Photosensitization Reactions of Biomolecules: Definition, Targets and Mechanisms

다국어 초록 (Multilingual Abstract)

Photosensitization reactions have been demonstrated to be largely responsible for the deleterious biological effects of UV and visible radiation, as well as for the curative actions of photomedicine. A large number of endogenous and exogenous photosensitizers, biological targets and mechanisms have been reported in the past few decades. Evolving from the original definitions of the type I and type II photosensitized oxidations, we now provide physicochemical frameworks, classifications and key examples of these mechanisms in order to organize, interpret and understand the vast information available in the literature and the new reports, which are in vigorous growth. This review surveys in an extended manner all identified photosensitization mechanisms of the major biomolecule groups such as nucleic acids, proteins, lipids bridging the gap with the subsequent biological processes. Also described are the effects of photosensitization in cells in which UVA and UVB irradiation triggers enzyme activation with the subsequent delayed generation of superoxide anion radical and nitric oxide. Definitions of photosensitized reactions are identified in biomolecules with key insights into cells and tissues.
What began with basic definitions of type I and type II mechanisms of photosensitized oxidations has served well, but is in need of frameworks defining physical‐chemical boundaries and biochemical consequences. We now provide sophisticated definitions of the type I process, with initial electron transfer or hydrogen abstraction and O2 participation, and expand on the type II mechanism involving energy transfer from the sensitizer excited state to O2 yielding 1O2. We also considered the oxygen‐independent reactions, including the formation of photoproducts by photocycloaddition and triplet–triplet energy transfer from the excited sensitizer to substrate. Definitions are provided with relevance to cellular processes.