title: 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.
abstract: 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.
authors.affiliation: Department of Physics, University of California, San Diego La Jolla, CA 92093-0354.
authors.forename: M L
authors.initials: ML
authors.lastname: Paddock
authors.forename: R A
authors.initials: RA
authors.lastname: Isaacson
authors.forename: E C
authors.initials: EC
authors.lastname: Abresch
authors.forename: M Y
authors.initials: MY
authors.lastname: Okamura