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Solar energy absorbed by plants can be redistributed between photosystems in the process termed "state transitions" (ST). ST represents a reversible transition of a part of the PSII light harvesting complex (L-LHCII) between photosystem II (PSII) and photosystem I (PSI) in response to the change in light spectral composition. The present work demonstrates a slower development of the state 1 to state 2 transition, i.e., L-LHCII transition from PSII to PSI, in the leaves of dicotyledonous arabidopsis (Arabidopsis thaliana) than in the leaves of monocotyledonous barley (Hordeum vulgare) plants that was assessed by the measurement of chlorophyll a fluorescence at 77 K and of chlorophyll a fluorescence at room temperature. It is known that the first step of the state 1 to state 2 transition is phosphorylation of Lhcb1 and Lhcb2 proteins; however, we detected no difference in the rate of accumulation of these phosphorylated proteins in the studied plants. Therefore, the parameters, which possibly affect the second step of this transition, i.e., the migration of L-LHCII complexes along the thylakoid membrane, were evaluated. Spin-probe EPR measurements demonstrated that the thylakoid membranes viscosity in arabidopsis was higher compared to that in barley. Moreover, confocal microscopy data evidenced the different size of chloroplasts in the leaves of the studied species being larger in arabidopsis. The obtained results suggest that the observed deference in the development of the state 1 to state 2 transition in arabidopsis and barley is caused by the slower L-LHCII migration rate in arabidopsis than in barley plants rather than by the difference in the Lhcb1 and Lhcb2 phosphorylation.

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The mechanism of bacteriochlorophyll photooxidation in light-harvesting complexes of a number of purple photosynthetic bacteria when the complexes are excited into the carotenoid absorption bands remains unclear for many years. Here, using narrow-band laser illumination we measured action spectrum of this process for the spectral ranges of carotenoid and bacteriochlorophyll. It is shown that bacteriochlorophyll excitation results in almost no photooxidation of these molecules, while carotenoid excitation leads to oxidation with quantum yield of about 0,0003. Low value of the yield enabled an assumption that the studied process is initiated by the triplet states of the main carotenoids of the complexes with the number of conjugated double-bond chain length of N = 11. Interaction of these states with oxygen facilitates formation, though with low efficiency, of the excited singlet oxygen, which oxidizes bacteriochlorophylls. The carotenoid triplet states are formed in the process of the earlier studied singlet-triplet fission. The obtained results point at the necessity of reconsidering the functions of carotenoids in the light-harvesting complexes of purple bacteria.

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In photosynthetic reaction centers (RCs) of purple bacteria, conserved histidine residues [His L173 and His M202 in Rhodobacter (Rba.) sphaeroides] are known to serve as fifth axial ligands to the central Mg atom of the bacteriochlorophyll (BChl) molecules (PA and PB, respectively) that constitute the homodimer (BChl/BChl) primary electron donor P. In a number of previous studies, it has been found that replacing these residues with leucine, which cannot serve as a ligand to the Mg ion of BChl, leads to the assembly of heterodimer RCs with P represented by the BChl/BPheo pair. Here, we show that a homodimer P is assembled in Rba. sphaeroides RCs if the mutation H(M202)L is combined with the mutation of isoleucine to histidine at position M206 located in the immediate vicinity of PB. The resulting mutant H(M202)L/I(M206)H RCs are characterized using pigment analysis, redox titration, and a number of spectroscopic methods. It is shown that, compared to wild-type RCs, the double mutation causes significant changes in the absorption spectrum of the P homodimer and the electronic structure of the radical cation P+, but has only minor effect on the pigment composition, the P/P+ midpoint potential, and the initial electron-transfer reaction. The results are discussed in terms of the nature of the axial ligand to the Mg of PB in mutant H(M202)L/I(M206)H RCs and the possibility of His M202 participation in the previously proposed through-bond route for electron transfer from the excited state P* to the monomeric BChl BA in wild-type RCs.

