Photodegradation pathways

The understanding of some photodegradation pathways appears to be still in its infancy. Photostability studies on a new ester of methylprednisolone... 

The described enhancement of carbamazepine photodegradation by Fe(III) oxide colloids could take place in deltas and estuaries, and our laboratory data suggest that the photochemical consequences of the chloride build-up could more than compensate the decrease of iron, due to colloid coagulation and sedimentation, which is usually observed in these environments. Interestingly, carbamazepine photodegradation on interaction between Fe(III) oxide colloids and chloride was more important than DOM-sensitized photolysis or direct photolysis, which are usually major photodegradation pathways for organic pollutants in surface waters [93]. 

K.K. Ngim, S.A. Mabury, D.G. Crosby, Elucidation of fipronil photodegradation pathways, J. Agric. Food Chem. 48 (2000) 4661-4665. 

Scheme 9 Photodegradation pathways of irgarol in natural waters...

Figure 11.7 Photodegradation pathways of esfenvalerate. (From Katagi, T., /. Agric. Food Chem., 39,1351,1991. With permission.)... 

Moore DE. Photochemistry in medicine. Techniques for the analysis of photodegradation pathways of medicinal compoimds. Trends Anal Chem 1987 6 234-238. 

Baertschi SW. IGH Option 1 and Option 2 sources potential for different photodegradation pathways. Photostability 01 4th International Conference on the Photostability of Drug Substances and Drug Products, Research Triangle Park, NC, July 16-19, 2001. 

Table 1 Photodegradative Pathways of Some Therapeutic and Nontherapeutic Agents...

Several reports have indicated that the nature of photodegradation pathway and rate of photochemical decomposition of drug substances are influenced by the type of the source, intensity, and wavelength. In addition, time of exposure and the distance between the source and the location of the exposed samples within a test chamber are other variables that can affect the noted rate of photodecomposition. 

The photodegradation pathway of a drug in the solid state does not necessarily follow the degradation in solution. The difference arises mainly from a lack of molecular mobility, which restricts diffusion-controlled reactions and interactions between molecules, thereby influencing the reactant conformation. The photochemical properties of molecules in the solid state or embedded in a solid matrix depend greatly on the organization (if any) and nature of the solid lattice. In a number of cases, it has been possible to correlate the reactivity with packing of the reactant molecule in the crystal (Hadjoudis et al., 1986). 

Sortino, S., Giuffrida, S., De Guidi, G., Chillemi, R., Petralia, S., Marconi, G., Condorelh, G., and Sciuto, S. (2001) The photochemistry of flutamide and its inclusion complex with 3-cyclodextrin. Dramatic effect of the microenvironment on the nature and on the efficiency of the photodegradation pathways, Photochem. Photobiol., 73, 6-13. 

Moore, D.E. (1987) Techniques for analysis of photodegradation pathways of medicinal compounds, Trends Anal. Chem., 6, 234—238. 

The majority of PCB photodegradation studies have been carried out in solution with organic or combined solvents preferred due to their low water solubility. Reductive dechlorination is the predominant photodegradation pathway in hydrocarbon solvents as well as alcohols. In aqueous media and alcohols photonucleophi 1 ic displacement reactions yielding phenols or alkoxy derivatives, respectively, also are detected. In the gas phase PCB are photochemically converted to hydroxylated derivatives (26). Photochemical reduction also is the major reaction pathway in thin films (7), solid phase (8) and on silica gel surfaces (9). 

Fig. 4.4. Initial photodegradation pathways for methyl parathion (after Dzyadevych etal., 2002).

Photolysis of Model Compound I. The bis-n-propyl ether of bisphenol-A (I) is a colorless liquid and easily miscible in 1,4-dioxane. Figure 3 shows the GC as drawn by the computer and Table I shows the various significant peaks that appeared in the GC spectrum and their corresponding m/e ratios for all three photolysis runs at various intervals of time. The proposed photodegradation pathways are shown in Scheme 1. 

The carbamate pesticides aldicarb, carbaryl and their degradation products, aU under research in photochemical degradation studies, were characterised by TSP-LC-MS and MS/MS. A tentative photodegradation pathway for the different pesticides in water was postulated [256]. 

In an interlaboratory study with nine participating laboratories a TSP-LC-MS method was evaluated in order to determine diuron and linuron. Depending on the compounds, the intralaboratory precisions range from 6.5-33 with a relative standard deviation (RSD) of 1% [254]. After linuron had been submitted to photochemical degradation, TSP-LC-MS and MS/MS was used to characterise the degradation products. A tentative photodegradation pathway for the different pesticides in water was postulated by Durand et al. [256]. 

The photodegradation of unstabilized bisphenol A polycarbonate films is described as an autoaccelerating process initiated by formation of products arising from a formal photo-Fries reaction. Oxidation also contributes as an additional photodegradation pathway. Photosensitive polymers... 

Figure 4. Proposed photodegradation pathway of 3,5,6-trichloro-2-pyridinol. (Reproduced with permission from reference 18, Copyright 1998 Society of Environmental Toxicology and Chemistry.)...

Bosca, R, Miranda, M. A., Vano, L., and Vargas, R, New photodegradation pathways for naproxen, a phototoxic non-steroidal anti-inflammatory drug, /. Photochem. Photobiol A Chem., 54, 131, 1990. 

Figure 32. Summary of main photodegradative pathways for polycarbonate.

The photodegradation pathway of the polymer is quite similar to the one abeady discussed before. The basic difference is in the initiation step. Scheme 7 represents a very simplified illustration of the initiation step of the photo-oxidation involving a polymer chain represented by R ... 

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