Marcus Theory for Electron Transfer in Photosystem II

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Marcus theory provides a framework for understanding the energy barriers to electron transfer by representing the electron donor and acceptor coordinates with parabolas. The difference between the troughs of each parabola represents the Gibbs free energy of the electron transfer reaction, and the difference between the trough and the intersection of the parabolas represents the energy of reorganizing the solution to accommodate the transfer of the electron. This, according to Marcus theory, contributes to the energy of activation of the reaction. This information can be applied to the Arrhenius equation to derive a rate constant for the reaction. Electron transfer in photosystem II, specifically from plastoquinone A to plastoquinone B, is modeled in this Demonstration, with electron transfer occurring under optimal conditions. When is mV, the first electron transfers from plastoquinone A to plastoquinone B. The second electron transfers when is mV.

Contributed by: Rebecca Brown and Aaron Walker (May 2018)
Open content licensed under CC BY-NC-SA



This Demonstration shows of a graph of two Marcus parabolas for a donor and an acceptor, with sliders to control their relative separation, and a graphic showing the movement of an electron from plastoquinone A to plastoquinone B under a certain set of conditions. One slider controls of the reaction, and the other slider controls the position of the second parabola. When is negative, an electron moves from plastoquinone A to plastoquinone B in photosystem II. This Demonstration utilizes Marcus theory to describe a specific electron transfer that occurs in photosystem II during photosynthesis. The output for values of the Gibbs free energy, reorganization energy and activation energy are shown above the parabolas.

Snapshots 1, 2, 3: images before electron transfer (when is )

Snapshots 4: image after electron transfer (when is )


[1] T. Engel and P. Reid, Physical Chemistry, San Francisco: Pearson Benjamin Cummings, 2006.

[2] S. Chaudhuri, S. Hedström, D. D. Méndez-Hernández, H. P. Hendrickson, K. A. Jung, J. Ho and V. S. Batista, "Electron Transfer Assisted by Vibronic Coupling from Multiple Modes," Journal of Chemical Theory and Computation, 13(12), 2017 pp. 6000–6009. doi:10.1021/acs.jctc.7b00513.

[3] Y. Kato, R. Nagao and T. Noguchi, "Redox Potential of the Terminal Quinone Electron Acceptor in Photosystem II Reveals the Mechanism of Electron Transfer Regulation," Proceedings of the National Academy of Sciences, 113(3), 2016 pp. 620–625. doi:10.1073/pnas.1520211113.

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