Photochemistry of a bacterial photosynthetic reaction center missing the initial bacteriochlorophyll electron acceptor.

A novel chromophore composition of the bacterial photosynthetic reaction center (RC) has been discovered: RCs lacking the L-side monomeric bacteriochlorophyll chromophore result from mutation of the native isoleucine at M204 to glutamine in Rhodobacter capsulatus . This conclusion is obtained from 77 K UV-vis spectroscopy and pigment extractions of the I(M204)Q mutant and seven variants containing the I(M204)Q plus other mutations. The oxidation potential of the primary electron donor P (a dimer of bacteriochlorophylls) was measured for three of the mutants and found to be 50-65 mV lower than in wild-type RCs. Ultrafast transient absorption measurements reveal (minimally) two subpopulations of P* that have distinct lifetimes and photochemical outcomes for all mutants containing I(M204)Q. In one subpopulation P* decays solely by internal conversion to the ground state. In the other subpopulation P* decays by electron transfer to the normally inactive M-side bacteriopheophytin (H(M)) in competition with internal conversion to the ground state. When a Tyr residue is substituted for the native Phe at L181 near the M-side monomeric bacteriochlorophyll (B(M)), the rate of electron transfer to H(M) is increased about 4-fold.