Radical pair state in photosystem II: (photosynthesis/electron paramagnetic resonance/electron transfer/pheophytin/donors)

Journal Article · Wed Dec 01 00:00:00 EST 1982 · Proc. Natl. Acad. Sci. U.S.A.; (United States)

DOI:https://doi.org/10.1073/pnas.79.23.7283· OSTI ID:6307300

Rutherford, A W; Thurnauer, M C

A stable light-induced EPR signal is reported in photosystem II particles and in chloroplasts at 5K. Characteristic spectral features indicate that the signal arises from dipole-dipole interactions of a radical pair triplet state. From its dependence on potential, its relationship to the spin-polarized triplet state, and the redox state of the pheophytin acceptor (Ph) and because it is present in Tris-washed chloroplasts but not in untreated chloroplasts, we conclude that the signal is formed when the reaction center is in the state D/sup +/P/sub 680/Ph/sup -/ (P/sub 680/ is the primary chlorophyll donor and D/sup +/ is an oxidized donor to P/sub 680/). The low-temperature photochemical sequence is thought to occur as follows. (i) Donation from D to the P/sub 680//sup +/Ph/sup -/ state occurs at liquid helium temperature in low quantum yield; this reaction is reversible at temperatures above 5 K. (ii) In normal chloroplasts, donation occurs to the D/sup +/P/sub 680/Ph/sup -/ state, but this does not occur in Tris-washed chloroplasts or in the photosystem II particles at 77 K or lower. (iii) Illumination, at 200K, of photosystem particles or Tris-washed chloroplasts results in donation to the D/sup +/P/sub 680/Ph/sup -/ state from another donor. From experiments in the absence of redox mediators and the temperature dependence of the splitting of the signal, it is suggested that the state D/sup +/P/sub 680/Ph/sup -/ itself may be the origin of the radical pair triplet signal. The signal has been simulated by assuming the presence of at least two distinct radical pairs that differ slightly in the distance separating the radicals of the pairs. The distance between the radicals of the pair is calculated to be 6-7 A.

• Research Organization: Univ. of Illinois at Urbana-Champaign, IL (United States)
• DOE Contract Number: W-31-109-ENG-38
• OSTI ID: 6307300
• Journal Information: Proc. Natl. Acad. Sci. U.S.A.; (United States), Vol. 79
• Country of Publication: United States
• Language: English

Related Subjects

59 BASIC BIOLOGICAL SCIENCES
PHOTOSYNTHESIS
BIOPHYSICS
REACTION INTERMEDIATES
BIOCHEMISTRY
CHEMICAL REACTIONS
CHLOROPHYLL
CHLOROPLASTS
MOLECULAR BIOLOGY
PHOTOCHEMISTRY
CARBOXYLIC ACIDS
CELL CONSTITUENTS
CHEMISTRY
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
PHYTOCHROMES
PIGMENTS
PORPHYRINS
SYNTHESIS
550500* - Metabolism
550200 - Biochemistry