Primary reactions, plastoquinone and fluorescence yield in subchloroplast fragments prepared with deoxycholate

Abstract

1. In subchloroplast fragments prepared with the detergent deoxycholate the primary reactions of Photosystem II could be studied at room temperature, because the secondary reactions were largely or completely inhibited. 2. The main quencher of chlorophyll fluorescence in these particles was the photosynthetically active pool of plastoquinone in its oxidized form. Its photoreduction in the presence of artificial electron donors was accompanied by a shift of a chlorophyll a absorption band. Its reoxidation in the dark was very slow, even in the presence of ferricyanide. 3. Of all the artificial electron donors tested MnCl 2 was by far the most efficient. 4. Measurements at room temperature of the C550 absorbance change confirmed its correlation with the primary electron acceptor. Its difference spectrum was broader and its extinction coefficient correspondingly lower than at liquid-N 2 temperature. In chloroplasts the C550 concentration was about 1:360 chlorophylls. 5. In the dark C550 was largely in the reduced state and its oxidation by plastoquinone took place in the presence of an artificial electron donor only, suggesting that the redox potential of C550 was increased by accumulated positive charges at the donor side of the reaction center. 6. The free radical 1,1′-diphenyl-2-picrylhydrazyl oxidized C550 directly in a 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-insensitive reaction. A DCMU-insensitive oxidation of C550 was observed at high ferricyanide concentrations as well, but probably in this case an endogenous electron donor was oxidized, which in turn oxidized C550 via the back reaction of the photochemical reaction. 7. The oxidized form of the primary electron donor, P680 +, accumulated in the light in the presence of deoxycholate and a low ferricyanide concentration. In chloroplasts the P680 concentration was about 1:360 chlorophylls. 8. The P 680 absorption difference spectrum and electron spin resonance could be explained by the oxidation of a chlorophyll a dimer. Repeated deoxycholate treatments progressively changed the spectra to those of a monomer. The monomer was still photochemically active. 9. A new interpretation of the difference spectrum of P700 is proposed: it may be the same as that of the difference spectrum of P680 if the bleaching at 700 nm is attributed to a band shift.