Abstract

The primary electron transfer (ET) is the key process in photosynthesis. It occurs in a membrane bound pigment protein complex called reaction center (RC). Fortunately x-ray structure analysis has revealed the arrangement of aminoacids and chromophores of the RC giving the basic information for an understanding of the primary ET on a molecular basis: The important actors in the primary ET are four bacteriochlorphyll (BChl) and two bacteriopheophytin (BPh) molecules. After optical excitation of the primary donor P (which is a "special pair" of BChl molecules) the electron travels to or passes by the accessory BChl B and arrives within less than 4 ps at the BPh H before it finally (on the picosecond time scale) reaches the quinone Q after 200 ps. While the role of quinone Q and BPh H as real electron carriers is well established there is no general agreement about the function of the BChl B. Recently femtosecond absorption data have revealed a weak additional 0.9 ps kinetic component compatible with the assignment that the radical pair state P⁺B^– is a real intermediate in the reaction (see model A in Fig. 1). However, alternative reaction models (B and C) - where the electron does not reside on the BChl B - could not be excluded by experimental data on native RC at room temperature.

© 1992 The Author(s)