Formation of a Semiquinone at the Q_BSite by A- or B-Branch Electron Transfer in the Reaction Center fromRhodobacter sphaeroides^†

In Rhodobacter sphaeroides reaction centers containing the mutation Ala M260 to Trp (AM260W), transmembrane electron transfer along the A-branch of cofactors is prevented by the loss of the Q(A) ubiquinone. Reaction centers that contain this AM260W mutation are proposed to photoaccumulate the P(+)Q(B)(-) radical pair following transmembrane electron transfer along the B-branch of cofactors (Wakeham, M. C., Goodwin, M. G., McKibbin, C., and Jones, M. R. (2003) Photoaccumulation of the P(+)Q(B)(-) radical pair state in purple bacterial reaction centers that lack the Q(A) ubiquinone. FEBS Lett. 540, 234-240). The yield of the P(+)Q(B)(-) state appears to depend upon which additional mutations are present. In the present paper, Fourier transform infrared (FTIR) difference spectroscopy was used to demonstrate that photooxidation of the reaction center's primary donor in Q(A)-deficient reaction centers results in formation of a semiquinone at the Q(B) site by B-branch electron transfer. Reduction of Q(B) by the B-branch pathway still occurs at 100 K, with a yield of approximately 10% relative to that at room temperature, in contrast to the Q(A)(-) to Q(B) reaction in the wild-type reaction center, which is not active at cryogenic temperatures. These FTIR results suggest that the conformational changes that "gate" the Q(A)(-) to Q(B) reaction do not necessarily have the same influence on QB reduction when the electron donor is the H-B anion, at least in a minority of reaction centers.