Hybrid system based on quantum dots and photosystem 2 core complex

We show that semiconductor nanocrystals (quantum dots, QD) can be used to increase the absorption capacity of pigment-protein complexes. In a mixture of photosystem 2 core complex (PS2) and QD, the fluorescence of the latter decreases several-fold due to the transfer of the absorbed energy to the PS2 core complex. We discuss Forster’s inductive-resonance mechanism as a possible way of energy transfer in donor-acceptor pairs QD-PS2 core complex. Calculations based on the experimental data show that the enhancement of PS2 fluorescence and the rate of Q_A reduction increase up to 60% due to efficient energy migration from QD to PS2.     

Temperature dependence of protein fluorescence in <Emphasis Type="Italic">Rb. sphaeroides</Emphasis> reaction centers frozen to 80 K in the dark or on the actinic light as the indicator of protein conformational dynamics

The differences in the average fluorescence lifetime (τ_av) of tryptophanyls in photosynthetic reaction center (RC) of the purple bacteria Rb. sphaeroides frozen to 80 K in the dark or on the actinic light was found. This difference disappeared during subsequent heating at the temperatures above 250 K. The computer-based calculation of vibration spectra of the tryptophan molecule was performed. As a result, the normal vibrational modes associated with deformational vibrations of the aromatic ring of the tryptophan molecule were found. These deformational vibrations may be active during the nonradiative transition of the molecule from the excited to the ground state. We assume that the differences in τ_av may be associated with the change in the activity of these vibration modes due to local variations in the microenvironment of tryptophanyls during the light activation.     

A Study of the Temperature Dependence of Tryptophan Fluorescence Lifetime in the Range of –170 to +20°С in Various Solvents

Biophysics - The temperature dependences (–170...+20°C) of the fluorescence lifetime of tryptophan molecules in an aqueous solution, in solutions of glycerol (50 and 75% by volume),...     

Probing the fluorescence emission kinetics of the photosynthetic apparatus of Rhodopseudomonas sphaeroides, strain 1760-1, on a picosecond pulse fluorometer

Using the pulse picosecond fluorometric technique the fluorescence properties of intact cells, isolated chromatophores and photosynthetic reaction centres were studied in bacteria Rhodopseudomonas sphaeroides, strain 1760-1.The fluorescent emission from reduced reaction centres excited by 694.3 nm light has a biphasic character, the lifetimes of the components being τ₁ = 15±8 ps and τ₂ = 250 ps. The faster component, τ₁, contributes to the integral fluorescence in the long wavelength region. It disappears with oxidation of the reaction centres and is attributed to photoactive bacteriochlorophyll P870. The slow component, τ, is apparently due to both bacteriochlorophyll P800 and bacteriopheophytin. The fluorescence from intact cells exhibits a monophasic pattern and decays with τ = 200 ps.The fluorescence emitted by chromatophores comprises two components with τ₃ = 200 ps and τ₄ = 4200 ps. The duration of fluorescence τ₃ increases to its maximum of 500–550 ps, as P870 is oxidized chemically or photochemically, while τ₄ remains unchanged. The fluorescence with a lifetime of 200 ps was ascribed to the photosystem and the 4200-ps fluorescence to bacteriochlorophyll which had lost its functional links with the photosystem.The rise time of the fluorescence emitted by chromatophores varies from 60 or 70 ps to 350 ps depending on the wavelength of the exciting light and the recorded spectral region. On the basis of our findings the rate for energy migration was estimated to be 10⁹ s^?1.     

The influence of hydrogen bonds on electron transfer rate in photosynthetic RCs

Hydrogen bonds formed between photosynthetic reaction centers (RCs) and their cofactors were shown to affect the efficacy of electron transfer. The mechanism of such influence is determined by sensitivity of hydrogen bonds to electron density rearrangements, which alter hydrogen bonds potential energy surface. Quantum chemistry calculations were carried out on a system consisting of a primary quinone Q(A), non-heme Fe(2+) ion and neighboring residues(.) The primary quinone forms two hydrogen bonds with its environment, one of which was shown to be highly sensitive to the Q(A) state. In the case of the reduced primary quinone two stable hydrogen bond proton positions were shown to exist on [Q(A)-His(M219)] hydrogen bond line, while there is only one stable proton position in the case of the oxidized primary quinone. Taking into account this fact and also the ability of proton to transfer between potential energy wells along a hydrogen bond, theoretical study of temperature dependence of hydrogen bond polarization was carried out. Current theory was successfully applied to interpret dark P(+)/Q(A)(-) recombination rate temperature dependence.     

