Aqueous extracts of the intercellular fluid from Sedum album L. leaves generated singlet oxygen chemiluminescence at 1270 nm when exposed to a nitrogen gas stream containing ozone at 21 +/- 2 ppm. The concentration of ascorbic acid in the intercellular fluid extracts was 310 +/- 40 microM. The intensity of the singlet oxygen chemiluminescence from the intercellular fluid extracts was comparable with the chemiluminescence from a control solution containing 300 microM ascorbic acid. The intensity of the singlet oxygen emission from intercellular fluid treated with ascorbate oxidase was 0.19 +/- 0.07 of the intensity of the singlet oxygen chemiluminescence from untreated samples of intercellular fluid extract. The simplest explanation for the effect of ascorbate oxidase is that ascorbic acid is the major ozone target generating singlet oxygen. Much weaker singlet oxygen chemiluminescence was detected at 1270 nm when intact S. album L. plant tips were exposed to a nitrogen gas stream containing ozone at 22 +/- 5 ppm. Various explanations for the relatively low intensity of the singlet oxygen chemiluminescence from intact S. album L. plant tips are discussed.

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Singlet-oxygen quenching constants were measured for 19 cyanine dyes in acetonitrile. The most efficient quenchers were 1-butyl-2-[2-[3-[(1-butyl-6-chlorobenz-[cd]indol-2(1H)- ylidene)ethylidene]-2-chloro-1-cyclohexen-1-yl]ethenyl]-6-chlorobenz[cd] indolium and 6-chloro-2-[2-[3-(6-chloro-1-ethylbenz[cd]indol-2(1H)-ylidene) ethylidene]-2-phenyl-1-cyclopenten-1-yl]ethenyl]-1-ethyl-benz[cd]indolium, having quenching constants with diffusion-controlled values of 2.0 +/- 0.1 x 10(10) and 1.5 +/- 0.1 x 10(10) M-1 s-1, respectively. There was a trend toward increased quenching constants for cyanine dyes with the absorption band maxima at longer wavelengths. However, the quenching constants correlated better with the oxidation potentials of the cyanine dyes, suggesting that quenching proceeds by charge transfer rather than energy transfer. The quenching constants for 1,1',3,3,3',3'-hexamethylindotricarbocyanine perchlorate and 1,1'-diethyl-4,4'-carbocyanine iodide were measured in several solvents as well as in aqueous solutions of detergent micelles. In different solvents, the quenching constants varied by as much as a factor of 50. The quenching constants were largest in solvents with the highest values on the pi* scale of Kamlet, Abboud, Abraham and Taft. This was consistent with quenching occurring by charge transfer. Within cells, cyanine dyes concentrate in membrane-bound organelles. The quenching constants were substantial within detergent micelles. To the extent that micelles are models for biological membranes, cyanine dyes may be effective biological singlet-oxygen quenchers.

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