Hydroperoxide initiated chemiluminescence

Table 2. Antioxidant Activity of Argentine Aqueous Plant Extracts IC50 and 95% Confidence Interval for Inhibition of Hydroperoxide-initiated Chemiluminescence (CL) and the Production of Thiobarbituric Acid-reactive Substances (TBARS) in Rat Liver Homogenates...

Hydroperoxides are formed in each polymer containing aliphatic hydrocarbon chains on standing by various initiators, including light, heat, etc. This may be well detected by the non-isothermal chemiluminescence method in nitrogen. The analytical possibilities of this approach have not been fully utilized until recently [17],... 

The degradation process has a free radical mechanism. It is initiated by free radicals P that appear due to, for example, hydroperoxide decomposition induced thermally or by trace amounts of metal ions present in the polysaccharide. One cannot exclude even direct interaction of the polysaccharide with oxygen in its ground triplet state with biradical character. Hydroperoxidic and/or peracid moieties are easily formed by oxidation of semiacetal chain end groups. The sequence of reactions on carbon 6 of polysaccharide structural unit that ultimately may lead to chemiluminescence is shown in Scheme 11. 

Hi. Lysine. Gamma radiolysis of aerated aqueous solution of lysine (94) has been shown, as inferred from iodometric measurements, to give rise to hydroperoxides in a similar yield to that observed for valine and leucine. However, attempts to isolate by HPLC the peroxidic derivatives using the post-column derivatization chemiluminescence detection approach were unsuccessful. This was assumed to be due to the instability of the lysine hydroperoxides under the conditions of HPLC analysis. Indirect evidence for the OH-mediated formation of hydroperoxides was provided by the isolation of four hydroxylated derivatives of lysine as 9-fluoromethyl chloroformate (FMOC) derivatives . Interestingly, NaBILj reduction of the irradiated lysine solutions before FMOC derivatization is accompanied by a notable increase in the yields of hydroxylysine isomers. Among the latter oxidized compounds, 3-hydroxy lysine was characterized by extensive H NMR and ESI-MS measurements whereas one diastereomer of 4-hydroxylysine and the two isomeric forms of 5-hydroxylysine were identified by comparison of their HPLC features as FMOC derivatives with those of authentic samples prepared by chemical synthesis. A reasonable mechanism for the formation of the four different hydroxylysines and, therefore, of related hydroperoxides 98-100, involves initial OH-mediated hydrogen abstraction followed by O2 addition to the carbon-centered radicals 95-97 thus formed and subsequent reduction of the resulting peroxyl radicals (equation 55). 

Abstract The oxidation of polymers such as polypropylene and polyethylene is accompanied by weak chemiluminescence. The development of sensitive photon counting systems has made it comparatively easy to measure faint light emissions and polymer chemiluminescence has become an important method to follow the initial stages in the oxidative degradation of polymers. Alternatively, chemiluminescence is used to determine the amount of hydroperoxides accumulated in a pre-oxidised polymer. Chemiluminescence has also been applied to study how irradiation or mechanical stress affects the rate of polymer oxidation. In recent years, imaging chemiluminescence has been established as a most valuable technique offering both spatial and temporal resolution of oxidation in polymers. This technique has disclosed that oxidation in polyolefins is non-uniformly distributed and proceeds by spreading. 

Under nitrogen, the emission of chemiluminescence from polymer samples corresponds to the initial concentration of peroxy radicals (POO") and it is proportional to the hydroperoxide content formed during the processing of the material, this being related to its thermal oxidation history. Under these conditions, Rt = 0. Hence, Equation 3.2 can be simplified as follows [26] ... 

TABLE 3.1 Chemiluminescence Under Nitrogen at 170°C of LDPE films (120 (Lin) That Contain 0.1% w/w of Different Hindered Phenols and Their Initial Hydroperoxide Content... 

An investigation of the kinetics of the decomposition of cyclohexyl hydroperoxide at 60-70 in the presence of vanadyl acetylacetonate was recently carried out [355]. Cyclohexanol and cyclohexanone were formed in roughly a 1 1 ratio. The initial rate of decomposition was first order in initial concentrations of hydroperoxide and vanadium complex at [ROOM] <5 x 10 M and [VO(acac)2] < 1 x 10" M. The initial rate of decomposition changed from first order in [ROOH] to zero order giving evidence of complex formation prior to hydroperoxide decomposition. Using a chemiluminescence method the authors [355] concluded that only about 20% of the cyclohexyl hydroperoxide which decomposed gave free radicals. 

That an intermediate with a lifetime of some minutes (in the absence of a fluorescer) plays a key role in oxalate chemiluminescence was demonstrated by delayed fluorescer addition. Even when the fluorescer was added 70 minutes after the preparation of oxalate/hydroperoxide mixture, 53% of the quantum yield obtained by initial addition of the fluorescer was observed. On the other hand, the fluorescer evidently acts as catalyst in the chemiluminescent decomposition of that intermediate [2]. This intermediate could be transported by an inert gas stream from the oxalate/hydrogen peroxide mixture into a fluorescer solution, producing the fluorescence of the latter. In this case no consumption of the fluorescer took place. A charge-transfer complex between the fluorescer and the intermediate was proposed as early as 1967. The intermediate, as was mentioned above was assumed to be dioxetanedione (4) but it was not possible to identify it [2]. 

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