V. Z. Pashchenko

Hybrid systems of quantum dots mixed with the photosensitive protein phycoerythrin

It is shown that semiconductor nanocrystals (or quantum dots) can be used to increase the absorbability of a pigment protein. In the mixture of phycoerythrin with quantum dots, the fluorescence of the quantum dots is suppressed several times due to the transfer of absorbed energy to phycoerythrin. The Forster resonance energy transfer is discussed as a possible mechanism of energy transfer in quantum dot-phycoerythrin donor-acceptor pairs. Calculations based on experimental data show that the efficiency of energy migration from quantum dots to phycoerythrin is 88% and the corresponding rate constant is 1.17 × 109 s−1.

Photophysical properties of hybrid complexes of quantum dots and reaction centers of purple photosynthetic bacteria Rhodobacter sphaeroides adsorbed on crystalline mesoporous TiO2 films

Complexes of CdSe/ZnS and CdTe quantum dots (QDs) with proteins of reaction centers (RCs) of purple bacteria Rhodobacter sphaeroides have been obtained. An efficient energy transfer from QDs (a donor of electron excitation energy) to RCs (acceptor) is observed for them both in the solution and in the films of crystalline mesoporous titanium oxide. The design of such hybrid structures allows us to increase the absorptive ability of the RC many times and, respectively, increase the efficiency of the light energy conversion into the electrical potential. Such hybrid structures can be used for the development of high-performance solar cells.

The nature of oscillations in the kinetics of electron transfer in the reaction center of purple bacteria.

87 The reaction centers (RCs) of purple bacteria are natural nanostructures able to transform electron excitation energy into the energy of separated charges with a high efficiency (~100%). The RC main unit is formed of two protein subunits, L and M, with four bacteriochlorophyll (BCl) molecules, two bacteriopheophytin ( H A and H B ) molecules, and two quinone ( Q A and Q B ) molecules attached to them. An iron atom is localized between the quinones. In turn, two of the four BCl molecules form a special pair, the primary electron donor P. Spatial organization of the Rhodobacter sphaeroides RC has been determined with a resolution of 2.65 A [1]. Study of the interaction between the excited states and the charge transfer states of RC cofactors provided for discovering oscillations in the kinetics of stimulated luminescence in the R. sphaeroides RC at the excitation at Q y absorption band of the special pair [2, 3]. Shuvalov et al. studied the oscillations in the absorption band of the reduced intermediate acceptor [4, 5]. These oscillations were explained by a wave packet formed on the potential energy surface for the interaction between the special pair and BCl in the active photosynthesis chain during electron transfer. The oscillation data were described by the Redfield super

Analysis of Absorption Spectra of RC from the Bacteria Blastochlorii viridis in Near IR Region Using High-Order Derivative Spectroscopy

Photosynthetic reaction center (RC) of purple bacteria is able to transform the energy of absorbed light into the energy of separated electric charges of different sign with quantum efficiency close to 100%. Bacterial photosynthetic RC is a transmembrane pigment–protein complex that incorporates four molecules of bacteriochlorophyll (BChl) molecules; two molecules of bacteriopheophytin (BPh), and quinone acceptors of electron. Two molecules of BChl are in strong exciton interaction with one another giving rise to formation of a photochemically active dimer (special pair, P), which fulfills the function of the primary electron donor. Monomeric molecules of BChl and BPh produce two branches of cofactors (L and M), which are arranged symmetrically relative to P in the structure of RC. Under normal conditions, only L-branch is involved in electron transfer to quinone acceptors. The conventional scheme of the primary processes in RC of purple bacteria can be represented as follows:

The influence of structural phase transition on the temperature dependence of the rate of charge recombination P+QA(-)-->PQA in Rhodobacter sphaeroides reaction centers.

Studies of the temperature dependence of the rate constants of the initial stages of light energy transformation in photosynthetic reaction centers (RCs) provide valuable information about the physical mechanisms of these processes. It was shown in our earlier works [1, 2] that the temperature dependence of the reaction of dark recombination between photooxidized bacteriochlorophyll (P + ) and reduced primary quinone

Role Played by Water–Protein Surrounding of Cofactors in Picosecond Electron Transport Steps in Photosynthesis Reaction Centers

Effect of the water–protein environment of cofactors on the rates and efficiency of conversion of light energy into the energy of a photochemical potential in reaction centers of purple bacterium Rhodobacter sphaeroides is studied. The environment is modified by isotopic replacement D2O → H2O or by adding glycerol and dimethyl sulfoxide (DMSO). The replacement D → H is shown to makes no impact on the midpoint potential Em of the electron donor, whereas addition of 70 vol % of glycerol or 35 vol % of DMSO raises Em by 30 and 45 mV, respectively. Rate constants of charge separation ke and electron transport onto quinone kQ remained unchanged following glycerol addition, while deuteration and DMSO addition diminished ke and kQ by two to three times. In addition to the known component with a characteristic time of about 10 ns, a component with a duration of 0.5–0.8 ns appears in the recombination kinetics of charges of deuterated and DMSO-treated preparations of reaction centers. The mechanism of the environment response on the emergence of nonequilibrium states of cofactors is analyzed theoretically. The energy model proposed for primary processes of photosynthesis accounts for the contribution made by the environment in the realization of a highly effective electron transport.

Deformation of hydrogen bonds as a mechanism of stabilization of nonequilibrium states of photosynthetic cofactors

Photosynthetic reaction centers (RCs) of purple bacteria have the following cofactors: a dimer of bacteriochlorophyll (BChl) molecules (P), monomeric molecules of BChl and bacteriopheophytin (BPheo), and a quinone acceptor Q A . These cofactors are bound with hydrogen bonds to protein subunits L and M of an RC. The protein–pigment interaction plays a key role in the ability of the RC to mediate highly efficient (with a quantum yield about 100%) electron transport [1]. This conclusion is supported by a variety of direct and indirect evidence. For example, isotopic substitution H 2 O → D 2 O causes a twoto threefold decrease in the rates of energy migration, charge separation, and electron transport in the Rhodobacter sphaeroides RCs [2–4] and a 10–12% decrease in the photosynthetic efficiency in the system of RC cofactors P → BPheo → Q A . In a native RC, there is only one hydrogen bond between histidine residue L168 and the P L acetylcarbonyl group. Site-directed mutagenesis makes it possible to form any possible number of hydrogen bonds between P and amino acid residues. In mutant RCs with different numbers of hydrogen bonds between P and protein, the characteristic time of charge separation ranges from 3.5 ps (one hydrogen bond) to 50 ps (four hydrogen bonds) [5–7]. Mutants with different numbers of hydrogen bonds between P and the RC protein are also characterized by decreased rates of charge recombination P + → PQ A [7] and electron transfer from cytochrome (Cyt) to P: P + Cyt 2+ → PCyt 3+ [8]. Thus, the pigment–protein interactions in RC mediated by hydrogen bonds represent a fine mechanism of regulation of the efficiency of the initial photosynthetic processes.