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

Electron transfer along both branches of excitonically coupled chlorophylls in PSI. Andrew Webber^*,1, V Ramesh¹, Krzysztof Gibasiewicz³, Su Lin¹, Neal Woodbury¹, ¹ School of Life Sciences, Tempe, AZ, USA³ Department of Physics, Poland

ABSTRACT- Six symmetrically arranged chlorophylls form the initial PSI electron transfer cofactors: P700 (eC-A1 and eC-B1) , A (eC-A2 and eC-B2), and A0 (eC-A3 and eC-B3). We have designed Chlamydomonas mutants that change the axial ligand to each chlorophyll to 1) determine which chorophylls are excitonically coupled and how this may effect excitation energy transfer and 2) determine which electron transfer branch is active. Results from ultrafast absorption spectroscopy support theoretical calculations that the 6 electron transfer chlorophylls interact to form a set of three excitonically coupled dimers: eC-A1/eC-B1, eC-A2/eC-A3, and eC-B2/eC-B3. Excitonic coupling leads to wide transient absorption changes: positive between 635 and 665 nm and four negative absorption bands at 667, 675, 683 and 695 nm. Analysis of mutants show that the bands at 675 and 695 are attributed to the eC-A2/eC-A3 and eC-A3/eC-B3 chlorophylls. The exitonic coupling and wide absorption spectra contributes to the highly efficient trapping of excitation energy in PSI Mutants of the axial ligand to either eC-A3 or eC-B3 show altered spectra 200 ps after excitation, with increased absorption at 680 nm. This represents accumulation of reduced A0. Decay associated spectra generated from a global analysis on a 4000 ps time scale reveals a new 1700 ps component with an absorption maximum of 680 nm identical to reduced A0. This shows that both eC-A3 and eC-B3 are active in electron transfer and that substituting Met for His alters the potential of A0 and slows electron transfer from A0 to A1 from approximately 20 ps in wild-type to approximately 1700 ps in the mutants.

KEY WORDS: Excitation energy transfer, Chlamydomonas, Photosystem I, Electron transfer