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 photosy stem 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 th e region 710-740 nm transfer excitation energy to the reduced WOO with the same efficiency as bulk antenna Chls, causing the oxidation of P7 00. 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 co mplexes from Spirulina. The relative quantum yield of F760 quenching i s 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 a nd the absorption band of the cation radical of P700 (P700(+)) is high er than that of the P700 band, it is suggested that Chl735 transfers e nergy to P700(+) more efficiently than to reduced P700; energy transfe r to P700(+) causes the quenching of F760. A linear relationship betwe en the photooxidation rate of P700 and the fraction of P700(+) at 293 K indicates that the energy exchange between PSI subunits of the trime r is negligible. Thus, the antenna of PSI trimers of Spirulina is orga nized in separate photosynthetic units.