Photochemistry reactive oxygen species

Keywords Photodynamic therapy Photosensitizer Photochemistry Reactive oxygen species Cancer Microorganism Infection... 

Other reactive species are or may be formed upon irradiation of CDOM, but their production rates were determined (or are considered) to be much lower than for the species represented in Scheme 1. Nevertheless, these species can be important for the transformation of organic contaminants that are not susceptible to reaction with the main reactive species. They comprise the hydroxyl radical, the carbonate radical and all the radicals derived from the DOM (carbon-centred, oxyl, peroxyl radicals). The following subsections describe in more detail the role that each reactive species plays in the transformation of aquatic organic contaminants. Hydrogen peroxide, although considered a reactive oxygen species (ROS) and an important player in aquatic photochemistry [7], is neglected because there is no evidence that... 

The four chapters within the Chemistry section discuss the effects of UVR on biogeochemical cycles of various elements (Chapter 5), the photochemistry of DOM (Chapter 6), the photo-activated toxicity of several natural and anthropogenic substances (Chapter 7) and the environmental implications of photoinduced formation of reactive oxygen species (Chapter 8). Once again, the central role of DOM emerges as a unifying theme. 

The high concentration of oxygen in the atmosphere plays a central role in the photochemistry and chemical reactivity of the atmosphere. Atmospheric oxygen also defines the oxidation reduction potential of surface waters saturated with oxygen. The presence of oxygen defines the speciation of many other aquatic species in surface waters. 

The most likely major photochemical products in marine aerosol particles are OH radicals, singlet oxygen, and hydrogen peroxide, all of which are reactive species that could potentially have some effects on atmospheric processes in the local environment. The purpose of this exercise is not to paint an accurate picture of the photochemistry occurring in marine aerosol particles, a complex and poorly understood environment. Rather it is to provide some possible pathways for future research, and to initiate discussion of the processes initiated by light in this heterogeneous environment. 

As we have noted often, photochemistry is a game of relative rates. The longer the intrinsic lifetime of a species, the more likely it is to react. Thus, the efficiency of singlet oxygen chemistry can be profoundly influenced by just the judicious choice of solvent. Note that the relatively short lifetime of singlet oxygen in water means that under biological conditions this potentially reactive species is fairly short lived. 

Sensitization was another word requiring definition and correct application. Thus, a stable molecule could be made photosensitive by the additirai of a small amount of another molecule, but this might involve different phenomena, as illustrated by Berthoud [62]. These ranged from the photochemical formation of a reactive compound to that of a catalyst. Atoms could be generated and transfer their energy to other species, as demonstrated for atoms of mercury or of oxygen [63]. Noyes pointed out to the reaction of molecules, e.g., NO2, at an irradiated mercury surface, and on the differences and similarity between photochemistry and photo-ionization [64]. 

It is known that photochemistry is the study of chemical changes produced by electronically excited species. Most of the photochemical transformations come from triplet excited state, due to its higher life-time. But oxygen molecule is a rare case, its ground state is a triplet while reactive state is electronically excited singlet state. The life time of electronically excited triplet state is relatively low, so significantly populated is singlet excited state. 

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