The Surface Metal Site in Blc. viridis Photosynthetic Bacterial Reaction Centers: Cu2+ as a Probe of Structure, Location, and Flexibility

Metal ion binding to a surface site on photosynthetic reaction centers (RCs) modulates light-induced electron and proton transfer events in the RC. Whereas many studies have elucidated aspects of metal ion modulation events in Rhodobacter sphaeroides RCs, much less is understood about the surface site in Blastochloris viridis (Blc. viridis) RCs. Interestingly, electron paramagnetic resonance studies revealed two spectroscopically distinct Cu²⁺ surface site environments in Blc. viridis RCs. Herein, Cu²⁺ has been used to spectroscopically probe the structure of these Cu²⁺ site(s) in response to freezing conditions, temperature, and charge separation. One Cu²⁺ environment in Blc. viridis RCs, termed Cu_A, exhibits temperature-dependent conformational flexibility. Different conformation states of the Cu_A²⁺ site are trapped when the RC is frozen in the dark either by fast-freeze or slow-freeze procedure. The second Cu²⁺ environment, termed Cu_B, is structurally invariant to different freezing conditions and shows resolved hyperfine coupling to three nitrogen atoms. Cu²⁺ is most likely binding at the same location on the RC, but in different coordination environments which may reflect two distinct conformational states of the isolated Blc. viridis RC protein.