Magnetic field-induced increase in chlorophyll a delayed fluorescence of photosystem II: A 100- to 200-ns component between 4.2 and 300 K.
At room temperature the delayed fluorescence (luminescence) of spinach chloroplasts, in which the acceptor Q is prereduced, consists of a component with a lifetime of 0.7 mus and a more rapid component, presumably with a lifetime of 100-200 ns and about the same integrated intensity as the 0.7- mus component. Between 4.2 and 200 K only a 100- to 200-ns luminescence component was found, with an integrated intensity appreciably larger than that at room temperature. At 77 K the 150-ns component approached 63% of saturation at roughly the same energy as the variable fluorescence of photosystem II at room temperature. At 77 K the emission spectra of prompt fluorescence but not that of the 150-ns luminescence had a preponderant additional band at about 735 nm. The 150-ns emission also occurred in the photosystem I-lacking mutant FL5 of Chlamydomonas. These experiments indicate that the 150-ns component originates from photosystem II. At room temperature a magnetic field of 0.22 T stimulated the 0.7-mus delayed fluorescence by about 10%. At 77 K the field-induced increase of the 150-ns component amounted to 40-50%, being responsible for the observed approximately 2% increase of the total emission; the magnetic field increased the lifetime about 20%. In order to explain these phenomena a scheme for photosystem II is presented with an intermediary acceptor W between Q and the primary donor chlorophyll P-680; recombination of P-680(+) and W(-) causes the fast luminescence. The magnetic field effect on this emission is discussed in terms of the radical pair mechanism.
Department of Biophysics, Huygens Laboratory of the State University, P.O. Box 9504, 2300 RA Leiden, The Netherlands.
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Duysens
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Moerdijk