Removal of the H subunit results in enhanced exposure of the semiquinone sites in the LM dimer from Rhodobacter sphaeroides to oxidation by ferricyanide and by O2

Bacterial reaction centers (RC) from Rhodobacter sphaeroides have been widely used to functionalize electrodes and to generate photocurrent. However, in most studies, direct electron transfer from the semiquinone to the electrode was not observed because the H subunit of the RC shields the semiquinone. It is demonstrated in the current work that removal of the H subunit effectively exposes the semiquinone sites in the LM dimer. This is demonstrated by measuring the second-order rate constant for the reaction between ferricyanide and the anionic semiquinone Q (A) (-) formed by an actinic flash. The rate constant increases 1000-fold for Q (A) (-) oxidation by ferricyanide in the LM dimer compared to the intact RC. The second-order rate constant approaches the diffusion limit of 6 x 10(9) M-1 center dot s(-1) at low pH, but it decreases steadily when the pH is above 6.5. This pH dependence suggests that the protonation state of the LM dimer plays an important role in controlling the electron transfer kinetics. It is also shown that the addition of exogenous ubiquinone to replenish the Q(B) site, which is mostly empty in the LM dimer, leads to oxidation of Q (A) (-) by O-2 following an actinic flash. It is concluded that removal of the H subunit results in exposure of the semiquinone sites of the LM dimer to externally added oxidants and may provide a strategy for enhancing direct electron transfer from the RC to an electrode.