Site Energies of Active and Inactive Pheophytins in the Reaction Center of Photosystem II from Chlamydomonas Reinhardtii

K. Acharya, B. Neupane, V. Zazubovich, R. T. Sayre, R. Picorel, M. Seibert, R. Jankowiak

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It is widely accepted that the primary electron acceptor in various Photosystem II (PSII) reaction center (RC) preparations is pheophytin a (Pheo a) within the D1 protein (PheoD1), while PheoD2 (within the D2 protein) is photochemically inactive. The Pheo site energies, however, have remained elusive, due to inherent spectral congestion. While most researchers over the past two decades placed the Qy-states of PheoD1 and PheoD2 bands near 678-684 and 668-672 nm, respectively, recent modeling [ Raszewski et al.Biophys. J. 2005, 88, 986-998; Cox et al.J. Phys. Chem. B 2009, 113, 12364-12374 ] of the electronic structure of the PSII RC reversed the assignment of the active and inactive Pheos, suggesting that the mean site energy of PheoD1 is near 672 nm, whereas PheoD2 (∼677.5 nm) and ChlD1 (∼680 nm) have the lowest energies (i.e., the PheoD2-dominated exciton is the lowest excited state). In contrast, chemical pigment exchange experiments on isolated RCs suggested that both pheophytins have their Qy absorption maxima at 676-680 nm [ Germano et al.Biochemistry 2001, 40, 11472-11482; Germano et al.Biophys. J. 2004, 86, 1664-1672 ]. To provide more insight into the site energies of both PheoD1 and PheoD2 (including the corresponding Q x transitions, which are often claimed to be degenerate at 543 nm) and to attest that the above two assignments are most likely incorrect, we studied a large number of isolated RC preparations from spinach and wild-type Chlamydomonas reinhardtii (at different levels of intactness) as well as the Chlamydomonas reinhardtii mutant (D2-L209H), in which the active branch PheoD1 is genetically replaced with chlorophyll a (Chl a). We show that the Qx-/Qy-region site energies of PheoD1 and PheoD2 are ∼545/680 nm and ∼541.5/670 nm, respectively, in good agreement with our previous assignment [ Jankowiak et al.J. Phys. Chem. B 2002, 106, 8803-8814 ]. The latter values should be used to model excitonic structure and excitation energy transfer dynamics of the PSII RCs.

Original languageAmerican English
Pages (from-to)3890-3899
Number of pages10
JournalJournal of Physical Chemistry B
Issue number12
StatePublished - 29 Mar 2012

NREL Publication Number

  • NREL/JA-2A00-55118


  • electron acceptor
  • inactive pheophytins


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