Energy Equilibration among the Chlorophylls in the Electron-Transfer System of Photosystem I Reaction Center from Spinach

Abstract

Excitation-wavelength dependence of the transient absorbance changes of photosystem I reaction center particles with a reduced number of antenna chlorophylls (16 chlorophylls/primary electron donor chlorophyll (P700)) hasbeen studied in an effort to understand the energy equilibration among the chlorophylls in the electron-transfer system. The photobleaching and stimulated-emission signals in the Q y band region of the chlorophylls upon the preferential excitation of chlorophyll spectral forms at the blue edge (662 nm) and red edge (697 nm) of the Q y band are analyzed. In the case of the red-edge excitation, spectral equilibration proceeds with a time constant of 0.34 (′0.07) ps, which is attributable to the energy equilibration between P700 and neighboring chlorophylls absorbing around 686 nm in the electron-transfer system. This equilibration seems to precede the fastest phase of the primary charge separation (apparent time constant of 0.8 ps) reported previously (Kumazaki et al. J. Phys. Chem. B 2001, 105, 1093). A slow decay of the excited states because of the slow phase of the primary charge separation proceeds with a time constant of 7.2 (′0.6) ps. In the case of the blue-edge excitation, vibrational relaxation and downhill energy transfer proceed with a time constant of 0.38 (′0.08) ps, which are followed by a slow downhill energy transfer from residual antenna chlorophylls to the electron-transfer system. Even with the slow energy transfer from the residual antenna chlorophylls to the electron-transfer system, the overall primary charge separation is completed with a time constant of 10 (′0.7) ps. These interpretations are in part supported by control experiments on chlorophyll a in ethanol under equivalent optical conditions. Implications of these results for understanding the primary processes in more intact photosystem I are discussed.