Sensitizers chlorophyll

In the somewhat special case of chlorophyll-sensitized oxygenation processes, Franck55 proposed a mechanism in which the electronically excited chlorophyll dissociates into a dehydrogenated chlorophyll radical and a hydrogen atom, the latter reacting with oxygen to a hydro-peroxy radical. 

Such ideas have initiated in vitro studies of chlorophyll-sensitized reactions in photosynthetic membranes from the laws of electrochemical kinetics. It has been possible to mimic the energy conversion and storage capacity of these membranes by experimental bilayer lipid membranes... 

It has been assumed so far that the sensitizer acts by an energy-transfer mechanism, but in some cases other modes of interaction may occur. It is possible that electron transfer takes place to give the radical anion or the radical cation of the alkene, which is the species that subsequently isomerizes. This is likely to be the case in the chlorophyll-sensitized isomerization of vitamin A acetate, which is used commercially to obtain the required all-trans isomer 12.8) from the mixture of Isomers resulting from the synthesis. Unlike triplet-sensitized reactions, electron-transfer isomerizations frequently lead to a predominance of the most thermodynamically stable isomer. 

Brandis A, Salomon Y, Scherz A. Chlorophyll sensitizers in photodynamic therapy. In Chlorophylls and Bacteriochlorophylls Biochemistry, Biophysics, Functions and Applications. Grimm B, Porra R, Rudiger W, Scheer H, eds. 2006. Springer, Dordrecht, The Netherlands, pp. 461-483. 

Chen, C.W., Lee, T.C., and Ho, C.T. 1997. Antioxidative effect and kinetics study of capsanthin on the chlorophyll-sensitized photoxidation of soybean oil and selected flavor compounds. In Spicies (S.J. Rich and C.T. Ho, eds), pp. 188-198. ACS Symposium Series 660, American Chemical Society, Washington, DC. 

Antioxidative Effect and Kinetics Study of Capsanthin on the Chlorophyll-Sensitized Photooxidation of Soybean Oil and Selected Flavor Compounds... 

Table II shows that as the number of conjugated double bond increased from 10 to 11, the API of soybean oil increased at the concentrations of 100 and 200 ppm carotenoids. It has been reported that as the number of conjugated double bonds of carotenoids increased, the peroxide values of chlorophyll-sensitized photooxidation of soybean oil decreased significantly 16,17). Therefore, the singlet oxygen quenching ability of carotenoids was dependent on the number of conjugated double bonds of the carotenoids as reported by Lee and Min 16), Jung and Min 17), and Hirayama el al. (23).

Soybean oil, role of capsanihin on chlorophyll-sensitized photoox id at ion, 188-197... 

Triincthyl-l,3-cydohcxadiene-l-carboxaidehyde. See Safranal 2,4,5-Trimethyloxazole, role of capsanthin on chlorophyll-sensitized photooxidation. 192,193/... 

Livingstone and Owens have shown that the triplet state is an essential intermediate in chlorophyll-sensitized photochemical reactions occurring in homogeneous solutions (17). It has not yet been satisfactorily proven whether or not the triple state participates in in viw chlorophyll reactions, but nevertheless there is considerable evidence that the sequence of biochemical reactions in photosynthesis is initiated by a chlorophyll molecule in its triplet state (5). 

Before proceeding it would be well to restate that our speculations on the fitness of chlorophyll relate principally to its triplet state reactions in solution. In determining its fitness, we should then ask what physical properties it possesses that make it such a good producer of long-lived triplets, which can act as intermediates in photochemical chlorophyll-sensitized reactions. From a cursory glance at the molecule [Fig. 10] four things are readily apparent ... 

Livingston, R., and K. E. Owens A Diffusion-controlled Step in Chlorophyll-sensitized Photochemical Auto-oxidations. J. Am. Chem. Soc. 78, 3301 (1956). 

Lee and Min conducted a study to assess the effects of lutein, zeaxanthin, lycopene, isozeaxan-thin, and astaxanthin on the chlorophyll sensitized photooxidation of soybean oil. They found that as the carotenoid concentration increased, the oil PVs decreased, whereas, the antioxidant effect increased with an increased number of double bonds. 

Lee, H.S. and Min, B.D. Effects, qnenching mechanisms, and kinetics of carotenoids in chlorophyll-sensitized photo-oxidation of soyahean. Journal of Agriculture and Food Chemistry, 38, 1630-1634. 1990. 

Interestingly, the here demonstrated influence of the nature of the absorption sites on the photochemical degradation of sensitizers has already been recognized in an early study, 35 years ago, on ZnO single crystals sensitized with chlorophyll [5]. From the degradation curve of chlorophyll sensitized electrodes, it was deduced that sensitizer molecules with widely different stabilities were active on the oxide surface. 

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