Department of Chemistry, Massachusetts Institute of Technology, Cambridge 02139.
Electron paramagnetic resonance (EPR) has been used to investigate the cation and triplet
states of Rhodobacter capsulatus reaction centers (RCs) containing amino acid
substitutions affecting the primary donor, monomeric bacteriochlorophylls (Bchls), and the
photoactive bacteriopheophytin (Bphe). The broadened line width of the cation radical in
HisM200----Leu and HisM200----Phe reaction centers, whose primary
donor consists of a Bchl-Bphe heterodimer, indicates a highly asymmetric distribution of
the unpaired electron over the heterodimer. A T0 polarized triplet state with
reduced yield is observed in heterodimer-containing RCs. The zero field splitting
parameters indicate that this triplet essentially resides on the Bchl half of the
heterodimer. The cation and triplet states of reaction centers containing HisM200----Gln,
HisL173----Gln, GluL104----Gln, or GluL104----Leu
substitutions are similar to those observed in wild type. Oligonucleotide-mediated
mutagenesis has been used to change the histidine residues that are positioned near the
central Mg2+ ions of the reaction center monomeric bacteriochlorophylls.
Reaction centers containing serine substitutions at M180 and L153 or a threonine
substitution at L153 have unaltered pigment compositions and are photochemically active.
The cation and triplet states of HisL153----Leu reaction centers are similar to
those observed in wild type. Triplet energy transfer to carotenoid is not observed at 100
K in HisM180----Arg chromatophores. These results have important implications
for the structural requirements of tetrapyrrole binding and for our understanding of the
mechanisms of primary electron transfer in the reaction center.