Light-induced generation of superoxide radicals and hydrogen peroxide in isolated thylakoids has been studied with a lipophilic spin probe, cyclic hydroxylamine 1-hydroxy-4-isobutyramido-2,2,6,6-tetramethylpiperidinium (TMT-H) to detect superoxide radicals, and the spin trap α-(4-pyridyl-1-oxide)-N-tert-butylnitron (4-POBN) to detect hydrogen peroxide-derived hydroxyl radicals. Accumulation of the radical products of the above reactions has been followed using electron paramagnetic resonance. It is found that the increased production of superoxide radicals and hydrogen peroxide in higher light is due to the enhanced production of these species within the thylakoid membrane, rather than outside the membrane. Fluorescent probe Amplex red, which forms fluorescent product, resorufin, in the reaction with hydrogen peroxide, has been used to detect hydrogen peroxide outside isolated chloroplasts using confocal microscopy. Resorufin fluorescence outside the chloroplasts is found to be suppressed by 60% in the presence of the inhibitor of aquaporins, acetazolamide (AZA), indicating that hydrogen peroxide can diffuse through the chloroplast envelope aquaporins. It is demonstrated that AZA also inhibits carbonic anhydrase activity of the isolated envelope. We put forward a hypothesis that carbonic anhydrase presumably can be attached to the envelope aquaporins. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Recent electron paramagnetic resonance (EPR) experiments (Proskuryakov, I. I.; Klenina, I. B.; Hore, P. J.; Bosch, M. K.; Gast, P.; Hoff, A. J. Chem. Phys. Lett. 1996, 257, 333) on the primary radical pair in bacterial reaction centers are interpreted in detail. The spin-correlated radical pair model is extended to include the time dependence of the EPR intensities, differential lifetime broadening of the basic four-line spectrum, and averaging over the inhomogeneous distributions of hyperfine interactions in the two radicals. Values of the singlet and triplet recombination rate constants and the magnitude and sign of the exchange interaction of the radical pair are obtained from the shape and linewidth of the spin-polarized EPR spectra and the kinetics of formation of the triplet state of the primary electron donor. For Rhodobacter sphaeroides at 70 K, k S = (1.2 ± 0.3) × 107 s-1, k T = (5.7 − 8.8) × 108 s-1, −J = 8 − 6 G; and for Rhodopseudomonas viridis at 190 K, k S = (8 ± 2) × 107 s-1, k T = (6.5 − 10.0) × 108 s-1, −J = 17 − 16 G. The reliability of these values is discussed in the light of the energetic heterogeneity of the radical pair energies (Ogrodnik, A.; Keupp, W.; Volk, M.; Aumeier, G.; Michel-Beyerle, M. E. J. Phys. Chem. 1994, 98, 3432−3439) and of the sensitivity and selectivity of the EPR measurements toward the three parameters.

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The first time-resolved magnetophotoselection study of the primary donor triplet state ( 3 P) in bacterial reaction centers from Rhodobacter sphaeroides R26 and Rhodopseudomonas Viridis is reported. With direct excitation of the primary donor, this approach provides the orientation (spherical coordinates δ, colatitude, and γ, longitude) of the excited optical transition moment in the principal triplet axis system. It is essentially free from the effects of spin-lattice relaxation of 3 P, enabling magnetophotoselection measurements over a wide temperature range. Two independently measured triplet-state spectra, excited with light polarized parallel and perpendicular to the EPR magnetic field, are simulated with the same set of parameters. This procedure results in a high precision (ca. (5°with sufficient signal/noise ratio) of the obtained spherical coordinates of the optical transition moment vector. We find δ ) 80 ( 5°, γ ) 70 ( 5°and δ ) 75 ( 5°, γ ) 70 ( 5°for Rb. sphaeroides R26 and Rps. Viridis, respectively. We demonstrate that excitation of the sample with nonpolarized light is essentially nonisotropic. Neglect of this effect in spectral simulations of light-induced signals may lead to considerable error in the parameters determined.

The triplet states of photosystem II core particles from spinach were studied using time-resolved cw EPR technique at different reduction states of the iron--quinone complex of the reaction center primary electron acceptor. With doubly reduced primary acceptor, the well-known photosystem II triplet state characterised by zero-field splitting parameters |D|=0.0286 cm(-1), |E|=0.0044 cm(-1) was detected. When the primary acceptor was singly reduced either chemically or photochemically, a triplet state of a different spectral shape was observed, bearing the same D and E values and characteristic spin polarization pattern arising from RC radical pair recombination. The latter triplet state was strongly temperature dependent disappearing at T=100 K, and had a much faster decay than the former one. Based on its properties, this triplet state was also ascribed to the photosystem II reaction center. A sequence of electron-transfer events in the reaction centers is proposed that explains the dependence of the triplet state properties on the reduction state of the iron--quinone primary acceptor complex.

a b s t r a c tAccumulation of nitroxide radicals, DCP Å or TMT Å , under illumination of a thylakoid suspension containing either hydrophilic, DCP-H, or lipophilic, TMT-H, cyclic hydroxylamines that have high rate constants of the reaction with superoxide radicals, was measured using ESR. A slower accumulation of TMT Å in contrast with DCP Å accumulation was explained by re-reduction of TMT Å by the carriers of the photosynthetic electron transport chain within the membrane. Superoxide dismutase suppressed TMT Å accumulation to a lesser extent than DCP Å accumulation. The data are interpreted as evidencing the production of intramembrane superoxide in thylakoids.

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