M-side electron transfer in reaction center mutants with a lysine near the nonphotoactive B bacteriochlorophyll

We report the primary charge separation events in a series of Rhodobacter capsulatus reaction centers (RCs) that have been genetically modified to contain a lysine near the bacteriochlorophyll molecule, BChl(M), on the nonphotoactive M-side of the RC. Using wild type and previously constructed mutants as templates, we substituted Lys for the native Ser residue at position 178 on the L polypeptide to make the S(L178)K single mutant, the S(L178)K/G(M201)D and S(L178)K/L(M212)H double mutants, and the S(L178)K/G(M201)D/L(M212)H triple mutant. In the triple mutant, the decay of the photoexcited primary electron donor (P*) occurs with a time constant of 15 ps and is accompanied by 15% return to the ground state, 62% electron transfer to the L-side bacteriopheophytin, BPhL, and 23% electron transfer to the M-side analogue, BPhM. The data supporting electron transfer to the M-side include bleaching of the Qx band of BPhM at 528 nm and a spectrally and kinetically resolved anion band with a maximum at 640 nm assigned to BPhM-. The decay of these features and concomitant similar to 20% decay of bleaching of the 850 nm band of P give a P+BPhM- lifetime on the order of 1-2 ns that reflects deactivation to give the ground state. These data and additional findings are compared to those from parallel experiments on the G(M201)D/L(M212)H double mutant, in which 15% electron transfer to BPhM has been reported previously and is reproduced here. We also compare the above results with the primary electron-transfer processes in S(L178)K, S(L178)K/G(M201)D, and S(L178)K /L(M212)H RCs and with those for the L(M212)H and G(M201)D single mutants and wild-type RCs, The comparison of extensive results that track the primary events in these eight RCs helps to elucidate key factors underlying the directionality and high yield of charge separation in the bacterial photosynthetic RC.