Photooxidation of Chlorophyll b by Quinones Studied by Chemically-Induced Dynamic Nuclear Polarization

Abstract

The photooxidation of chlorophyll b by 1,4-benzoquinone and 2,6-dimethyl-1,4-benzoquinone was investigated by the technique of chemically-induced dynamic nuclear polarization. Polarization of the proton magnetic resonance lines of the quinone was detected. A mechanism for the photooxidation was postulated that invokes the reaction of the quinone with the excited singlet state of the chlorophyll to form a radical pair. This mechanism, together with a theoretical model and parameters taken from the literature, yields a theoretical proton magnetic resonance spectrum for the polarized quinone that agrees well with that observed experimentally.The photosensitized oxidation of chlorophyll by different oxidizing agents is being investigated in several laboratories (1-17), in order to provide guidelines for understanding the primary steps in photosynthesis. Among the various oxidants, the reaction of chlorophyll with various quinones is of special interest. It is well established that this reaction proceeds through free-radical intermediates, but the mechanistic details are still controversial (16). There are conflicting data on the nature of the excited states of chlorophyll with which the quinones react. The main sources of information about the nature of the precursors involved were obtained from flash photolysis, electron spin resonance (ESR), and fluorescence studies with 3-carotene as a competing quencher of the excited states. It was assumed that P-carotene reacts only with the triplet state of the excited chlorophyll. Addition of quinone reduced the concentration of the excited triplet of the chlorophyll, but not its lifetime (13,14), suggesting that the quinone reacts with a precursor of the chlorophyll triplet.