Type I and Type II Photosensitized Oxidation Reactions: Guidelines and Mechanistic Pathways

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 oxygen-dependent and involve unstable species such as the initial formation of radical cation or neutral radicals from the substrates and/or singlet oxygen (O-1(2) (1)Delta(g)) by energy transfer to molecular oxygen. In addition, superoxide anion radical (O-2(.-)) can be generated by a charge-transfer reaction involving O-2 or more likely indirectly as the result of O-2(.)- mediated oxidation of the radical anion of type I photosensitizers. In subsequent reactions, O-2(.-) may add and/or reduce a few highly oxidizing radicals that arise from the deprotonation of the radical cations of key biological targets. O-2(.-) can also undergo dismutation into H2O2, the precursor of the highly reactive hydroxyl radical ((OH)-O-.) 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.