Singlet oxygen quenching photooxidation reactions

Pyrene can sensitize the photooxidation of 1,3-diphenylisobenzofuran (72) in DTAC micellar solutions 61>. The reaction involves sensitization of singlet oxygen by pyrene which diffuses into another micelle and reacts with (72). Indole and tryptophan, which also react with singlet oxygen, quench the above reaction in ethanol solutions. However, in micellar systems they enhance the rate of reaction. Because of the high local concentrations of the quencher, the pyrene excited state is quenched by indole and tryptophan which leads to the photooxidation of (72) by a Type I process. 

MPT reacts somewhat more slowly with singlet oxygen than PTH. The combined rate constants of physical quenching (e.g. Equation 32) and chemical reaction (e.g. Equation 27) have been calculated from photooxidation experiments in bromobenzene/methanol (2/1) to be 1.2x10 M s-1 for MPT and 4.2x10 for PTH (26). Hovey has calculated... 

The singlet oxygen reacts directly with the double bond by addition, and shifts the double bond one carbon away. The singlet oxygen attack on linoleate produces four hydroperoxides as shown in Figure 2-20. Photooxidation has no induction period, but the reaction can be quenched by carotenoids... 

Telfer A, De Las Rivas J, and Barber J (1991) j5-Carotene within the isolated Photosystem II reaction centre photooxidation and irreversible bleaching of this chromophore by oxidized P680. Biochim Biophys Acta 1060 106-114 Telfer A, Dhami S, Bishop SM, Phillips D and Barber J (1994)/3-carotene quenches singlet oxygen formed by isolated Photosystem II reaction centers. Biochemistry 33 14469-14474... 

In addition to these two primary processes, there are also possibilities of sensitization of the photooxidation by a variety of mechanisms. The first of these is the formation of an excited state of a ketone or a polynuclear aromatic compound by absorption of light followed by quenching with oxygen to form singlet oxygen (102) which later adds to a double bond in the polymer to form an allylic hydroperoxide by the scheme shown below. This reaction has been demonstrated to occur quantitatively in... 

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