1,721,048 research outputs found
Interpretation and chronostratigraphic mapping of multichannelseismic reflection profile I95167,Eastern Falkland Plateau (South Atlantic)
No full textThe Falkland Plateau (FP) is a submarine high representing the eastern prolongation of the South America continent. Itseparates the Argentine Basin of the South Atlantic Ocean from the Scotia Sea of the Antarctic region. A south–north seismicsection across the Falkland Plateau has been interpreted using information from two wells sited on the margin of the MauriceEwing Bank (MEB). On the Plateau, the sedimentary cover has a regionally horizontal configuration with high lateral continuityof the reflecting horizons. The older sequence is dominated by a normal fault system of the Mesozoic Gondwana break-uptectonics, partially reactivated by the following tectonic events. Between them, the one mainly affecting the studied area is thetranspression of the North Scotia Ridge (NSR), producing some thrusting in the sedimentary cover of the Falkland Trough (FT,visible only on the south-extremity of the interpreted section) and a prominent tilting of its whole crust. The interpreted seismicsection outlines a gradually thinner crust from the Maurice Ewing Bank Margin to the Falkland Plateau and to the FalklandTrough. During the interpretation, particular attention was given to picking the main unconformities, to analysing therelationships of reflections within the sequences to the unit boundaries, and to identifying, by the seismic facies analysis, thedifferent depositional units. These last, labelled with symbols AH G, from Mesozoic to Present age, represent the wholesedimentary cover lying on the pre-Cambrian basement of the Falkland Plateau. In the reconstructed chronostratigraphic section,the whole data set has been displayed as a diagram wherein age decreases upward and the lateral extent of units is on thehorizontal scale, illustrating the main sedimentary and erosional events in the area. Erosion, nondeposition and deposition ofsediment drifts by the sea bottom currents, particularly active from the Cainozoic to the Present, are mostly linked to theGondwana breakup, with the Mesozoic opening of the South Atlantic Ocean and the Cainozoic opening of the Drake Passage.During the Mesozoic time, the Proto-South Atlantic and Indian Oceans connection caused extensive erosion and nondeposition,resulting in absence of important portions of the sedimentary cover
Binding of ubiquinone to photosynthetic reaction centers. 2: Determination of enthalpy and entropy changes for the binding to the QA site in reverse micelles
No full textThe Q(A) Site binding properties of the purple non-sulfur bacterium Rhodobacter sphaeroides reaction centers solubilized in phospholipid-based reverse micelles have been determined. By means of time-resolved absorbance measurements, the binding of the ubiquinone-10 to the Q(A) Site has been followed at different temperatures and quinone concentrations yielding the relative binding constants. A global fit of the experimental data allowed us to get quite reliable values of the thermodynamic parameters joined to the binding process. Enthalpy and entropy changes obtained for the binding at the Q(A) Site (Delta H degrees(bind) = -75.3 +/- 3.4 kJ mol(-1) and Delta S degrees(bind) = -181 +/- 11 J mol(-1) K-1) confirm that the quinone binding to the primary site is stronger with respect to that at the Q(B) site. A Monte Carlo simulation of both the classical Van't Hoff and global analysis approaches is also presented, showing the higher reliability of the thermodynamic parameters derived with the latter method (uncertainty less than 1% with respect to more than 40% of the Van't Hoff analysis). Such an analysis indicates also that the enthalpy-entropy compensation previously observed through the ubiquinone series is likely due to a statistical artifacts
Binding of ubiquinone to photosynthetic reaction center: Determination of enthalpy and entropy changes in reverse micelles
No full textThe photosynthetic reaction center from the purple non-sulfur bacterium Rhodobacter sphaeroides has been solubilized in reverse micelles of phospholipids in n-hexane. The rate of the charge recombination from the secondary quinone acceptor (Q(B)) to the bacteriochlorophyll dimer (P) has been investigated, by flash absorption spectroscopy, as a function of the concentration of ubiquinone-10 at eight temperatures between 37 and 6 degrees C. Deconvolution of P+ decay shows that, in reverse micellar solutions, quinone molecules are in fast exchange between the Q(B) site of the protein and the bulk organic phase, with the exception of a small fraction of reaction centers lacking the Q(B) functionality, Global analysis of the kinetics of charge recombination permits proper separation of the contribution of the binding at Q(B) from that of the P(+)Q(A)(-)Q(B)-->P(+)Q(A)Q(B)(-) electron transfer. Enthalpy and entropy changes obtained for the electron transfer process (Delta H(AB)degrees = -0.140 +/- 0.016 meV; Delta S(AB)degrees = -2.01 x 10(-4) +/- 6.7 x 10(-5) meV/K) are in agreement with previous evaluations in detergent suspensions of reaction centers. Thermodynamic parameters of the enthalpically driven quinone binding at Q(B) (Delta H(bind)degrees = -0.526 +/- 0.058 meV; Delta S(bind)degrees = -1.36 x 10(-3) +/- 2.3 x 10(-4) meV/K) compare favorably with data in aqueous systems when considering a free energy change of about -4.77 kJ/mol for the transfer of ubiquinone-10 from a direct detergent micellar phase to a n-hexane reverse micellar solution. This comparison indicates that the quinone affinity for the Q(B) site is approximately the same in the (dark) PQ(A) and (light) P(+)Q(A)(-) states of the reaction center
