Type II photooxidation

Figure 3 Type I and type II photooxidation processes with a porphyrin sensitizer illustrated with a modified Jablonski diagram. (S0 = ground singlet state, Si = first excited singlet state, S2 = second excited singlet state, T,i— ground triplet state, Ti = first excited triplet state, i.s.c. — intersystem crossing.)...

Whereas several l-aryl-4-nitroimidazoles are found to be good sensitizers for superoxide ion formations85 (Type I photooxidation), only l-phenyl-2-methyl-4-nitroimidazole 140 is a photosensitizer for singlet oxygen, i.e. by energy transfer of type II photooxidation (equation 70). 

Reactions of para- or mefa-substituted aromatic compounds hydrolysis, hydration of alkenes, substitution, oxidation, enzyme-catalyzed oxidations some type II photooxidations Hydrolysis and many other reactions of aliphatic organic compounds... 

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... 

In type I photosensitized oxidation, the triplet state sensitizer abstracts a hydrogen or electron from the unsaturated oil, producing radicals that initiate chain propagation as in autoxidation. However, chain-breaking antioxidants do not stop this reaction as new radicals are produced photochemically. In type II photooxidation, the energy of the triplet sensitizer is transferred to molecular oxygen, converting it... 

Type II photooxidations involve energy transfer from triplet sensitizers to 302 to form what is now recognized as 02. Dyes such as rose bengal and methylene blue [187] and metal complexes such as ruthenium(bipyr)3 [188—189] are very efficient sensitizers in visible light with quantum efficiencies close to 1. The process is generally described by the reactions... 

It is well established that singlet oxygen is able to oxidise DNA with a much higher specificity than hydroxyl radical. The main stable oxidation products of the reaction of O2 with 2-deoxyribose guanine were identified as the and 4S diastereoisomers of 4-hydroxy-8-oxo-4,8-dihydro-2 -deoxy-guanosine on the basis of extensive NMR and mass spectrometry measurements (Ravanat et al. 1992, Ravanat and Cadet 1995). Similar oxidation products were generated by the type II photooxidation reaction of 2 -deoxyguanosylyl -(3 -5 )-thymidine (Buchko et al. 1992). 

Table 3.28 shows that the composition of hydroperoxide isomers derived from an unsaturated acid by autoxidation ( 02) differs from that obtained in the reaction with 02- The isomers can be separated by analysis of hydroperoxides using high performance liquid chromatography and, thus, one can distinguish Type I from Type II photooxidation. Such studies have revealed that sensitizers, such as chlorophylls a and b, pheophytins a and b and riboflavin, present in food, promote the Type II oxidation of oleic and linoleic acids. 

The quenching effect of carotenoids (transition of 02 to 02) is very fast (k = 3 X 10 1 mole s ). They also prevent energy transfer from excited-state chlorophyll to 02- Therefore, carotenoids are particularly suitable for protecting fat (oil)-containing food from Type II photooxidation. 

Attaching the ketone groups to the polymer backbone is more efficient on a chain scission/ketone basis because some of the light energy that the pendent ketone absorbs leads direcdy to chain scission via the Norrish type II mechanism, as well as photooxidation via the Norrish type I mechanism (see... 

Several research groups have focused their attention on the photooxidation of 2 -deoxyguanosine that is used as a model compound for DNA. The major photooxidation products of this nucleoside were identified and classified according to their formation through a radical mechanism (type I) or a singlet oxygen-mediated mechanism (type II). The major type I product was identified as 2,2-diamino-[(2-deoxy-p-D-e/7f/zro-pentofuranosyl)-4-amino]-5(27/)-oxazolone 98 (Fig. 7.16). ... 

PAHs photooxidized by the type II singlet oxygen mediated mechanism include acenaphthylene, whose oxidized products and yields are shown in Fig. 10.29B (Barbas et al., 1994), phenanthrene (Barbas et al., 1996), anthracene (Dabestini et al., 1995), and tetracene (Dabestini et al., 1996). An additional photochemical process, the formation of photodimers, is also observed for acenaphthylene, anthracene, and tetracene. 

Adam W, Saha-Moller CR, Schonberger A (1996) Photooxidation of 8-oxo-7,8-dihydro-2 -deoxy-guanosine by thermally generated triplet-excited ketones from 3-(hydroxymethyl)-3,4,4,-tri-methyl-1,2-dioxetane and comparison with type I and type II photosensitizers. J Am Chem Soc 118 9233-9238... 

As mentioned above, the photooxidation was discovered by exposure of compound 22 to sunlight. The reaction proved to be of great value for angucycline synthesis because the -hydroxy group present in most natural products which is easily eliminated under basic or acidic conditions (see Scheme 2) and the carbonyl group at C-1 can thus be introduced under mild neutral conditions. We assume that the reaction is initiated by Norrish type II y-hydrogen abstraction of the excited carbonyl in 25 to yield a diradical 26 as shown in Scheme 8 with 1-deoxyrabelomycin (25) as the example [39]. The H-abstraction requires a very definite steric environment in which the benzylic protons have to be in proximity of the excited carbonyl group. Subsequent addition of the diradical 26 with... 

The photooxidation rates of nylon polymers have been monitored using Fourier Transform Infra-red spectroscopy. Norrish type-II reactions of carbonyl groups were considered to be the most important process. Another group of workers have studied the photooxidations rates of a nylon 6- polypropylene glycol copolymerIn this case polyether sequences are the major source of free radical attack resulting in high levels of hydroperoxides. Irradiation of nylon and polyester fibres with an excimer laser resulted in... 

Various 4-(arylazo)phenols and naphthols have been photooxidized using 02( A ) involving a type II mechanism . l,l -Binaphthol undergoes enantioselective oxidation (5.2% ee) when the chiral complex A-[Ru(4,4 -dimenthoxycarbonyl-2,2 -bipyridine)3] + is used as the photocatalyst . 

The photooxidation of oxopurines such as caffeine, theophylline, theobromine, and 1,3,7-trimethyluric acid using Rose Bengal as sensitizer occurs by a type II mechanism. 3-Methyl-5-(methylamine)-l,5-dehydrohydantoin has been characterised as a reaction product, and evidence is presented which suggests that the initial exciplex formed between 02( Ag) and the oxopurine evolves into a zwitterionic transition state. 

---