Femtosecond Spectroscopy of the Primary Electron Transfer in Photosynthetic Reaction Centers

Abstract

In the primary photosynthetic process of bacterial reaction centers (RC) light energy is stored by a rapid electron transfer (ET). The structural arrangement of the reaction centers with the six chromophores kept in two symmetry related branches A and B predetermines the ET path. The branches begin at two strongly interacting bacteriochlorophyll molecules forming the special pair P which acts as a primary electron donor. Subsequently each branch contains a monomeric bacteriochlorophyll (BChl) molecule BA and BB, a bacteriopheophytin (BPhe) (HA, HB) and a quinone (QA, QB) [1, 2]. Spectroscopy on reaction centers has revealed that the two pigment branches are spectroscopically non-equivalent and that electron transfer uses predominantly the A branch. It is generally accepted that the ET starts after excitation by light from the special pair P and that the primary reaction is finished by the ET from the bacteriopheophytin HA to the quinone QA which occurs with a time constant of 200 ps. However, there exist different opinions on the first part of the ET reaction: In the super-exchange ET model the electron is transferred directly from the special pair P to the bacteriopheophytin HA on the A branch. The monomeric bacteriochlorophyll is only used as a virtual electron carrier [3–7]. In the stepwise ET model the monomeric bacteriochlorophyll BA is a real electron carrier and the electron undergoes two reaction steps before it reaches the bacteriopheophytin. This model is supported by recent experimental results on RC from Rhodobacter (Rb.) sphaeroides which indicate that the electron transfer to BA occurs in approximately 3.5 ps while the second transfer step to the bacteriopheophytin HA should be faster taking less than one picosecond (0.9 ps) [8, 9].