PARENT SESSION

Symposium S2A Type I reaction centres
Monday August 30th, 2004 2:40 PM-4:40 PM Room 511D
Chair: John Golbeck
Co-Chair: Kevin Redding

Secondary electron transfer in Photosystem I: What transient absorption can tell. Martin Byrdin^*,1, Rachel Cohen², Wendy Fairclough³, Feifei Gu⁴, John Golbeck², Peter Heathcote³, Kevin Redding⁴, Fabrice Rappaport¹, ¹ IBPC Paris, France² Pennsylvania State University, USA³ University of London, UK⁴ University of Alabama, USA

ABSTRACT- Whereas the structural asymmetry at the quinone level in type II reaction centers reflects functional differences, for type I reaction centers there is an ongoing debate on the extent of functionality of the two structurally symmetric quinones suggested to play the role of the secondary electron acceptor A1 in photosystem I. Optical transient absorption spectroscopy reveals two phases with of lifetimes about 20ns and 200ns and somewhat different spectra, the amplitude ratio of which varies somewhat between species and preparations. The assignment of the fast phase as artifactual could be ruled out by measurements on whole cells of Chlamydomonas reinhardtii. Mutations in the binding pockets of the two quinones selectively influenced the lifetimes of the two phases, which served as evidence to assign the two phases to the reoxidation of the two quinones. Activation energy measurements and calculations rationalized the different rates as being due to a difference in the redox potential of the two quinones. In an effort to understand the partition ratio between the two electron transfer (ET) branches, we constructed a series of mutations modifying the environments of upstream ET cofactors and have demonstrated a change in the relative weight of the phases upon modification of the A0 binding pocket (with spectra and lifetimes largely conserved). Due to the different form and position of the two phases′ spectra, the notion of their ′′ratio′′ is problematic, as it differs with wavelength. There is evidence that the slow phase contains contributions from more than one process: mutants with a retarded slow phase still displayed a distinct 150-ns decay component with amplitude at 445 nm, where both fast and slow phases are zero. Moreover, after normalization to equal maximal amplitudes (around 380nm), comparison of the slow and fast phases revealed that the slow spectrum had significantly more amplitude in the 340-nm region, where absorption changes due to FeS redox changes have been observed. We thus propose a contribution of downstream ET to the 150-ns spectrum.

KEY WORDS: transient absorption, mutations, photosystem I, electron transfer