•-. electrons from photosystem II and, similarly to respiration, leads to the formation of water. Abiotic, photochemical processes, by contrast, proceeds through several, chemically distinct paths of O 2 elimination (Figure ). species also contribute to further reductive oxygen elimination; on the one hand through oxygenation reactions such as the formation of endo-and hydroperoxides with olefinic and aromatic compounds. On the other hand, 1 O 2 enables the one-electron oxidation of organic compounds and formation of O