Properties of the Primary Radicals

The properties of the primary radicals produced by the radiolysis of water are collected in... 

Primary steps of photoinduced electron transfer have been studied in plant reaction centers (PS-I and PS-II), by flash absorption and EPR. In PS-I two questions wereinvestigated i) the properties of the primary radical pair P-700+, A0 (kinetics of decay nature of A0, presumably a specialized chlorophyll a decay by recombination to populate the P-700 triplet state) and ii) the nature of the secondary acceptor A,. Extraction-reconstitution experiments indicate that A, is very probably a molecule of vitamin K,. 

Reaction (6) produces the radical T with properties identical to those of the primary radicals RJ(T = R ). Both particles R and T react quantitatively with the monomer at a rate controlled by kp, and they do not terminate (thus they do not contribute to changes in kt with macroradical length). 

In the purines, there are more sites for OH attack, and the situation as to where OH adds preferentially is not as clear-cut as it is in the pyrimidines. On the basis of the oxidizing/reducing properties of the primary OH adduct radicals, additions at C4 and C8 have been suggested to dominate dAdo 50% at C4 (oxidizing), 37% at C8 (reducing)X dGuo 60-70% at C4 (oxidizing), 17% at C8... 

Fig. 3 Energy diagram of strongly doped n-Si/Si02 electrode in contact with aqueous electrolyte solution. The diagram contains the formal redox potentials of the primary radicals and combined redox and luminescence properties of Ru(bpy)3, and 2,6-bisPM,N-bis(carboxymethyl)amino-methyl]-4-benzoylphenol (ligand L). The diagram demonstrates clearly that on thermodynamical basis, both of these luminopbores could be excited even to their singlet excited states in the simultaneous presence of hydrated electrons and sulfate radicals via both ox-red and red-ox pathways. However, in practice the ox-red pathway seems to be dominant in both cases. E-type electron centers in silica fully correspond to F and F electron centers in alumina [36]...

Stabilizers, or more precisely antioxidants, are used to capture radicals in the system and thereby inhibit the degradation process. Mostly O-radicals are captured, but there are also some developments towards capturing C-radicals. Primary types (e.g. phosphites) mainly show activity in processing (melt temperature), secondary types (e.g. sterically hindered phenolics) in the normal application temperature range (long term stabilizers). Ultraviolet stabilizers are an additional means of preserving material properties by inhibition of the primary radical creation through radiation. 

The hydrogen abstraction from the Si-H moiety of silanes is fundamentally important for these reactions. Kinetic studies have been performed with many types of silicon hydrides and with a large variety of radicals and been reviewed periodically. The data can be interpreted in terms of the electronic properties of the silanes imparted by substituents for each attacking radical. In brevity, we compared in Figure 1 the rate constants of hydrogen abstraction from a variety of reducing systems by primary alkyl radicals at ca. 80°C. ... 

The primary conclusion that follows from the effect of base sequence on the efficiency of radical cation migration through duplex DNA is that base pairs cannot be considered in isolation. For example, the effect of placing a T in a sequence of purines depends critically on the nature and number of purines. In this regard, the effect of base sequence on radical cation transport emerges from examination of collective properties of the DNA. This is a clear indication that the charge is delocalized over several base pairs, a conclusion that is supported by extensive quantum calculations. 

However, surface segregation (cs / Cb) has a radical effect on AE, as can clearly be seen in Fig. 6.3(b). Cu/Ni alloys are known (Kelley and Ponec 1981, Ouannasser et al 1997) to have an enriched Cu concentration in the surface layer for all bulk concentrations. As a result, the alloy shows a more Cu-like behaviour than it would if it were non-segregated. In particular, AE has a value significantly closer to that for pure Cu than in the case where cs = Cb, and this occurs at all bulk concentrations c. The smallest change in AE occurs in Cu-rich alloys, which is understandable, because these alloys have mostly Cu in the surface layer anyway, so the effect of surface segregation is relatively small. Thus, surface segregation has a lesser effect in these alloys than in Ni-rich ones, which have mostly Ni in the bulk, but may have a Cu majority in the surface layer. Clearly, then, the concentration cs of the surface layer is the primary parameter in determining the chemisorption properties of the DBA. 

As we have seen in Sections 1 and 4, the principal primary products of the radiolysis of water are powerful oxidizing and reducing radicals in approximately equal yields. For water radiolysis to be a useful tool in general chemistry, it is desirable to convert the primary radicals to a single kind of secondary radical to achieve either totally oxidizing or reducing conditions. Moreover, there is the possibility of designing the system to have the required redox properties by suitable selection of the secondary radicals. Some useful systems that meet these requirements are described below. 

When the antioxidants were used in the cooked/stored samples, data indicated that they were very effective in inhibiting lipid oxidation and MFD. The chemical and off-flavor indicators were reduced and the on-flavor notes were increased. Thus, phenolic-type primary antioxidants that function as free radical scavengers are very effective tools for preventing lipid oxidation and MFD in ground beef. It should also be noted that the intensity of the desirable flavor notes remained at very high levels, which meant that the patties retained their beefy tastes. Therefore, for an antioxidant to be highly effective, it should not only prevent lipid oxidation, but it should also retain the desirable flavor properties of the food commodity. 

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