Abstract

The primary reaction of bacterial photosynthesis — an electron transfer via several prosthetic groups in the so-called reaction center — proceeds extremely rapid on the time-scale of picoseconds. A series of recent experiments gave the following picture of this charge transfer process (data taken for reaction centers from Rhodobacter (Rb.) sphae-roides /1–4/): After excitation of the lowest excited singlet state of the primary electron donor (a “special pair” of bacteriochlorophyll molecules) the excited electronic state P* lives for approximately 3.5ps. The decay of P* is related with the electron transfer away from P. From several time-resolved experiments it was concluded that this first charge transfer carries the electron directly to the bacteriopheophytin H /1,2/. Only very recently we could demonstrate the existence of an additional short-lived intermediate prior to the reduction of the bacteriopheophytin H /4/. We interpreted this intermediate as P+B‒, i.e. the state where the electron from the special pair P has reduced the monomeric bacteriochlorophyll B to the anion radical B‒. In the final picosecond reaction the electron arrives (with a time constant of 200 ps) at the quinone Qa. It is the purpose of this paper to present additional experimental data supporting a sequential electron transfer via the accessory bacteriochlorophyll.

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