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Multiple Charge-Separation Pathways in Photosystem II: Modeling of Transient Absorption Kinetics

Authors

  • Prof. Vladimir I. Novoderezhkin,

    1. Institute of Physico-Chemical Biology, Moscow State University, Leninsky Gory, 119992, Moscow (Russia)
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  • Dr. Elisabet Romero,

    1. Department of Physics and Astronomy, Faculty of Exact Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081 HV, Amsterdam (The Netherlands), Fax: (+31) 20-598-7999
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  • Prof. Jan P. Dekker,

    1. Department of Physics and Astronomy, Faculty of Exact Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081 HV, Amsterdam (The Netherlands), Fax: (+31) 20-598-7999
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  • Prof. Dr. Rienk van Grondelle

    Corresponding author
    1. Department of Physics and Astronomy, Faculty of Exact Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081 HV, Amsterdam (The Netherlands), Fax: (+31) 20-598-7999
    • Department of Physics and Astronomy, Faculty of Exact Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081 HV, Amsterdam (The Netherlands), Fax: (+31) 20-598-7999
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Abstract

We explain the transient absorption kinetics (E. Romero, I. H. M. van Stokkum, V. I. Novoderezhkin, J. P. Dekker, R. van Grondelle, Biochemistry2010, 49, 4300) measured for isolated reaction centers of photosystem II at 77 K upon excitation of the primary donor band (680 nm). The excited-state dynamics is modeled on the basis of the exciton states of 6 cofactors coupled to 4 charge-transfer (CT) states. One CT state (corresponding to charge separation within the special pair) is supposed to be strongly coupled with the excited states, whereas the other radical pairs are supposed to be localized. Relaxation within the strongly coupled manifold and transfer to localized CT’s are described by the modified Redfield and generalized Förster theories, respectively. A simultaneous and quantitative fit of the 680, 545, and 460 nm kinetics (corresponding to respectively the Qy transitions of the red-most cofactors, Qx transition of pheophytin, and pheophytin anion absorption) enables us to define the pathways and time scales of primary electron transfer. A consistent modeling of the data is only possible with a Scheme where charge separation occurs from both the accessory chlorophyll and from the special pair, giving rise to fast and slow components of the pheophytin anion formation, respectively.

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