Spin trapping studies of photochemical reactions

Spin trapping is a very useful technique to study the photochemistry of chemical photosensitizers that undergo electron or hydrogen-transfer (Type I) reactions upon UV or visible irradiation. The Type I photochemistry of chlorpromazine [2-chloro-N-(3-dimethylaminopropy1)-phenothiazine], 3,3'4'5-tetrachlorosalicylanilide and 3,4',5tribromosalicylanilide has been investigated using 2-methyl-2-nitrosopropane (MNP) and 5,5-dimethyl-l-pyrroline-N-oxide (DMPO) as spin traps. All compounds underwent dehalogenation upon UV-irradiation and the resultant aryl radicals were trapped and identified. INTRODUCTION Light is known to interact with chemical agents present in exposed tissues, such as the skin or eyes, to produce photosensitization ( 1 ) . The chemical agent may be endogenous (protoporphyrin in erythropoietic protoporphyria), a drug (sulfonamides, declomycin, chlorpromazine), a topical agent (4-aminobenzoic acid and its esters in sunscreens; halogenated salicylanilides in soaps) or an environmental agent (polycyclic aromatic hydrocarbons in coal tar; amyl esters of 2-aminobenzoic acid in printer's ink) (2,3). The photosensitivity response may be one of two types, phototoxic or photo-allergic. The phototoxic reaction generally occurs during a subject's first exposure to sunlight, after the administration or topical application of a chemical or drug, and usually takes the form of an exaggerated erythemal response ("sunburn"). Phcltoallergic individuals may also exhibit an initial erythemal reaction. As this subsides, delayed abnormal responses may begin to appear including papular, eczematous and urticaria1 reactions. Such reactions to light may persist for months after avoidance of the photoallergen. The initial step in all forms of photosensitivity must be the absorption of light by the chemical or its metabolites. When the ground state ( S o ) of a molecule absorbs a photon of light it is converted to a very short-lived singlet excited state ('S*) which can rapidly convert to a long-lived excited triplet state ( 'S*). Most photosensitized reactions in biological systems are mediated by the triplet state of the sensitizer. Subsequent reactions can proceed by a number of different pathways, depending on the chemical nature of the sensitizer and the substrate as well as the reaction conditions. Two major types of processes may occur, termed Type I and Type 11. the triplet sensitizer molecule may abstract an electronor hydrogen atom) from the substrate molecule ( R ) to give a semi-reduced (free raqical) form of the ( S ' ) and a semi-oxidized (free radical) form of the substrate ( R ' ) , These processes may be summarized as follows: In Type I (free radical or redox) reactions The semi-reduced form of the sensitizer may also react directly with oxygen to give the superoxide radical (02--) s * + 02 -> s + 02.In Type I1 (energy transfer) reactions, electronic excitation energy is transferred from the triplet state sensitizer to ground state oxygen to give a highly electrophilic excited singlet state (lo2) of oxygen: 3s" + 3 0 2 -> so+ lo2 This form of oxygen reacts with many kinds of biomolecules much more rapidly than ground state oxygen.