photooxidative

Photooxidative describes chemical and biological processes in which oxidation is initiated or accelerated by light. It commonly involves light-absorbing molecules that transfer energy or electrons to oxygen or other substrates, generating reactive oxygen species such as singlet oxygen, superoxide, and hydrogen peroxide. This can occur through direct photolysis or more often through photosensitized oxidation in which a sensitizer absorbs light and initiates the reaction.

There are two broad photosensitized pathways. In Type I reactions, a photoexcited sensitizer transfers electrons or hydrogen atoms to substrates, creating radicals that react with oxygen to form reactive species. In Type II reactions, energy transfer from the excited sensitizer converts ground-state oxygen into singlet oxygen, a highly reactive form that can attack lipids, proteins, and nucleic acids. The specific outcomes depend on the sensitizer, light wavelength, oxygen availability, and the surrounding environment. Common sensitizers include natural pigments such as chlorophyll and flavins, as well as industrial photocatalysts like titanium dioxide.

In biology and medicine, photooxidative stress occurs when ROS accumulation exceeds protective defenses, causing lipid peroxidation, protein modification, and DNA damage. This contributes to skin aging, photodamage, and various diseases, while plants experience photoinhibition under excess light. In materials science and environmental chemistry, photooxidation drives aging of polymers and pigments, degradation of coatings, and the atmospheric formation of oxidants from volatile organic compounds under sunlight.

Mitigation strategies include antioxidants, UV stabilizers, and controlling light exposure to limit ROS formation.