Magnetic field affects the fluorescence yield in reaction center preparations from Rhodopseudomonas spaeroides R-26.

Purified photochemical reaction centers from Rhodopseudomonas sphaeroides R-26 were reduced with Na2S2O4 so as to block their photochemical electron-transfer reaction. The magnetic field induced an increase in the emission yield. Our results support the hypothesis that under these conditions, charge recombination in the singlet radical pair composed of the oxidized primary donor and reduced primary acceptor predominantly generates the excited singlet state of the reaction cnter bacteriochlorophyll. The maximum relative fluorescence change and the value of the magnetic field at which half-saturation of the effect is achieved (B1/2) at room temperature are 5.5% and 75 G, respectively. For the whole cells of Rps. sphaeroides R-26 these parameters are 1.2% and 120G. The relative fluorescence change at 600 G, deltaF/F(600), and B1/2 are studied as functions of temperature. The temperature dependencies of deltaF/F(600) for reaction centers and whole cells of Rps. sphaeroides R-26 are qualitatively the same, with the maximum effect (8% for reaction centers) occurring at 230 K. However, the B1/2 curves for the two preparations are different.

Magnetic field-induced fluorescence changes in chlorophyll-proteins enriched with P-700.

Fluorescence yield dependence on external magnetic field (0-600 G) was measured for chlorophyll-protein complexes enriched with Photosystem I. Maximal relative changes of fluorescence yield at room temperature (1.0-2.5%) were dependent on the chlorphyll a:P-700 ratio. Magnetic field-induced changes were observed only in the presence of dithionite. At low temperatures (down to -160 degrees C) the magnetic field-induced effect decreased. The effect is obviously connected with the functions of reaction centers in Photosystem I. An explanation of the effect is proposed based on the hypothesis of radical pairs recombination within the reaction center. For the radical pair (P-700+. A-.), an intermediate acceptor, A-., with a g-value approximately equal to that of P-700+. is proposed.

[Effect of a magnetic field on the fluorescence yield of Rhodopseudomonas sphaeroides cells of the wild strain and strain R-26. Relationship to temperature]. / Vliianie magnitnogo polia na vykhod fluorestsentsii kletok Rhodopseudomonas sphaeroides dikii shtamm i R-26. Temperaturnaia zavisimost'.

Fluorescence yield of Rhodopseudomonas sphaeroides cells whose primary acceptor is chemically reduced depends on the magnetic field tension. The maximal value of relative magnetic effect and the field value, at which the effect amounts to a half of the maximal value for R. sphaeroides wild strain and R-26, equal 3.6 and 1.1%; 260 and 120 G correspondingly at 295 K. An effect of th magnetic field on fluorescence yield for R. sphaeroides wild strain at the medium potential 300 mV was observed. The values of relative magnetic effect were shown to change within the temperature range 295-120 K. The results obtained are discussed in terms of the relationship between these parameters and the velocity constant values of the radical paris recombination (P 870(+) . Bph(-) . in singlet and triplet states and the value of metabolic interaction in the radical pairs (P 870(+) . Bph(-) .) and (Bph(-) . --X(-) .).

[Mechanism of photochemical oxidation of bacterioviridin]. / Issledovanie mekhanizma fotokhimicheskogo okisleniia bakterioviridina

The mechanism of bacterioviridin photochemical oxidation has been studied by the methods of ESR, flash-photolysis and low-temperature spectrophotometry. ESR spectrum of pigment cation-radical, a singlet line with H=11 G, g = 2.0027, has been recorded. The bands with maxima at 370, 470, 525, 590, 840 nm correspond to bacterioviridin cation -- radical in the absorption spectra. When -- benzoquinone is used as an electron acceptor with excitation light 640 nm the product of bacterioviridin irreversible oxidation is formed with the absorption band maximum 760 nm and absorption between 350 and 370 nm. It is suggested that this product is of double-oxidized non-radical nature and the mechanism of its formation through oxidation of the pigment cation-radical is discussed. The regeneration reaction of double-oxidized bacterioviridin up to cation-radical form in the presence of triphenylamine as a reducing agent has been carried out. The rate constants of cation-radical decay in the dark and desactivation of triplet state have the following values: K1=(1,64+/-0,15)-10(3) sec-1, K2=(13+/-2,0)-10(3) sec-1 correspondingly. The activation energy of the radical decay in the dark is Eact =(13,2-0,5) kcal/mole.