Triplet states in photosystem I of spinach chloroplasts and subchloroplast particles.

Abstract
Light-induced EPR triplet spectra are reported from samples of chloroplasts or digitonin photosystem I particles that depend upon the dark redox state of the bound acceptors of photosystem I. If the reaction centers are prepared in the redox sate P-700 A X- FdB- FdA-, then upon illumination at 11K a polarized chlorophyll triplet species was observed which is interpreted as arising from radical pair recombination between P-700+ and A-. This chlorophyll triplet is apparently the analog of the PR state of photosynthetic bacteria. If the reaction centers are prepared in the dark, redox state P-700 A X FdB- FDA-, then upon illumination at 11K a different triplet species of uncertain origin, possibly pheophytin or carotenoid, was observed. This species is closely associated with the photosystem I reaction center and it traps excitation when P-700 is oxidized.

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