Abstract
Abstract— Since 1972 the protective (quenching) effect of N3 and the enhancing effect of D2O have been used in combination to demonstrate 1O2 (1Δg) as the major reactive intermediate in the dye‐sensitized photooxygenation of biomolecules in in vitro systems. Extended application to in vivo systems has recently begun, producing some results which generally support the involvement of 1O2 pathways in the photosensitization processes. The use of other 1O2 quenchers and acceptors has also been increasing. In the application of these techniques as diagnostic tests for 1O2 participation in in vivo systems, careful examination of the experimental conditions should be made with respect to the quenching capability and accessibility of D2O in the specified cellular environment. Furthermore, the diffusive nature of 1O2, generation sites, generation efficiencies, the location and type of sensitizers, and the reactivity of 1O2 with presumed target structures in the cell should also be taken into account in the interpretation. Recent studies illustrating the importance of these factors are discussed. Finally, a tentative picture of the mode of in vivo photodynamic activity of common dye sensitizers is outlined.

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