Quantum yield of P700+ photodestruction in isolated photosystem I complexes of the cyanobacteriumArthrospira platensis

Abstract

The photostability of P700 cation radical (P700⁺) was studied by evaluating the quantum yields of P700⁺ photodestruction in photosystem I (PSI) complexes of the cyanobacteriumArthrospira platensis. The time courses of P700⁺ photodestruction in PSI trimers and monomers have been measured in aerobic conditions under selective excitation of far-red absorption band of P700⁺ by intense light of laser diodes. Long-term exposure of PSI complexes to 808 or 870 nm laser light caused destruction of P700⁺ and antenna chlorophylls. The true integral quantum yield of P700⁺ photodestruction calculated from these data was less than 0.7–1.4 × 10⁻⁸. Illumination of PSI complexes by 650 nm light caused destruction of antenna chlorophylls with true quantum yield of about 6–7 × 10⁻⁶ and damage of P700 with apparent quantum yield 2–3 × 10⁻⁸. Preferential photodestruction of the long-wavelength antenna chlorophyll absorbing at 710 nm as compared with bulk chlorophylls was observed. About three orders of difference in magnitude between quantum yields of P700⁺ and bulk chlorophyll photodestruction indicates that P700⁺ is extremely photostable for functioning as an efficient quencher of singlet excitation energy in PSI.