Resolution radical stabilization

A similar connection between copper coordination to peptides and medical consequences exists for the Alzheimer disease.79 The amyloid-p peptide in senile plaque is the site of copper binding and as before there is interest to study details of the coordination and the stability. The neurotoxicity seems to be related to free radical damage and Cu2+ chelators are probed as therapy. So far, there seems to be no investigation at a resolution possible to pinpoint copper ligands as derived from ENDOR or related pulse techniques. We mention some relevant EPR studies as introductory sources into the topic.80 81,82... 

In the continuation of our work a study has also been made of the system benzene/silica gel. When irradiating this system at 77 °K. it was found that the silica gel could stabilize both monomeric and dimeric cation radicals of benzene (6). Furthermore, the high resolution of the electron spin resonance lines indicated a high degree of mobility for the benzene molecules in the adsorbed layer. No spectrum from trapped electrons could be observed although this could very well be hidden behind the strong cation absorption. However, ethylene, and isobutylene in the adsorbed state at low temperature gave spectra from shortlived species identified as trapped electrons (7). 

The requirement of the presence of the polymerization catalyst in the depolymerization process stems from the principle of microscopic reversibility. If, for example, all free radicals are removed from the system by simply endcapping the polymer, the thermodynamic equilibrium dictates that depolymerization state cannot be reached and the system will be stable. This was the approach employed by Ito and Willson in stabilizing polyphthaldehyde resists. [See for example, C.G. Willson, H. Ito, J.M.J. Frechet, T.G. Tessier, F.M. Houlihan, Approaches toward the design of radiation sensitive polymeric imaging systems with improved sensitivity and resolution, J. Electro chem. Soc. 133, 181 (1986)]. 

Hence, our calculations emphasized a step-wise mechanism over most oxides H-abstraction leads to a surface hydroxyl, in conjunction with the alkyl radical formation, which rapidly rebounds to a nearby oxygen to form a surface alkoxy. This step-wise one-electron oxidation mechanism not only offers an energetically more favorable route than the one-step two-electron oxidation mechanism such as (5+2), but also provides a plausible solution to the puzzle as contrasting the EPR results [50] to the IR results [49] (cf. Fig. 6). We propose that EPR with higher time resolution (10 -10 s) detected the radical formation, whose stability was enhanced by the higher acidity of the tungstated zirconia catalyst which retarded the rebound process. On the contrary, IR with lower time resolution (10 -10 s) was unable to capture the active alkyl intermediates, showing the peaks related to the existence of both surface hydroxy and alkoxy after rebound. 

In spin trapping experiments, relatively stable ESR-active compounds, the spin adducts, are formed by reaction of radicals with ESR-silent compounds, the spin traps, added to the smnpie. The most commonly used spin traps are nitroxides and nitrones, which form stabilized radicals by reaction with other radicals (23). Based on the characteristics of the spin adduct (e.g. hyperfine pattern, coupling constants, and g-value), an assignment of the radical in question is often possible. However, due to lack of specificity of the often-used nitroxides, like N-r-butyl-a-phenylnitro-ne (PBN), a valid verification of the radicals trapped depends on identification by tecimiques such as HPLC-MS. Despite the lack of spectral resolution, spin tr q>ping seems to be a promising technique for prediction of the oxidative susceptibility of dairy products (see later sections). 

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