EFFICIENT ENERGY-TRANSFER FROM THE LONG-WAVELENGTH ANTENNA CHLOROPHYLLS TO P700 IN PHOTOSYSTEM-I COMPLEXES FROM SPIRULINA-PLATENSIS

To study the role of the long-wavelength chlorophylls (Chl) in photosystem I (PSI), the action spectra of P700 photooxidation at 293 and 77 K have been measured for PSI trimeric and monomeric complexes isolated from Spirulina platensis. The long-wavelength Chls which absorb in the region 710-740 nm transfer excitation energy to the reduced WOO with the same efficiency as bulk antenna Chls, causing the oxidation of P700. The relative quantum yield of P700 photooxidation is about unity (293-77 K) even under the direct excitation of Chi absorbing at 735 nm (Chl735). At 77 K Chl735 exhibits a fluorescence band at 760 nm (F760) whose intensity is quenched under illumination of the PSI trimeric complexes from Spirulina. The relative quantum yield of F760 quenching is not dependent on the wavelength of excitation in the region 620-750 nm. Since the value of the overlap integral between the band of F760 and the absorption band of the cation radical of P700 (P700(+)) is higher than that of the P700 band, it is suggested that Chl735 transfers energy to P700(+) more efficiently than to reduced P700; energy transfer to P700(+) causes the quenching of F760. A linear relationship between the photooxidation rate of P700 and the fraction of P700(+) at 293 K indicates that the energy exchange between PSI subunits of the trimer is negligible. Thus, the antenna of PSI trimers of Spirulina is organized in separate photosynthetic units.