Fluorescence of Photosystem I

Fluorescence of chlorophylls in the Photosystem I (PS I) reaction center (RC) of plants and cyanobacteria in vivo and in vitro is reviewed. The purified PS I-RC has a structure with 90 antenna chlorophyll (Chi) a molecules surrounding the six core Chi a molecules that function in the fast electron transfer steps. The RC forms a system highly efficient for energy capture and charge separation. Measurements by the new confocal microfluorimetry technique have revealed features of ‘high’ PS I fluorescence in single intact heterocyst cells that are specialized for nitrogen fixation, compared to that in single vegetative cells of a filamentous cyano-bacterium Nostoc sp. Typical PS I fluorescence at room temperature and at 77K are reviewed, with examples taken from measurements by a comprehensive streak camera-imaging of fluorescence decay dynamics in (1) intact Nostoc cells, (2) isolated spinach PS I particles, (3) chlorophyll-depleted PS I and (4) Chi d-containing PS I of a newly found cyanobacterial type unicellular orgamsmAcaryochloris marina. Ultra-fast energy transfer and electron transfer in the reaction center is reviewed to obtain a general view for the energy conversion in the RC. Excitation equilibration among most of Chls in the PS I-RC is very fast (~0.5ps), and the shift of excitation to several Chi a molecules absorbing at longer wavelength occurs in 3–15 ps in parallel with the dissipation of energy in the charge separation in P700 at the center of the reaction center. The overall charge separation occurs apparently in 10–50 ps, but the intrinsic charge separation time is estimated to be ~0.5 ps. Results obtained with the Chl-depleted or Chi (d-containing PS I, which has a similar structure but lacks long-wavelength forms of Chls, also indicates similar fast excitation equilibration and the ultra-fast intrinsic charge separation times.