred to as a chemical (or pseudo) capacitance. This capacitance can be as large as 0.1 — 1 F/cm and thus easily detected by current-interruption or impedance techniques. Thus, capacitance is a strong indicator (independent of resistance) as to what degree the interface, surface, and/or bulk are playing in the... 

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. 

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). 

Sulfur mustard-induced lipid peroxidation is a function of glutathione (GSH) depletion. For this mechanism, depletion of GSH results in an accumulation of reactive oxygen species via hydrogen peroxide-dependent processes (Miccadei et al., 1988). The oxygen radicals react with membrane phospholipids forming lipid peroxides that alter membrane structure resulting in membrane breakdown. 

Many varieties of saturated ions are involved in the fruitful reactions of cracking, all participating in cycles of reactions which preclude deactivation. In order to explain catalyst decay, we must envision the formation of some other type of surface species. This must be a species which occasionally arises from the carbenium ions which normally participate in the mainline reactions. Where else could it come from Such a species - the species we believe to be responsible for decay - is unsaturated carbenium ions. Unsaturated ions may be expected to differ in their desorption and reactivity properties from the more common saturated ions perhaps these differences are sufficient to explain the accumulation of deactivating species. 

Tubuloside B (19) significantly attenuated MPP+-induced cytotoxicity, DNA fragmentation and intracellular accumulation of reactive oxygen species (ROS). These results indicated that tubuloside B (19) prevent MPP+-induced apoptosis and oxidative stress and may be applied as an anti-Parkinsonian agent [59]. 

Fester T, Hause G (2005) Accumulation of reactive oxygen species in arbuscular mycorrhizal roots. Mycorrhiza 15 373-379... 

The underlying assumption for using C X t as the quantitation of mutagen exposure is that the amount of mutation is related to the extent of reaction with DNA. The reaction should have first-order kinetics in which reaction rate is proportional to the concentration of the active species. If the first-order rate constant for the reaction of reactive species with DNA is k and the concentration of reactive species in the nucleus is Cn, then the reaction rate is Cn- If reaction products are not removed from the DNA, damage will accumulate. The total amount of reaction is then... 

---