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  • Title: [Photooxidation of P700 in photosystem 1 preparations with various amounts of antenna chlorophyll a].
    Author: Il'ina MD, Borisov AIu.
    Journal: Biokhimiia; 1982 Dec; 47(12):1954-62. PubMed ID: 6760906.
    Abstract:
    A number of membrane fragments and pigment-protein complexes of photosystem 1 was obtained from pea chloroplasts, using ionic and non-ionic detergents (SDS, digitonin, Triton X-100, lauryldimethylamine-N-oxide). The ratio of chlorophyll (Chl) a to P700 varied from 220 to 30. For non-dialyzed preparations the quantum yield of P700 photooxidation (phi e) measured by the initial rate of photobleaching at 696-698 nm with excitation at the Soret band of Chla was equal to 40-60%. When the P700 photooxidation was measured at 432 nm, the phi e value showed a further decrease to 20-40% during red light excitation over the range of 660-680 nm but rose to 70-90% at the exciting light wavelengths of greater than or equal to 695 nm. On the basis of the observed dependences the red absorption band was approximated by a sum of two spectra: the spectrum of Chla photoactive in P700 photooxidation and that of photoinactive Chla. Both spectra had maxima near the absorption peak of the object. The photoinactive fraction was additionally enriched by the long-wavelength absorption forms of Chla with an absorption maximum over the range of 684-690 nm. The amount of the bulk Chla in the photoinactive fraction was no less than 40%. The phi e value for freshly dialyzed preparation at a Chla/P700 ratio of 30 was equal to 50-60% independent of the exciting light wavelength. An addition of 0.05% Triton X-100 to this preparation caused: i) a blue shift of the absorption and fluorescence maxima; ii) a decrease of the long-wavelength absorption forms content of Chla and, iii) a considerable increase in fluorescence lifetime and quantum yield due to deaggregation of Chla and its solubilization by detergent micelles. The same phenomenon seems to be responsible for the formation of photoinactive fraction of a pigment, since after addition of a detergent the above-mentioned spectral dependence of phi e appeared, i.e. phi e showed a 3-fold decrease (down to 18%) within the region of 660-680 nm and a 1,6-fold increase (up to 90%) at 705-730 nm. These results suggest that the detergents destroy the intact construction of a light-harvesting antenna rather than that of the photosystem 1 reaction center.
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