title: Electrochemical and spectro-kinetic evidence for an intermediate electron acceptor in photosystem I.
abstract: Absorption changes accompanying light-induced P-700 formation and its decay in the dark at 15 K in Photosystem-I particles poised at various redox potentials have been examined. In unpoised samples, the light-induced absorption change is practically irreversible. At increasingly negative potentials, an increasing fraction of the absorption change, proportional to the fraction of bound iron-sulfur protein chemically reduced, becomes reversible, and the titration curve has a midpoint potential of --530 mV (vs. normal hydrogen electrode). At --66 mV, the P-700 absorption change is 97% reversible. The total P-700-signal amplitude decreases over the same potential span and levels off at about 43% (to slightly over 50% at a substantially higher excitation intensity). These results provide additional support to previous suggestions of an existence of an intermediate electron acceptor located between the primary donor, P-700, and the more stable primary electron acceptor (P-430 or bound iron-sulfur protein).
mesh_terms: D004563:Electrochemistry; D004579:Electron Transport; D066298:In Vitro Techniques; D007501:Iron / Q000378:metabolism; D008027:Light; D010084:Oxidation-Reduction; D010788:Photosynthesis; D011506:Proteins / Q000378:metabolism; D013053:Spectrophotometry; D013455:Sulfur / Q000378:metabolism
chemical_list: D011506:Proteins; D013455:Sulfur; D007501:Iron