Photooxidation of Substrates

The majority of photooxidation reactions carried out with metalloporphyrins can be classified as atom transfer types. Under thermal conditions such reactions are exemplified by the oxidation reactions that can be carried out with cytochrome P-450. Upon photolysis the //-oxo-bis(tetraphenylporphinato)iron(III) complex (FeTPP)20 undergoes disproportionation to give the Fe(II) complex FeTPP, and the Fe(rV) complex Fe(0)TPP (Ref. 49)  

This highly reactive complex Fe(0)TPP will abstract a hydrogen atom from an alkene to give Fe(OH)TPP. Photolysis of a mixture of (FeTPP)20 and cyclohexene in the presence of oxygen results in the formation of cyclohex-2-en-l-one when the photolysis is carried out at low light intensities, and cyclohex-2-en-1 -ol at high light intensities (Fig. 9.4). When the photolysis of (FeTPP)20 is carried out in the presence of triphenylphosphine, the products are FeTPP and triphenylphosphine oxide 

Coordinately unsaturated metalloporphyrins can be incorporated into catalytic systems that involve substrate photooxidation. The photodehydrogenation of cy-clohexanol and isopropanol can be carried out using a tetraphenylporphyrin complex of rhodium. This complex has absorption bands at 417 nm and at 532 nm, and irradiation into either band results in photodehydrogenation. For irradiation at 417 nm the reaction rate is independent of catalyst concentration, but for irradiation at 532 nm the rate depends on the catalyst concentration because a bimolecular quenching occurs between the excited and ground states of Rh CTP ) . The 

Electron transfer reactions from metalloporphyrin dimers have been studied using molecules that have a quinone moiety chemically attached to the porphyrin ring. These metalloporphyrin complexes are designed such that charge separation can be accomplished between the porphyrin and the quinone, and that these transients can then be used as oxidants. 

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Further support for the idea of complex formation was provided by measurements of the photooxidation of substrate amounts of cytochrome c in continuous light. The observed time courses were exponential rather than linear even at the highest cytochrome concentrations used. A similar result is well-established in the analogous assay with mitochondrial cytochrome oxidase and is believed to be caused by binding of both oxidised and reduced cytochrome to the oxidase with equal affinity (reviewed by Wikstrom et al, 1981). The effect of cytochrome concentration on the rate is shown in Fig. 3. In contrast to the usual observation with the mitochondrial enzyme, there is evidence for only one binding site, with a K... 

Kinetics of the photooxidation of organic water impurities on illuminated titania surfaces has been generally regarded to be based on the Langmuir-Hinshelwood equation with first-order reaction kinetics vs. initial substrate concentration was established univocally by many authors... 

Photolytic. Fukuda et al. (1988) studied the photodegradation of acenaphthene and alkylated naphthalenes in distilled water and artificial seawater using a high-pressure mercury lamp. Based upon a rate constant of 0.23/h, the photolytic half-life of acenaphthene in water is 3 h. Behymer and Hites (1985) determined the effect of different substrates on the rate of photooxidation of acenaphthene using a rotary photoreactor equipped with a 450-W medium pressure mercury lamp (X = 300-410 nm). The photolytic half-lives of acenaphthene absorbed onto silica gel, alumina, and fly ash were 2.0, 2.2, and 44 h, respectively. The estimated photooxidation half-life of acenaphthene in the atmosphere via OH radicals is 0.879 to 8.79 h (Atkinson, 1987). 

Behymer and Hites (1985) determined the effect of different substrates on the rate of photooxidation of acenaphthylene using a rotary photoreactor equipped with a 450-W medium pressure mercury lamp X = 300-410 nm). The photolytic half-lives of acenaphthylene absorbed onto silica gel, alumina, fly ash, and carbon black were 0.7, 2.2, 44, and 170 h, respectively. 

Overall, while the combinations of substrate effects, ambient NOz levels, and other gas-particle phenomena preclude a definitive answer, the formation of significant amounts of nitroarenes in heterogeneous particle-phase N02-PAH, atmospheric reactions seems unlikely, e.g., much slower than photooxidation or ozonolysis. This conclusion also applies to heterogeneous reactions of N205 with particle-bound PAHs on diesel and wood soot (Kamens and co-workers, 1990 see also Pitts et al., 1985c, 1985d, 1985e). 

In the catalysis of the lyase from C. perfringens, the participation of lysine residues forming intennediary Schiff bases between enzyme and substrate molecules, and of histidine residues, has been demonstrated with the aid of photooxidation, reagents for histidine modification, and borohydride reduction in the presence of substrate.408-418 Thus, according to Frazi and coworkers,414 the lyase belongs to the class I lyases (aldolases). The catalytic mechanism proposed is outlined in Scheme 3. Evidence has been educed for the existence of a similar mechanism of cleavage of sialic acid by the lyase enriched from pig kidney.411... 

Since reduced flavins, pteridine derivatives, and PQQ can be readily oxidized by oxygen to regenerate the oxidized forms [59-62], these coenzyme analogs can act as photocatalysts when the oxidation of substrates by the coenzymes occurs photochemically. No appreciable photooxidation of benzyl alcohol by oxygen occurs when aminopterin (AP), lumazine (Lu), or riboflavin-tetraacetate (FI) is used as a photocatalyst in the absence of acid in MeCN. When HC104 is added to this system, however, the flavin and pteridine derivatives are protonated as described above, and each proton-ated species (catH+) can act as an efficient photocatalyst for the oxidation of benzyl alcohol derivatives (X-C6H4CH2OH) by oxygen [70] ... 

Pteridines can also function as catalysts in photooxidations whereby the protonated species of lumazine and aminopterin, respectively, catalyze the substrate-selective photooxidation of benzyl alcohol derivatives by oxygen in the presence of perchloric acid in acetonitrile <89CC816>. 

No photooxidation of the original organic substrate occurs in the absence of 02. Therefore the efficiency of electron trapping, ST, is a function of the dissolved oxygen concentration and is given by ... 

These results suggested that superoxide ion is indeed involved as a reactive intermediate, at least in the electron-transfer processes, leading to 1,4-diketones and oxiranes, whereas the ring-contracted derivatives arise from a process not requiring superoxide ion as the oxygen reactive species. In this regard, a comparative study on the sensitized photooxidation of the same substrates with... 

Competition between Type I and Type II photooxidations are affected by micellar media. Type I photooxidation involves initial quenching of the sensitizer excited state by substrate, while Type II photooxidations involve initial quenching of the sensitizer excited state by oxygen. Since, competition between Types I and II photooxidations are altered by the concentration of the substrate, local concentration effects in micelles play an important role. The photooxidation of tryptophan and tryptamine... 

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