Radical accumulators

The kinetic curve of the accumulation and consumption of o-DNB anion-radicals is S-shaped and that of the accumulation of o-nitrophenolate is parabolic. The anion-radical accumulation curve starts to go down when the o-nitrophenolate accumulation curve reaches its maximum (Abe and Ikegame 1978). 

In this connection, attention should be paid to an unusual NMR technique called nuclear magnetic relaxation dispersion (NMRD). In contrast with NMR spectroscopy, the NMRD signal arises from the nuclei of the abundant solvent molecules and not from the dissolved substances. The relaxation properties of the solvent molecules are profoundly modified if the solvent contains paramagnetic particles (see a review by Desreux 2005). A solvent molecule sails in the vicinity of an ion-radical and finds itself in the local magnetic field of this paramagnetic particle. Then, induced magnetism of the solvent molecule dissipates in the solvent bulk. This kind of relaxation seems to be registered by NMR. NMRD is applicable to studies on ion-radical solvation/desolvation, ion-pair dynamics, kinetics of ion-radical accumulation/consumption, and so on. 

Exposure of rat primary mixed hippocampal cell cultures to either sodium nitroprusside (SNP 100 xM) or 3-morpholinosydnonimine resulted in both a decrease in cell survival and an increase in free-radical accumulation. These SNP-induced events were blocked by either EGb 761 (10 to 100 pg/ml) or its flavonoid fraction CP 205 (25 pg/ml), as well as by inhibitors of protein kinase C (PKC chelerythrine) and 1-type calcium channels (nitrendipine). In contrast, the terpenoid constituents of EGb 761, known as bilobalide and ginkgolide B, as well as inhibitors of phospholipases A [3-[4-octadecyl)benzoyl]acrylic acid (OBAA)] and C (U-73122), failed to display any significant effects. Moreover, EGb 761 (50 pg/ml), CP 205 (25 pg/ml), and chelerythrine were also able to rescue hippocampal cells preexposed to SNP (up to 1 mM). Finally, EGb 761 (100 g/ml) was shown to block the activation of PKC induced by SNP (100 xM). These data suggest that the protective and rescuing abilities of EGb 761 are not only attributable to the antioxidant properties of its flavonoid constituents but also by their ability to inhibit NO-stimulated PKC activity (Figure 36.1). 

Since free radical accumulation was proposed to mediate Ap toxicity,27 we studied the effects of EGCG on intracellular reactive oxygen species (ROS) using DCF assay. As expected, a 24-hour exposure to Ap, 42 resulted in a small but significant increase in DCF fluorescence (+18% relative to control), which was reduced by EGCG (10 xM) and by EC (101xM), a tea catechin that failed to protect cells. 

Figure 5.6 Kinetic dependencies of intermediate radical accumulation and accumulation on the contact time. T = 400 °C cH202 = 30%.

Since the main reason for selectivity decrease of oxidation with hydrogen peroxide is associated with OH radical accumulation in the system, the search for ways to eliminate this event, i.e. liquidating the reason but not the consequence, should consist in the following ... 

The calculated kinetics of charge and radical accumulation was in qualitative agreement with what was expected. However, the theory does not fit the experimental data obtained by Beckert et al. [183,184]. The reaction time reported there is 0.8 ns, while the calculated buildup time of radical accumulation varies from 60 to 560 ns. The alcohols used by Dinse et al. can be scarcely considered as nonpolar solvents. Their polarity is so high as to render the Coulomb well insignificant and facilitates evacuation of products. The accumulation of radicals must end in a few nanoseconds, but in fact it lasts about 100 ns. This is actually an indication that there is some binding potential for ions or radicals that is deeper than that originating from the Coulomb attraction. There is room for the exciplex formation discussed in a recent review in 2000 [32]. 

Radical accumulators whose presence might facilitate addition to / -mono and / ,/ -disubstituted olefins were conceived. It seemed to us that alkylaryl or dialkyl tellurides should react with alkyl radicals and give an intermediate radical of type R1R2R8Te (an expanded valence shell) which might have a relatively longer life on the radical time scale. A secondary objective would be the exchange of one radical against another. In this way, the special nucleophilic properties of the aryl telluride anion for example, could be exploited to make complex natural product derived radicals. 