The time course of non-photochemical quenching in phycobilisomes of Synechocystis sp. PCC6803 as revealed by picosecond time-resolved fluorimetry

As high-intensity solar radiation can lead to extensive damage of the photosynthetic apparatus, cyanobacteria have developed various protection mechanisms to reduce the effective excitation energy transfer (EET) from the antenna complexes to the reaction center. One of them is non-photochemical quenching (NPQ) of the phycobilisome (PB) fluorescence. In Synechocystis sp. PCC6803 this role is carried by the orange carotenoid protein (OCP), which reacts to high-intensity light by a series of conformational changes, enabling the binding of OCP to the PBs reducing the flow of energy into the photosystems. In this paper the mechanisms of energy migration in two mutant PB complexes of Synechocystis sp. were investigated and compared. The mutant CK is lacking phycocyanin in the PBs while the mutant ΔPSI/PSII does not contain both photosystems. Fluorescence decay spectra with picosecond time resolution were registered using a single photon counting technique. The studies were performed in a wide range of temperatures — from 4 to 300 K. The time course of NPQ and fluorescence recovery in darkness was studied at room temperature using both steady-state and time-resolved fluorescence measurements. The OCP induced NPQ has been shown to be due to EET from PB cores to the red form of OCP under photon flux densities up to 1000 μmol photons m^− 2 s^− 1. The gradual changes of the energy transfer rate from allophycocyanin to OCP were observed during the irradiation of the sample with blue light and consequent adaptation to darkness. This fact was interpreted as the revelation of intermolecular interaction between OCP and PB binding site. At low temperatures a significantly enhanced EET from allophycocyanin to terminal emitters has been shown, due to the decreased back transfer from terminal emitter to APC. The activation of OCP not only leads to fluorescence quenching, but also affects the rate constants of energy transfer as shown by model based analysis of the decay associated spectra. The results indicate that the ability of OCP to quench the fluorescence is strongly temperature dependent. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.     

PS II model based analysis of transient fluorescence yield measured on whole leaves of Arabidopsis thaliana after excitation with light flashes of different energies

Our recently presented PS II model (Belyaeva et al., 2008) was improved in order to permit a consistent simulation of Single Flash Induced Transient Fluorescence Yield (SFITFY) traces that were earlier measured by Steffen et al. (2005) on whole leaves of Arabidopsis (A.) thaliana at four different energies of the actinic flash. As the essential modification, the shape of the actinic flash was explicitly taken into account assuming that an exponentially decaying rate simulates the time dependent excitation of PS II by the 10 ns actinic flash. The maximum amplitude of this excitation exceeds that of the measuring light by 9 orders of magnitude. A very good fit of the SFITFY data was achieved in the time domain from 100 ns to 10 s for all actinic flash energies (the maximum energy of 7.5 × 10¹⁶ photons/(cm² flash) is set to 100%, the relative energies of weaker actinic flashes were of ∼8%, 4%, ∼1%). Our model allows the calculation and visualization of the transient PS II redox state populations ranging from the dark adapted state, via excitation energy and electron transfer steps induced by pulse excitation, followed by final relaxation into the stationary state eventually attained under the measuring light. It turned out that the rate constants of electron transfer steps are invariant to intensity of the actinic laser flash. In marked contrast, an increase of the actinic flash energy by more than two orders of magnitude from 5.4 × 10¹⁴ photons/(cm² flash) to 7.5 × 10¹⁶ photons/(cm² flash), leads to an increase of the extent of fluorescence quenching due to carotenoid triplet (³Car) formation by a factor of 14 and of the recombination reaction between reduced primary pheophytin (Phe⁻) and P680⁺ by a factor of 3 while the heat dissipation in the antenna complex remains virtually constant.The modified PS II model offers new opportunities to compare electron transfer and dissipative parameters for different species (e.g. for the green algae and the higher plant) under varying illumination conditions.     

Membrane fluidity controls redox-regulated cold stress responses in cyanobacteria

Membrane fluidity is the important regulator of cellular responses to changing ambient temperature. Bacteria perceive cold by the transmembrane histidine kinases that sense changes in thickness of the cytoplasmic membrane due to its rigidification. In the cyanobacterium Synechocystis, about a half of cold-responsive genes is controlled by the light-dependent transmembrane histidine kinase Hik33, which also partially controls the responses to osmotic, salt, and oxidative stress. This implies the existence of some universal, but yet unknown signal that triggers adaptive gene expression in response to various stressors. Here we selectively probed the components of photosynthetic machinery and functionally characterized the thermodynamics of cyanobacterial photosynthetic membranes with genetically altered fluidity. We show that the rate of oxidation of the quinone pool (PQ), which interacts with both photosynthetic and respiratory electron transport chains, depends on membrane fluidity. Inhibitor-induced stimulation of redox changes in PQ triggers cold-induced gene expression. Thus, the fluidity-dependent changes in the redox state of PQ may universally trigger cellular responses to stressors that affect membrane properties.