Abstract
In this review, I outline the indirect evidence for the formation of singlet oxygen (1O2) obtained from experiments with the isolated PSII reaction center complex. I also review the methods we used to measure singlet oxygen directly, including luminescence at 1,270 nm, both steady state and time resolved. Other methods we used were histidine-catalyzed molecular oxygen uptake (enabling 1O2 yield measurements), and dye bleaching and difference absorption spectroscopy to identify where quenchers of 1O2 can access this toxic species. We also demonstrated the protective behavior of carotenoids bound within Chl–protein complexes which bring about a substantial amount of 1O2 quenching within the reaction center complex. Finally, I describe how these techniques have been used and expanded in research on photoinhibition and on the role of 1O2 as a signaling molecule in instigating cellular responses to various stress factors. I also discuss the current views on the role of 1O2 as a signaling molecule and the distance it might be able to travel within cells.
Highlights
Singlet oxygen (1O2) is an electronically excited state of molecular oxygen which is extremely reactive (Ogilby 2010)
The fact that when two Cars were present [i.e. as seen in the native structure, Loll et al (2005) and see Fig. 2], they could not quench all of the singlet oxygen (1O2) formed is due to the fact that the 1O2 formed at primary electron donor in PSII (P680) can diffuse in all directions within the complex and, because of the distance of the Cars from the source of the 1O2, a certain amount of damage will be done by 1O2 not scavenged by them (Telfer 2002)
3P680 is inevitably formed within the PSII reaction center (RC) when operating in oxygenic organisms, which are continuously evolving molecular oxygen, both at low light intensities and under high light, i.e. photoinhibitory, conditions
Summary
Singlet oxygen (1O2) is an electronically excited state of molecular oxygen which is extremely reactive (Ogilby 2010). It attacks and oxidizes proteins, lipids and nucleic acids, and it is an important reactive oxygen species (ROS) in biological systems It is less stable than triplet oxygen (3O2), and may be formed in a variety of ways; a common way is by electronic energy transfer from the triplet state of a photosensitized pigment or dye molecule. One of the earliest experiments demonstrating the transfer of energy from the triplet excited state of Chl to Car was the so-called ‘valve reaction’ of Witt (1971) in which an increase in the size of an absorbance change (due to 3Car formation) was seen only once photosynthetic electron transfer was light saturated. Plant Cell Physiol. 55(7): 1216–1223 (2014) doi:10.1093/pcp/pcu040 ! The Author 2014. 1217
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