Another obstacle to effective using of antioxidants is their extreme concentration-effect dependence. As noted above, antioxidants applied in high concentrations produce an opposite effect and do not inhibit but accelerate free radical reactions. The phenomenon may be attributed either to a high activity of radicals accumulated from inhibitors or to the prevailing consumption of natural antioxidants as compared with synthetic ones introduced. Many of these effects depend on the initial characteristics of free radical processes and the initial level of antioxidants. 

Lancelot, E., Revaud, M.L., Boulee, R.G. (1997). Alpha-N-tert-buty Ini tone attenuates excitotoxicity in rat striatum by preventing hydroxyl radical accumulation. Free Radio. Biol. Med. 23 1031-4. 

The total rate of absorption is obtained by assuming a rapidly established equilibrium distribution for the radical between the inner and the outer surface of the particle. Assuming no radical accumulation at the interface the value of in Eq. (14) is equal to that in Eq. (17). The equilib-iu ra distribution between the inner and outer surface of the particles is expres by... 

Quantitative study of kinetics of radicals accumulation has required solution of auxiliary problem - definition of the rate of photoinitiation Win. In the case of solid-phase reactions there are experimental difficulties in solution of this problem. Measurement of Win according to consumption of inhibitor is complicated by possible photochemical reactions of inhibitor itself and specific solid-phase effects of kinetic stop type and so on [10]. Measurement of Win according to initial rate of radicals accumulation is also tactless in solid polymer, as the latter may be much lower than Wm [164]. 

Presence of the processes of structurization in undyed film during the first period of irradiation may be observed while investigating viscosity of irradiated PVA-films, that proves some increase in intrinsic viscosity (see Figure 2.41). Such increase in intrinsic viscosity is not observed in dyed films. Probably PVA structurization after ultra-violet irradiation takes place by recombination radicals of polymer adjacent chains. The rate of radicals accumulation in dyed PVA-films is less (Figure 2.41) and at the same time probability of structurization by recombination PVA macroradicals decreases. 

Rates of iminoxyl radical accumulation and phenoxyl radical decay are approximately equal at various NO2 concentrations (Fig. 6). 

The case a > 1, i.e., the case where the frequency of fragmentation is smaller than that of addition, is shown in Figure 3. The radical concentration, r, is initially growing due to radical initiation as in Figure 2, while the intermediate radicals accumulate due to addition to the dormant chains. At r = ti, which corresponds to the characteristic time of the addition reaction, the produc-... 

The characteristic time intervals mentioned above, selected from the kinetic trajectories of the value contributions, correlate with the kinetics of phenoxyl radical accumulation. It follows fiom Figure 7.8 drat completion of the first time interval (ti) corresponds to establishing die quasi-stationary mode of pora-methylphenoxyl radical accumulation. Over the time interval t2 - h the growth in the concentration of phenoxyl radicals is observed as a consequence of increasing the role of degenerate chain branching steps. Finally, mounting to the maximum value, the concentration of phenoxyl radicals decreases because of inhibitor consumption. 

At the last stages of autoxidation the peroxyl radicals accumulate. At relatively high levels the peroxyl radicals interact with each other to form non-radical products by the termination reaction (11),... 

Knockout mice lacking the gene encoding the mitochondrial manganese superoxide dismutase develop metabolic acidosis and die a few days after birth of severe heart failure due to dilated cardiomyopathy (Li etal. 1995). Several lines of evidence demonstrate a decline of mitochobdrial enzyme activities with age, particularly complex I (Trounce etal. 1989). Lack of an efficient repair mechanism and absence of histones make mtDNA more susceptible to the toxic effects of free radicals. Accumulation of mtDNA mutations are proposed to cause progressive impairment of mitochondrial respiration. Deletion of mtDNA accumulate with age in the human heart (Hattori et al. 1991). 

Now let us consider a system composed of PS-TEMPO (denoted as P-N) adduct and monomer styrene with no extra N added. When the system is heated to a suf ciently high temperature where the adduct dissociation takes place, the concentration of P and N will start to increase from 0 up to the values determined by the equilibrium constant K. Since P /P biradical termination will continually reduce [P ] relative to [N ], polymerization will eventually stop because of numerous N radicals accumulated in the system. However, if initiation also takes place in the system, the newly formed radicals will combine with N preventing its accumulation and a stationary state for both [N ] and P"] will be reached. Setting d P ]jdt = 0 in Eq. (11.12) and d[N ]/<(/ = 0inEq. (11.13), and using Eq. (11.11), one thenhas... 

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