Photochemistry und Photobiology Vol. 54, No. 5, p. 659, 1991 Printed in Great Britain

0031-8655191 $03.00+0.00 Pcrgamon Press plc

GUEST EDITORIAL DEFINITION OF TYPE I AND TYPE I1 PHOTOSENSITIZED OXIDATION Photosensitized oxidations are the basis for photodynamic action. There has come to be some confusion about the definition of Type I and Type I1 photosensitized oxidation reactions. It is generally true that experimentally-based definitions are superior to mechanistic ones because mechanisms change but experimental observations are subject to vertification. For this reason, I suggest that the definition given below should prove the most useful. This editorial briefly reviews the history and mechanism of photosensitized oxidations in order to provide the background for the definition. The first step of these reactions is absorption of light by a sensitizer (Sens) to produce an excited state (Sens*). In the presence of oxygen, two competing reactions of the excited sensitizer can occur, as originally noted by Schenck and Gollnick (Gollnick, 1968; Schenck, 1963; Schenck and Koch, 1960). These processes are called Type I and Type I1 reactions.

direction, but most commonly, the excited sensitizer acts as an oxidant. The Type I1 leads mainly to singlet molecular oxygen by energy transfer. Electron transfer from sensitizer to oxygen can also occur in some cases, giving oxidized sensitizer and superoxide ion (Lee and Rodgers, 1987). This is also a Type I1 reaction under this definition, although it has occasionally been described as a Type I process. Thus the definition of reaction type rests on the primary interaction of the photosensitizer, which is relatively easily determined experimentally, rather than on whether the reaction proceeds through singlet oxygen or a radical pathway, which may not always be trivial to establish. S. FOOTE CHRISTOPHER Department of Chemistry and Biochemistry University of California, Los Angeles CA 90024-1569, U S A REFERENCES

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Because the mechanisms were incompletely understood at first, the original definitions of the two processes were subsequently refined (Foote, 1976, 1987). Sens* (a singlet or, more commonly, a triplet) can either react with the substrate or solvent (Type I) or with oxygen (Type 11); this is the experimental definition of the two processes (Foote, 1990). The Type I reaction results in either hydrogen atom or electron transfer, yielding radicals or radical ions. Transfer can actually occur in either

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Foote, C. S. (1976) Photosensitized oxidation and singlet oxygen: consequences in biological systems. In Free Radicals in Biology (Edited by W. A. Pryor), Vol. 2, pp. 85-133. Academic Press, New York. Foote, C. S . (1987) Type I and Type I1 mechanisms in photodynamic action. In Light-activated Pesticides (Edited by J . R. Heitz, K. R. Downum), Vol. ACS Symposium Ser. 339, pp. 22-38. American Chemical Society, Washington, DC. Foote, C. S . (1990) Chemical mechanisms of photodynamic action. In Future Directions and Applications in Photodynamic Therapy (Edited by C. J. Gomer), Vol. SPIE Institutes for Advanced Optical Technologies, Vol. IS6. SPIE Optical Engineering Press, Bellingham, WA. Gollnick, K. (1968) Type I1 photooxygenation reactions in solution. A d v . Photochem. 6, 1-122. Lee, P. C. C. and M. A. J. Rodgers (1987) Laser flash photokinetic studies of Rose Bengal sensitized photodynamic interactions of nucleotides and DNA. Phorochem. Photobiol. 45, 79-86, Schenck, G. 0. (1963) Photosensitization. Znd. Eng. Chem. 55, 4C-43. Schenck, G. 0. and E. Koch (1960) Zwischenreaktionen bei photosensibilisierten Prozessen in Losungen. Z. Elektrochem. 64, 170-177.

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Definition of type I and type II photosensitized oxidation.

Photochemistry und Photobiology Vol. 54, No. 5, p. 659, 1991 Printed in Great Britain 0031-8655191 $03.00+0.00 Pcrgamon Press plc GUEST EDITORIAL DE...
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