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Light-dependent oxygen reduction in the photosynthetic electron transfer chain, i.e. the Mehler reaction, has been studied using isolated pea thylakoids. The role of the plastoquinone pool in the Mehler reaction was investigated in the presence of dinitrophenyl ether of 2-iodo-4-nitrothymol (DNP-INT), the inhibitor of plastohydroquinone oxidation by cytochrome b6/f complex. Oxygen reduction rate in the presence of DNP-INT was higher than in the absence of the inhibitor in low light at pH 6.5 and 7.6, showing that the capacity of the plastoquinone pool to reduce molecular oxygen in this case exceeded that of the entire electron transfer chain. In the presence of DNP-INT, appearance of superoxide anion radicals outside thylakoid membrane represented approximately 60% of the total superoxide anion radicals produced. The remaining 40% of the produced superoxide anion radicals was suggested to be trapped by plastohydroquinone molecules within thylakoid membrane, leading to the formation of hydrogen peroxide (H2 O2 ). To validate the reaction of superoxide anion radical with plastohydroquinone, xanthine/xanthine oxidase system was integrated with thylakoid membrane in order to generate superoxide anion radical in close vicinity of plastohydroquinone. Addition of xanthine/xanthine oxidase to the thylakoid suspension resulted in a decrease in the reduction level of the plastoquinone pool in the light. The obtained data provide additional clarification of the aspects that the plastoquinone pool is involved in both reduction of oxygen to superoxide anion radicals and reduction of superoxide anion radicals to H2 O2 . Significance of the plastoquinone pool involvement in the Mehler reaction for the acclimation of plants to light conditions is discussed.

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We report the realization of the optically induced inverse population of the ground-state spin sublevels of the silicon vacancies (V(Si)) in silicon carbide (SiC) at room temperature. The data show that the probed silicon vacancy spin ensemble can be prepared in a coherent superposition of the spin states. Rabi nutations persist for more than 80 µs. Two opposite schemes of the optical alignment of the populations between the ground-state spin sublevels of the silicon vacancy upon illumination with unpolarized light are realized in 4H- and 6H-SiC at room temperature. These altogether make the silicon vacancy in SiC a very favorable defect for spintronics, quantum information processing, and magnetometry.

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Polyacrylamide gel electrophoresis of chernozem soil humic acids (HAs) followed by observation under UV (312 nm) excitation light reveals new low molecular weight (MW) fluorescent fractions. Ultrafiltration of HAs sample in 7 M urea on a membrane of low nominal MW retention (NMWR, 5 kDa) was repetitively used for separation of fluorescent and non-fluorescent species. Thirty ultrafiltrates and the final retentate R were obtained. Fluorescence maxima of separate ultrafiltrates were different and non-monotonously changed in the range of 475-505 nm. Fluorescence maxima of less than 490 nm were detected only in the four first utrafiltrates. For further physical-chemical analyses all utrafiltrates were combined into a fraction called UF<5 (NMW<5 kDa). Retentate R demonstrated very weak fluorescence under 270 nm excitation, while fluorescence intensity of UF<5 was about six times higher than of the bulk HAs. Fraction UF<5 was further ultrafiltrated on membranes of MNWR 3 kDa and 1 kDa, yielding three subfractions UF3-5, UF1-3 and UF<1 with NMW 3-5 kDa, 1-3 kDa and <1 kDa, respectively. The validation of the UF procedure was performed by size exclusion chromatography on Sephadex G-25 column. The fluorescence maxima were found to be at 505, 488 and 465 nm for UF3-5, UF1-3 and UF<1, respectively, with increasing of fluorescence intensity from UF3-5 to UF1-3 to UF<1 fraction. EPR analysis showed that the amount of free radicals was the largest in retentate R and drastically decreased in fluorescent ultrafiltrates. The results demonstrate that more than one fluorophore is present in chernozem soil HAs complex.

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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.

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