PHOTOCHEMICAL MECHANISMS IN PHOTOSENSITIZATION

Abstract
Action of light in chemical transformations can be classified in two main groups : - Direct photochemical reactions : molecules which absorb lightare that which are transformed. - Photosensitized reactions : the light is absorbed by a different molecule to that we wish to transform. In the second case the sensitizer which absorbs a photon may react in two ways : . Electron transfer. . Energy transfer. So for photo-oxidations it is possible to speak about two types : - Type I photo-oxidations : there is either hydrogen atom abstraction, or electron transfer. - Type II photo-oxidations : energy transfer occurs between excited sensitizer and oxygen giving excited singlet oxygens, it is a very reactive species. Rose Bengal, Acridin Orange, Methylene Blue, Chlorophyll and Hematoporphyrin are the most current sensitizers. It is Hematoporphyrin that is employed in phototherapy, in consideration of the two properties : . it is a good sensitizer without important toxicity ; . its life time in carcinogenic cells is longer than in normal cells. The action of light on Hematoporphyrin in ill cells gives singlet oxygen (strong oxidant) which destroys them. An example of photochemical studies usefulness is given.