Excitation and Electron Transfer from Selectively Excited Primary Donor Chlorophyll (P700) in a Photosystem I Reaction Center

Abstract

The primary processes in a photosystem I reaction center were studied by fluorescence up-conversion with a subpicosecond time resolution at room temperature. The samples were P700(primary donor chlorophyll)-enriched particles which retained ≈14 chlorophylls per P700. Upon selective excitation of P700 at 701 nm at ≈5 °C, anisotropy of the fluorescence at 749 nm decayed from ≈0.3 to ≈0.15 with a time constant of 1 ps. The dynamic depolarization is attributed to electronic excitation equilibration between P700 and the surrounding chlorophylls. In the isotropic fluorescence kinetics, at least two decaying components of 2.2 ps (≈35%) and 15 ps (≈55%) were found. The fast and slow components indicate the charge separation before and after full equilibration of excitation energy, respectively. A kinetic model calculation based on the above results suggests that the intrinsic rate constant of the primary electron transfer from P700* is > 0.25 ps-1.