Type I and Type II Photosensitized Oxidation Reactions

Abstract

Here, 10 guidelines are presented for a standardized definition of type I and type II photosensitized oxidation reactions. Because of varied notions of reactions mediated by photosensitizers, a checklist of recommendations is provided for their definitions. Type I and type II photoreactions are oxygendependent and involve unstable species such as the initial formation of radical cation or neutral radicals from the substrates and/or singlet oxygen (¹O₂ ¹Δ_g) by energy transfer to molecular oxygen. In addition, superoxide anion radical (O2←) can be generated by a charge-transfer reaction involving O2 or more likely indirectly as the result of O2-mediated oxidation of the radical anion of type I photosensitizers. In subsequent reactions, O2← may add and/or reduce a few highly oxidizing radicals that arise from the deprotonation of the radical cations of key biological targets. O2← can also undergo dismutation into H₂O₂, the precursor of the highly reactive hydroxyl radical (OH) that may induce delayed oxidation reactions in cells. In the second part, several examples of type I and type II photosensitized oxidation reactions are provided to illustrate the complexity and the diversity of the degradation pathways of mostly relevant biomolecules upon one-electron oxidation and singlet oxygen reactions.