Effects of Quasi-Equilibrium States on the Kinetics of Electron Transfer and Radical Pair Stabilisation in Photosystem I

  • Stefano SantabarbaraEmail author
  • Robert Jennings
  • Giuseppe Zucchelli
Part of the Biophysics for the Life Sciences book series (BIOPHYS, volume 11)


Reaction centres are the sites of primary energy conversion in photosynthesis. The energy of the absorbed photon is trapped photochemically, with high quantum yields, in the form of electrochemical potential in a radical pair, which is then stabilised by a cascade of electron transfer (ET) reactions. These occur through a chain of redox-active cofactors that are coordinated by the reaction centre protein subunits. It is commonly considered that each of the electron transfer steps in the redox chain is associated with a significant driving force and, hence, a relatively large, negative, standard free energy difference (ΔG 0 ≤ − 100 meV). In this scenario, the rate of the reverse reaction which describes the repopulation of the precursor is several orders of magnitude smaller than that of forward electron transfer. Hence, the actual electron transfer rate between each given pair of donor and acceptor molecules is determined almost exclusively by the molecular rate constant of the forward reaction.

However, at least for a few steps in the redox chains of both photosystem I and photosystem II, the driving force is not large and often comparable to or lower than the thermal energy at physiological temperatures. In this case, the rate constant of the backward reaction, which is determined by the equilibrium constant, is of the same order of magnitude as that of the forward electron step and hence cannot be neglected. This, in turn, has a profound impact in determining the effective electron transfer rate that can be significantly slower than that of the molecular rate for forward ET.

In this chapter we discuss some aspects of electron transfer reactions in photosystem I, in particular the steps in which reversibility determines the effective ET rate.


Electron transfer Reversible reaction Photosystem I Photochemical reactions Phyllo(semi)quinone oxidation Kinetic modelling 



S.S. wishes to thank Drs. Saul Purton (University College London), Fabrice Rappaport (Institut de Biologie Physico-Chimique, Paris) and Kevin Redding (Arizona State University) for useful and constructive discussion on some of the issues discussed here in the course of collaborative studies. We also wish to thank Dr. Anna Paola Casazza (IBBA, CNR, Milan) for discussion and useful suggestion on editing this chapter.


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© Springer Science+Business Media New York 2014

Authors and Affiliations

  • Stefano Santabarbara
    • 1
    • 2
    Email author
  • Robert Jennings
    • 2
  • Giuseppe Zucchelli
    • 1
  1. 1.Consiglio Nazionale delle RicercheIstituto di Biofisica, Sede di MilanoMilanoItaly
  2. 2.Dipartimento di BiologiaUniversitá degli Studi di MilanoMilanoItaly

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