Efficient energy transfer from the long-wavelength antenna chlorophylls to P700 in photosystem I complexes from Spirulina platensis
✍ Scribed by V.V. Shubin; I.N. Bezsmertnaya; N.V. Karapetyan
- Publisher
- Elsevier Science
- Year
- 1995
- Tongue
- English
- Weight
- 736 KB
- Volume
- 30
- Category
- Article
- ISSN
- 1011-1344
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✦ Synopsis
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 P700 with the same efficiency as bulk antenna Chls, causing the ,~xidation of P700. The relative quantum yield of P700 photooxidation is about unity (293-77 K) even under the direct excitation of Chl absorbing at 735 nm (Ch1735). At 77 K Ch1735 exhibits a fluorescence band at 760 nm (F760) whose intensity is quenched under illumination ,~f the PSI trimeric complexes from Spirulina. The relative quantum yield of F760 quenching is not dependent on the wavelength of excitation tn 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 Ch1735 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 9pirulina is organized in separate photosynthetic units.