Sensitized Photooxidation

As well as influencing chromophore types and levels, variations in production conditions result in fibers of varying morphology (orientation, crystallinity, and supramolecular order) (3). Because morphology itself may markedly effect photooxidative sensitivity, the extent of this effect was explored by first studying the susceptibility of the various filaments to y-initiated oxidation. The y-initiation is believed to be completely random and it is not dependent on trace quantities of chromo-phores or the build-up of oxidation products, in contrast to photooxidation, yet it is followed by the same propagation and termination steps as photo-initiated oxidations (2,4). 

White GJ (1965) Nonantigenicity of lincomycin. Antimicrob Agents Chemother 398-402 Wiebe JA, Moore DE (1977) Photooxidation sensitized by tetracyclines. J Pharm Sci 66 186-189... 

HPLC-CL (Section C) and HPLC-thermospray MS were used to analyse hydroperoxides generated from PC with either the free radical initiator azobis-dimethylvaleronitrile (AMVN), or by photooxidation sensitized with methylene blue. Mono-and di-PC hydroperoxides were established by HPLC-CL in the sample produced by photosensitized oxidation. Over 90% of the hydroperoxide peak was shown to be PC hydroperoxides and reacted preferentially with the cytochrome c-luminol CL cocktail (Section C). In contrast, the sample prepared with AMVN showed that 70-95% of the hydroperoxides was not PC-OOH, but was largely due to spurious AMVN-derived hydroperoxides (Table 6.2). 

Possible Mechanism of Water Photooxidation Sensitized by Chlorophyll Reaction Thermodynamics... 

Valorso R, Aumont B, Camredon M, Raventos-Duran T, Mouchel-Vallon C, Ng NL, Seinfeld JH, Lee-Taylor J, Madronich S (2011) Etqilicit modelling of SOA formation from alpha-pinene photooxidation sensitivity to vapour pressure estimation. Atmos Chem Phys 11(14) 6895-6910... 

The chlorophylls produced by bacteria, algae and plants are a natural source of chlorins. The isolation of chlorophylls from natural material is known to be difficult because of their extreme sensitivity to various reactions, such as enolization, epimerization, allomerization, de-methoxycarbonylation, solvolysis, demctalation, dephytylation, photooxidation, etc. Often the... 

Fig. 3. a) First order plot of oxygen uptake in the Methylene-blue (MB)-sensitized photooxidation of GA 8.4 pM and 1.3 mM histidine (control) in phosphate buffer pH 7. b) Percentage radical scavenging activity for the control molecule Trolox and GA at pH 7.4 in phosphate buffer 10 mM (hydroxyl radical) and pH 10 in sodium carbonate buffer 50 mM (anion superoxide radical). 

Pigment-Sensitized Dye Photooxidation. Dye degradation sensitized by titanium dioxide is a recognized phenomenon (16). 

As a beginning toward understanding and preventing this phenomenon, the effect of selected amines and phenol stabilizers on the pigment-sensitized photooxidation of the leuco triphenylmethane dye I to the blue dye II was determined (Scheme 2). 

Inhibition of T102 - Sensitized Photooxidation of a Leuco Dye by Amines and Phenols... 

Tetrahydrofuran has been reported to exhibit an absorption maximum at 280 nm (52,56), but several workers have shown that this band is not produced by the purified solvent (30,41,57). Oxidation products from THF have been invoked in order to account for the appearance of the 280-nm band in PVC films that are solvent-cast from THF in air (57. 581. However, in some reported cases (56,59), this band was undoubtedly produced, at least in part, by a phenolic antioxidant (2.6-di-tert-butyl-p-cresol)(59) in the solvent. Since certain -alkylphenols have now been shown to be powerful photosensitizers for the dehydrochlorination of PVC (60), it is clear that antioxidant photosensitization might well have been responsible for some of the effects attributed previously (56) to THF alone. On the other hand, enhanced rates of photodegradation under air have also been observed for PVC films cast from purified THF (57), a result which has been ascribed to radical formation during the photooxidation of residual solvent (57,61). Rabek et al. (61) have shown that this photooxidation produces a-HOO-THF, a-HO-THF, and y-butyro-lactone, and they have found that the hydroperoxide product is an effective sensitizer for the photodehydrochlorination of PVC at X = 254 nm (61). 

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