Modeling electron transfer in photosystem I

• Photosystem I with eight different quinones incorporated into the A1 binding site has been studied.
• Photosystem I was studied using time-resolved absorption spectroscopy at 298 and 77 K.
• Room and low temperature transient absorption data were modeled simultaneously.
• Redox potentials of eight quinones in A1 binding site were obtained.

Nanosecond to millisecond time-resolved absorption spectroscopy has been used to study electron transfer processes in photosystem I particles from Synechocystis sp. PCC 6803 with eight different quinones incorporated into the A1 binding site, at both 298 and 77 K. A detailed kinetic model was constructed and solved within the context of Marcus electron transfer theory, and it was found that all of the data could be well described only if the in situ midpoint potentials of the quinones fell in a tightly defined range. For photosystem I with phylloquinone incorporated into the A1 binding site all of the time-resolved optical data is best modeled when the in situ midpoint potential of phylloquinone on the A/B branch is − 635/− 690 mV, respectively. With the midpoint potential of the FX iron sulfur cluster set at − 680 mV, this indicates that forward electron transfer from A1− to FX is slightly endergonic/exergonic on the A/B branch, respectively. Additionally, for forward electron transfer from A1− to FX, on both the A and B branches the reorganization energy is close to 0.7 eV. Reorganization energies of 0.4 or 1.0 eV are not possible.For the eight different quinones incorporated, the same kinetic model was used, allowing us to establish in situ redox potentials for all of the incorporated quinones on both branches. A linear correlation was found between the in situ and in vitro midpoint potentials of the quinones on both branches.

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