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

A new model for the energy transfer and early electron transfer processes in Photosystem I. Alfred Holzwarth^*,1, Marc Müller¹, Jens Niklas¹, Wolfgang Lubitz¹, ¹ Max-Planck-Institut für Bioanorganische Chemie, Mülheim a.d. Ruhr, Germany

ABSTRACT- The energy transfer and early electron transfer processes in photosystem I are still only poorly understood. We will present evidence that the interpretation of the energy trapping and early electron transfer processes in terms of both kinetics and mechanisms has to be revised substantially as compared to current interpretations in the literature. We resolved for the first time i) the transient difference spectrum for the excited reaction center state, and ii) the formation and decay of the primary radical pair and its intermediate spectrum directly from measurements on open PSI reaction centers. It is shown that the dominant energy trapping lifetime due to charge separation is only 6-9 ps, i.e., by a factor of about 3 shorter than assumed so far for intact PS I. Likewise, energy equilibration between the RC and the core antenna of PS I also occurs in a much shorter time than assumed so far. Comparison of the transient absorption data with the fluorescence kinetics lets us propose a reversible first charge separation step which has been excluded so far for open photosystem I reaction centers. These data and the new kinetic model have important implications for the understanding of the antenna functions, the trapping of energy, the interpretation of the so-called "red antenna" states, and the mechanism of the early electron transfer steps in photosystem I.

KEY WORDS: lifetime, ultrafast, fluorescence, picosecond