Modified Bacterial Reaction Centers: 3. Chemical Modified Chromophores at Sites B_A, B_B and H_A, H_B

Abstract

The recently demonstrated exchange of the “monomeric” bacteriochlorophylls (BChl) B_A and B_B, has opened a new route to investigate structure-function relationships in photosynthetic reaction centers (RC) of Rhodobacter sphaeroides R26 [1–4]. The function of these pigments is still controversial [5–22]. However, the finding of a new kinetic intermediate at ambient temperature between the initial excited state P* of the primary donor, and the charge separated state P^*+H_A ^*− in which the photoejected electron resides on the bacteriopheophytin (BPhe) of the A-branch, has provided strong evidence that one of them (B_A) is an intermediate acceptor in the primary charge separation process [15, 20]. A similar role has been proposed for B_M in triplet energy transfer from P to the carotenoid in wild-type RC [18]. To evaluate the details of electron transfer and pigment interaction it seemed desirable to introduce pigments with altered spectral and/or redox properties. As first pigments showing considerably blue-shifted absorption spectra, we have incorporated bacteriochlorophylls containing 3-vinyl- or 3-hydroxyethyl-substituents instead of the 3-acetyl group [4]. Here, we wish to summarize results on these and some other pigment modifications, including in particular the exchange of bacteriopheophytins as well. An overview is given on the pigment modifications tested and some steady state spectra are shown.