Binding of ubiquinone to photosynthetic reaction centers. 2: Determination of enthalpy and entropy changes for the binding to the QA site in reverse micelles
No full textThe QA site binding properties of the purple non-sulfur bacterium Rhodobacter sphaeroides reaction centers solubilized in phospholipid-based reverse micelles have been determined. By means of time-resolved absorbance measurements, the binding of the ubiquinone-10 to the QA site has been followed at different temperatures and quinone concentrations yielding the relative binding constants. A global fit of the experimental data allowed us to get quite reliable values of the thermodynamic parameters joined to the binding process. Enthalpy and entropy changes obtained for the binding at the QA site (ΔH°bind = −75.3 ± 3.4 kJ mol-1 and ΔS°bind = −181 ± 11 J mol-1 K-1) confirm that the quinone binding to the primary site is stronger with respect to that at the QB site. A Monte Carlo simulation of both the classical Van't Hoff and global analysis approaches is also presented, showing the higher reliability of the thermodynamic parameters derived with the latter method (uncertainty less than 1% with respect to more than 40% of the Van't Hoff analysis). Such an analysis indicates also that the enthalpy−entropy compensation previously observed through the ubiquinone series is likely due to a statistical artifacts
Effect of detergent concentration on the thermal stability of a membrane protein: the case study of bacterial reaction center solubilized by N,N,-dimethyldodecylamine-N-oxide
No full textWe report on the response of reaction center (RC) from Rhodobacter sphaeroides (an archetype of membrane proteins) to the exposure at high temperature. The RCs have been solubilized in aqueous solution of the detergent N,N,-dimethyldodecylamine-N-oxide (LDAO). Changes in the protein conformation have been probed by monitoring the variation in the absorbance of the bacteriochlorine cofactors and modification in the efficiency of energy transfer from tryptophans to cofactors and among the cofactors (through fluorescence measurements). The RC aggregation taking place at high temperature has been investigated by means of dynamic light scattering. Two experimental protocols have been used: i) isothermal kinetics, in which the time evolution of RC after a sudden increase of the temperature is probed; ii) T-scans, in which the RCs are heated at constant rate. The analysis of the results coming from both the experiments indicate that the minimal kinetic scheme requires an equilibrium step and an irreversible process. The irreversible step is characterized by a activation energy of 20514 kJ/mol and is independent from the detergent concentration. Since the temperature dependence of the aggregation rate was found to obeys to the same law, the aggregation process is unfolding-limited. On the other hand, the equilibrium process between the native and a partially unfolded conformations was found to be strongly dependent on the detergent concentration. Increasing the LDAO content from 0.025 wt% to 0.5 wt% decreases the melting temperature from 49°C to 42°C. This corresponds to a sizeable ( 22 kJ/mol at 25°C) destabilization of the native conformation induced by the detergent. The nature of the aggregates formed by the denatured RCs depends on the temperature. For temperature below 60°C compact aggregates are formed while at 60°C the clusters are less dense with a scaling relation between mass and size close to that expected for diffusion-limited aggregation. The aggregates final size of formed at different temperatures indicate the presence of an even number of proteins suggesting that these clusters are formed by aggregation of dimers
Kinetics of charge recombination and distribution of the ubiquinone pool in reaction center - Light harvesting complexes purified from Rhodobacter sphaeroides
No full textWe have recently shown that in the reaction center – light harvesting complex 1 (RC-LH1) purified from the photosynthetic bacterium Rhodobacter sphaeroides, at pH=7.8, flash-induced P+QB-
recombines with an average rate constant ( = 0.3 s-1), significantly smaller than that measured in RC deprived of the LH1 ( = 1 s-1). Since the kinetics of P+QA- recombination is unaffected by the presence of the antenna, the P+QB- state appears to be energetically stabilized in core complexes
(1). The pH dependence of the P+QB- recombination kinetics has been analyzed in dimeric and monomeric forms of RC-LH1 and compared with that observed in RC deprived of the antenna. At 6.5 10.5, indicating the involvement of protonatable groups. The recombination kinetics, moreover, becomes progressively distributed at pH > 9. The width of the rate constant distribution (sigma = 0.3 s-1
at pH < 9.0) increases by more than one order of magnitude at pH 11.0, suggesting a variety of conformations, possibly differing in the protonation state. The observed pH dependence of sigma could
be explained when assuming that such conformations interconvert at a rate which is comparable to the rate of charge recombination at physiological pH values but is considerably lower at high pH
values. Under this condition the conformational heterogeneity becomes therefore observable. A similar behaviour was observed in chromatophores of Rhodobacter capsulatus FJ2, a c2- and cyminus
strain, in which the kinetics of P+QB- recombination could be accurately studied by avoiding any interference due to exogenous electron donors/acceptors.
