Light induced EPR spectra of reaction centers from Rhodobacter sphaeroides at 80K: Evidence for reduction of Q(B) by B-branch electron transfer in native reaction centers.

Photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides capture solar energy by electron transfer from primary donor, D, to quinone acceptor, Q(B,) through the active A-branch of electron acceptors, but not the inactive B-branch. The light induced EPR spectrum from native RCs that had Fe(2+) replaced by Zn(2+) was investigated at cryogenic temperature (80K, 35 GHz). In addition to the light induced signal due to formation of D(+•)Q(A) (-•) observed previously, a small fraction (~5%) of the signal displayed very different characteristics: (1) The signal was absent in RCs in which the Q(B) was displaced by the inhibitor stigmatellin. (2) Its decay time (τ=6 s) was the same as observed for D(+•)Q(B) (-•) in mutant RCs lacking Q(A,) which is significantly slower than for D(+•)Q(A) (-•) (τ=30 ms). (3) Its EPR spectrum was identical to that of D(+•)Q(B) (-•). (4) The quantum efficiency for forming the major component of the signal was the same as that found for mutant RCs lacking Q(A) (Φ =0.2%) and was temperature independent. These results are explained by direct photochemical reduction of Q(B)via B-branch electron transfer in a small fraction of native RCs.