title: Trapped conformational states of semiquinone (D+*QB-*) formed by B-branch electron transfer at low temperature in Rhodobacter sphaeroides reaction centers.
abstract: The reaction center (RC) from Rhodobacter sphaeroides captures light energy by electron transfer between quinones QA and QB, involving a conformational gating step. In this work, conformational states of D+*QB-* were trapped (80 K) and studied using EPR spectroscopy in native and mutant RCs that lack QA in which QB was reduced by the bacteriopheophytin along the B-branch. In mutant RCs frozen in the dark, a light induced EPR signal due to D+*QB-* formed in 30% of the sample with low quantum yield (0.2%-20%) and decayed in 6 s. A small signal with similar characteristics was also observed in native RCs. In contrast, the EPR signal due to D+*QB-* in mutant RCs illuminated while freezing formed in approximately 95% of the sample did not decay (tau >107 s) at 80 K (also observed in the native RC). In all samples, the observed g-values were the same (g = 2.0026), indicating that all active QB-*'s were located in a proximal conformation coupled with the nonheme Fe2+. We propose that before electron transfer at 80 K, the majority (approximately 70%) of QB, structurally located in the distal site, was not stably reducible, whereas the minority (approximately 30%) of active configurations was in the proximal site. The large difference in the lifetimes of the unrelaxed and relaxed D+*QB-* states is attributed to the relaxation of protein residues and internal water molecules that stabilize D+*QB-*. These results demonstrate energetically significant conformational changes involved in stabilizing the D+*QB-* state. The unrelaxed and relaxed states can be considered to be the initial and final states along the reaction coordinate for conformationally gated electron transfer.
authors.affiliation: Department of Physics, University of California, San Diego, La Jolla, California 92093, USA. mpaddock@physics.ucsd.edu
authors.forename: M L
authors.initials: ML
authors.lastname: Paddock
authors.forename: M
authors.initials: M
authors.lastname: Flores
authors.forename: R
authors.initials: R
authors.lastname: Isaacson
authors.forename: C
authors.initials: C
authors.lastname: Chang
authors.forename: E C
authors.initials: EC
authors.lastname: Abresch
authors.forename: P
authors.initials: P
authors.lastname: Selvaduray
authors.forename: M Y
authors.initials: MY
authors.lastname: Okamura
mesh_terms: D003080:Cold Temperature; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D006860:Hydrogen Bonding; D008958:Models, Molecular; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D011487:Protein Conformation; D011809:Quinones / Q000737:chemistry*; D012242:Rhodobacter sphaeroides / Q000737:chemistry*
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D011809:Quinones