The lipid complement of the examined RC-LH1 complexes, determined by Inductively Coupled Plasma Emission Spectroscopy of phosphorous, ranges between 200 and 400 phospolipid molecules
per RC. A large ubiquinone (UQ) pool, varying from 15 to 30 UQ molecules per RC was systematically found to be associated with the core complexes. When similar determinations are performed in chromatophores, it appears that the effective UQ concentration in the lipid phase of core complexes is at least three times higher than the average UQ concentration in the intact membrane. This finding argues strongly in favour of an in vivo heterogeneity in the distribution of
the quinone pool within the chromatophore bilayer
Interactions of photosynthetic reaction center with quinones in vesicles and reverse micelles
No full textElectron Transfer Kinetics in Photosynthetic Reaction Centres Embedded in Trehalose Glasses: Trapping of Conformational Substates at Room Temperature
No full textWe report on room temperature electron transfer in the reaction center (RC) complex purified from Rhodobacter sphaeroides. The protein was embedded in trehalose-water systems of different trehalose/water ratios. This enabled us to get new insights on the relationship between RC conformational dynamics and long-range electron transfer. In particular, we measured the kinetics of electron transfer from the primary reduced quinone acceptor (Q(A)(-)) to the primary photo oxidized donor (P(+)), by time-resolved absorption spectroscopy, as a function of the matrix composition. The composition was evaluated either by weighing (liquid samples) or by near infrared spectroscopy (highly viscous or solid glasses). Deconvolution of the observed, nonexponential kinetics required a continuous spectrum of rate constants, The average rate constant ( = 8.7 s(-1) in a 28% (w/w) trehalose solution) increases smoothly by increasing the trehalose/water ratio. In solid glasses, at trehalose/water ratios greater than or equal to97%, an abrupt increase is observed ( = 26.6 s(-1) in the driest solid sample). A dramatic broadening of the rate distribution function parallels the above sudden increase. Both effects fully revert upon rehydration of the glass. We compared the kinetics observed at room temperature in extensively dried water-trehalose matrices with the ones measured in glycerol-water mixtures at cryogenic temperatures and conclude that, in solid trehalose-water glasses, the thermal fluctuations among conformational substates are inhibited. This was inferred from the large broadening of the rate constant distribution for electron transfer obtained in solid glasses, which was due to the free energy distribution barriers having become quasi static. Accordingly, the RC relaxation from dark-adapted to light-adapted conformation, which follows primary charge separation at room temperature, is progressively hindered over the time scale of P(+)Q(A)(-) charge recombination, upon decreasing the water content. In solid trehalose-water glasses the electron transfer process resulted much more affected than in RC dried in the absence of sugar. This indicated a larger hindering of the internal dynamics in trehalose-coated RC, notwithstanding the larger amount of residual water present in comparison with samples dried in the absence of sugar
Confinement of cardiolipin and ubiquinone in reaction center core complexes purified from the photosynthetic bacterium Rhodobacter sphaeroides
No full textThe core complex formed by the reaction center and the light harvesting complex 1 (RC-LH1) was purified from the photosynthetic bacterium Rhodobacter sphaeroides. We analyzed the lipid and ubiquinone (UQ) complements copurifying with the RC-LH1 complex and with the peripheral antenna (LH2). ln RC-LH1 UQ was almost ten times more concentrated than in the LH2 and in the native membranes from which the complexes were extracted. The fractional lipid composition of the RC-LH1 complex also differed from that of lhe intact membranes, exhibiting a marked increase in cardiolipin concentration. We propose that the confinement of cardiolipin and ubiquinone observed within the RC-LH1 complex, plays a role in vivo in lhe
stabilization of the light-induced charge separation catalyzed by the RC
- …
