[Kinetics of pigment-acceptor interaction induced by steady-state illumination in Synechocystis spaeroides photosystem I preparations cooled to 160 K in the dark and on light].

The kinetics of dark reduction of chlorophyll P700 oxidized by steady-state illumination in photosystem I reaction center preparations of cyanobacterium Synechocystis sp. coolled in the dark to 160 K is greatly nonexponential. The characteristic times for the components of the reaction are from fractions of a second to minutes and more. During cooling reaction center preparations on actinic light, a great part of chlorophyll P700 is fixed at 160 K in oxidized state. The kinetics of dark reduction of P700+ in the fraction of reaction centers that retain the photochemical activity in these conditions is faster than the kinetics in samples cooled in the dark. A theoretical analysis of the substantial deceleration of the P700+ dark recovery kinetics was done for photosystem I reaction center preparations oxidized by steady-state illumination to 160 K in contrast with situation that arises after the oxidation of reaction centers by single short light pulses. The deceleration of the kinetics in samples activated by steady-state illumination can be explained by processes of microconformational relaxation, connected with proton shifts in the reaction center structure.

Effect of oxygen on temporary stabilization of photoreduced quinone acceptors in Rhodobacter sphaeroides reaction centers.

The effect of molecular oxygen on the photochemical activity of the Rhodobacter sphaeroides reaction centers frozen to 160 K under actinic illumination was investigated by the ESR method. About 90% of initially photochemically active bacteriochlorophyll (P) were fixed at 160 K for a long time in aerobic samples in an inactive form. In anaerobic samples, not more than 65% were fixed in an inactive form under the same conditions. In aerobic preparations, a small portion of photochemically active bacteriochlorophyll (about 10%) that retains its photochemical activity at 160 K after freezing under illumination has dark reduction kinetics similar to that of samples at room temperature after several seconds of actinic illumination. In anaerobic samples frozen under illumination, the remaining photochemically active reaction centers (35%) have the same dark reduction kinetics as samples illuminated at 295 K for 1-2 min. The conclusion is that the irreversible stabilization of bacteriochlorophyll P in the oxidized inactive state formed in the reaction centers frozen under illumination is brought about by light-induced conformational changes fixed under low temperatures.

Effect of low temperatures on photochemical activity of PS1 reaction centers from Synechocystis sp. frozen under illumination.

After cooling of Synechocystis sp. photosystem 1 (PS1) reaction centers (RC) to 160 K under illumination most of the photoactive pigment is fixed for a long time in the oxidized state. The same effect is observed in purple bacteria RC. The dark reduction kinetics of PS1 P700 chlorophyll, which still retains its photochemical activity, in these samples was similar to that in samples cooled in the dark. We suggest that the photoinduced charge separation in PS1 RC, as well as in purple bacteria RC, is accompanied by conformational changes that can be fixed in samples cooled under illumination. As a result, the electrons photomobilized in RC cooled under illumination are unable to return backward the process of electron transfer to P700(+) after cessation of actinic illumination. Such irreversible trapping of electrons can take place in different parts of the PS1 RC electron acceptor chain.