Abstract
(1) In photosystem I (PS I) particles in the presence of dithionite and intense background illumination at 290 K, an external magnetic field (0–0.22 T) induced an increase, ΔF, of the low chlorophyll a emission yield, F ( ΔF F ⋍ 1–1.5% ). Half the effect was obtained at about 35–60 mT and saturation occurred for magnetic fields higher than about 0.15 T. In the absence of dithionite, no field-induced increase was observed. Cooling to 77 K decreased ΔF at 685 nm, but not at 735 nm, to zero. Measuring the emission spectra of F and ΔF, using continuous excitation light, at 82, 167 and 278 K indicated that the spectra of F and ΔF have about the same maximum at about 730, 725 and 700 nm, respectively. However, the spectra of ΔF show more long-wavelength emission than the corresponding spectra of F. (2) Only in the presence of dithionite and with (or after) background illumination, was a luminescence (delayed fluorescence) component observed at 735 nm, after a 15 ns laser flash (530 nm), that decayed in about 0.1 μs at room temperature and in approx. 0.2 μs at 77 K. A magnetic field of 0.22 T caused an appreciable increase in luminescence intensity after 250 ns, probably mainly caused by an increase in decay time. The emission spectra of the magnetic field-induced increase of luminescence, ΔL, at 82, 167 and 278 K coincided within experimental error with those of ΔF mentioned above. The temperature dependence of ΔF and ΔL was found to be nearly the same, both at 685 and at 735 nm. (3) Analogously to the proposal concerning the 0.15 μs luminescence in photosystem II (Sonneveld, A., Duysens, L.N.M. and Moerdijk, A. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 5889–5893), we propose that recombination of the oxidized primary donor P-700 + and the reduced acceptor A −, probably A − 1, of PS I causes the observed fast luminescence. The effect of an external magnetic field on this emission may be explained by the radical pair mechanism. The field-induced increase of the 0.1–0.2 μs luminescence seems to be at least in large part responsible for the observed increase of the total (prompt + delayed) emission measured during continuous illumination in the presence of a magnetic field.
Published Version
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