PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

Exploring the rate of forward electron transfer in PSI with a series of anthraquinones in the A₁ binding site. Sarah Brown^*,1, Julia Pushkar², Irina Karygina², Dietmar Stehlik², Art van der Est¹, ¹ Brock University, St. Catharines, Ontario, Canada² Institut für Experimentalphysik, Berlin, Germany

ABSTRACT- The role of the midpoint potential of the quinone acceptor in determining the rate of forward electron transfer to the iron-sulfur clusters in Photosystem I (PS I) was investigated using transient EPR spectroscopy. The native phylloquinone was replaced with 9, 10-anthraquinone (AQ) and a series of disubstituted AQ's. Transient EPR spectroscopy was used to estimate the rate of forward electron transfer, the orientation and degree of incorporation of the AQ's into the A₁ site. All of the AQ's were found to have a high degree of incorporation into the A₁ site in approximately the same orientation as phylloquinone in the native system. Consistent with the more negative mid-point potential of AQ, the rate of electron transfer to F_X estimated from the EPR data was found to be almost an order of magnitude faster with a significantly lower activation energy than the corresponding rate in native PS I. An analysis of the spin polarization patterns also suggests that electron transfer from A₀ to the AQ's may be slower than in the native system. Preliminary results using disubstituted anthraquinones suggests that the introduction of electron withdrawing groups slows the forward electron transfer to F_X and increases the rate from A₀ to the quinone. Together all of the results suggest that the rate of electron transfer through the quinone is determined largely by its redox potential.

KEY WORDS: Secondary Electron Acceptor, Electron Transfer, Photosystem I, Anthraquinone