Characterization of the secondary electron-transfer pathway intermediates of photosystem II containing low-potential cytochrome b559

Cara A. Tracewell, Gary W. Brudvig

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14 Citations (Scopus)


β-carotene (Car) and chlorophyll (Chl) function as secondary electron donors in photosystem II (PS II) under conditions, such as low temperature, when electron donation from the O2-evolving complex is inhibited. In prior studies of the formation and decay of Car•+ and Chl •+ species at low temperatures, cytochrome b559 (Cyt b559) was chemically oxidized prior to freezing the sample. In this study, the photochemical formation of Car•+ and Chl •+ is characterized at low temperature in O2-evolving Synechocystis PS II treated with ascorbate to reduce most of the Cyt b 559. Not all of the Cyt b559 is reduced by ascorbate; the remainder of the PS II reaction centers, containing oxidized low-potential Cyt b559, give rise to Car•+ and Chl•+ species after illumination at low temperature that are characterized by near-IR spectroscopy. These data are compared to the measurements on ferricyanide-treated O2-evolving Synechocystis PS II in which the Car•+ and Chl•+ species are generated in PS II centers containing mostly high- and intermediate-potential Cyt b559. Spectral differences observed in the ascorbate-reduced PS II samples include decreased intensity of the Chl•+ and Car•+ absorbance peaks, shifts in the Car•+ absorbance maxima, and lack of formation of a 750 nm species that is assigned to a Car neutral radical. These results suggest that different spectral forms of Car are oxidized in PS II samples containing different redox forms of Cyt b559, which implies that different secondary electron donors are favored depending on the redox form of Cyt b559 in PS II.

Original languageEnglish
Pages (from-to)189-197
Number of pages9
JournalPhotosynthesis Research
Issue number1-3
Publication statusPublished - Oct 2008


  • Carotenoid radical cation
  • Chlorophyll radical cation
  • Cytochrome b
  • Photosystem II

ASJC Scopus subject areas

  • Biochemistry
  • Plant Science
  • Cell Biology

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