Model compounds, photooxidations

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). ... 

Hydroperoxides are much more efficient than ketones for initiating photooxidation of ethylene-propylene copolymers [19]. This fact was confirmed by the results from photolysis of low-molecular model compounds and isotactic polypropylene [20]. 

Berthomieu C, Boussac A, Mantele W, Breton J, Nabedryk E. (1992) Molecular changes following oxidoreduction of cytochrome b559 characterized by Fourier transform infrared difference spectroscopy and electron paramagnetic resonance Photooxidation in photosystem II and electrochemistry of isolated cytochrome b559 and iron protoporphyrin IX-bisimidazole model compounds. Biochemistry 31 11460-11471. 

Photooxidation reactions only take place in the presence of oxygen (15, 17). These reactions are the primary source of most of the sunlight damage to textiles. Photooxidation of nylon and model compounds have shown that oxidative attack usually produces free radicals, peroxides, and ultimately polymer chain scission. This results in lower tensile strength and ultimately a shorter useful lifetime of the textile product. 

Reinisch et al. (113) investigated photolysis and photooxidation of polycaprolactam using model compounds. 

The contribution made to these studies by my co-workers is gratefully acknowledged. In particular I wish to thank J. Vyvoda who carried out the study of PVC, A. Scott for the work on the photodegradation of HIPS, K. B. Chakraborty for his studies of the photooxidation and stabilization of the polyolefins and M. Humphrey for his studies of model compound autoxidation. 

The rates and products of poly(vinyl alcohol) (PVA) photooxidation were measured over a temperature range of 30°-90°C. Oxidation was initiated with 253.7-nm light, and several model compounds were included. The PVA photooxidation rate was autocatalytic and relatively insensitive to temperature with the major products being carbon dioxide, acids, peroxides, polymeric p-hydroxyketone, and hydrogen peroxide. Acids were mostly formic acid and carboxylic acids at the polymer chain ends. Several molecules of carbon dioxide and acid were formed per statistical chain scission. The mechanism of the photooxidation of PVA is discussed. 

However, the mechanism and products of PVA oxidation by molecular oxygen are not clear. The primary objective of this study is to measure rates and products of photooxidation of PVA and some model compounds in aqueous solution and to elucidate the oxidation mechanism. Such a fundamental understanding is needed to learn how to retard, accelerate, and direct photooxidation. 

Tables II and III summarize representative photooxidations of model compounds.

Ranby et have also used model compounds such as 3-phenylpentane for investigating the mechanisms of photo-oxidation of polystyrene. The photooxidation of polystyrene has been investigated in solution by a number of workers. Woolinski has observed the development of unsaturation and implicated the involvement of singlet oxygen in the mechanism of photo-oxidation, whereas Easton and MacCallum " invoke the involvement of polymer-solvent complexes in initiation. The photo-oxidation of polystyrene has been found to have a kinetic chain length of 10 —... 

The ring opening reaction was also supported by the studies of photooxidation of low molecular model compounds of polystyrene, e.g. 2-phenyl butane (5.85) [1360, 1361] ... 

In this review of the mechanisms now most commonly accepted as occurring in the photooxidation of polyolefins, we all especially consider the studies made on polyethylene and polsqMropylene, on some liquid hydrocarbons, models of the former, and on oxygen-containing model compounds (hydroperoxides (4), ketones (5), etc.) which supply much of the information we have concerning the photodiemical reactions of polymers. 

Although porphyrins and especially phthalocyanines are stable compounds, both will undergo photooxidative degradation or photoexcited ET reactions . An additional problem with magnesium complexes is their low stability in aqueous solution, as they demetallate quite easily. This is one of the main reasons that many photochemical studies targeted at modeling the natural situation use the more stable zinc(II) complexes. In addition, past years have seen increasing evidence that both Mg(II) and Zn(II) chlorophylls do exist in nature. 

Brunow and Sivonen (34) obtained similar results on a lignin model system, ethylguaiacylcarbinol. This compound did not undergo any oxidation when irradiated with near-uv light until p-methoxyacetophenone, a triplet sensitizer, was added to the solution. As the molar ratio of p-methoxyacetophenone to ethylguaiacylcarbinol was increased to 0.32 the rate of photooxidation of this benzyl alcohol increased. This result is consistent with the mechanism of light-induced yellowing shown in Scheme 3. 

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