Chemically induced dynamic electron polarization in chloroplasts at room temperature: evidence for triplet state participation in photosynthesis.

A transient electron paramagnetic resonance emission is observed after flash excitation of chloroplasts at room temperature. The spectrum of the emission signal is centered at g = 2.0037 and has a linewidth deltaHpp = 4G (4 X 10(-4) tesla). Inhibitor studies and chemical oxidation indicate that the signal is associated with Photosystem I, but the spectrum and kinetics indicate that it is neither P700 nor an iron-sulfur protein. The emission signal rises with the 2-musec time response of the instrument, and decays during the actinic flash. The emission signal is produced on only the first of a pair of strong flashes separated by 100 musec, indicating that the precursor has not been regenerated in that time. The results are discussed with reference to the two currently accepted mechanisms for chemically induced dynamic electron polarization: the radical pair and the photochemical triplet. For several reasons the photochemical triplet mechanism is the more attractive of the two. It is suggested that at room temperature the primary photochemistry of photosystem I proceeds via a triplet state of chlorophyll, and that the species giving rise to the emission signal is the primary electron acceptor of Photosystem I.