Search results for "centre"
showing 10 items of 478 documents
Visualizing a protein quake with time-resolved X-ray scattering at a free-electron laser
2014
We describe a method to measure ultrafast protein structural changes using time-resolved wide-angle X-ray scattering at an X-ray free-electron laser. We demonstrated this approach using multiphoton excitation of the Blastochloris viridis photosynthetic reaction center, observing an ultrafast global conformational change that arises within picoseconds and precedes the propagation of heat through the protein. This provides direct structural evidence for a 'protein quake': the hypothesis that proteins rapidly dissipate energy through quake-like structural motions. peerReviewed
Ultrafast Electron Transfer in Photosynthesis: Reduced Pheophytin and Quinone Interaction Mediated by Conical Intersections
2007
The mechanism of electron transfer (ET) from reduced pheophytin (Pheo−) to the primary stable photosynthetic acceptor, a quinone (Q) molecule, is addressed by using high‐level ab initio computations and realistic molecular models. The results reveal that the ET process involving the (Pheo−+Q) and (Pheo+Q−) oxidation states can be essentially seen as an ultrafast radiationless transition between the two hypersurfaces taking place via conical intersections (CIs) and it is favoured when the topology of the interacting moieties make possible some overlap between the lowest occupied molecular orbitals (LUMO) of the two systems. Thus, it is anticipated that large scale motions, which are difficul…
Red Spectral Forms of Chlorophylls in Green Plant PSI - A Site-Selective and High-Pressure Spectroscopy Study
2003
One of the special spectroscopic characteristics of photosystem I (PSI) complexes is that they possess absorption and emission bands at lower energy than those of the reaction center. In this paper, the red pigment pools of PSI-200, PSI-core, and LHCI complex from Arabidopsis thaliana have been characterized at low temperatures by means of spectrally selective (hole-burning and fluorescence line-narrowing) and high-pressure spectroscopic techniques. It was shown that the green plant PSI-200 complex has at least three red pigment pools, from which two are located in the PSI-core and one, in the peripheral light-harvesting complex I (LHCI). All of the red pigment pools are characterized by st…
Enhanced electron-transfer properties of cofacial porphyrin dimers through pi-pi interactions
2009
pi-pi assisted: Photoinduced electron transfer from cofacial porphyrin dimers to electron acceptors is prominently accelerated, whereas the back electron transfer is decelerated, relative to the corresponding porphyrin monomer (see figure).The radical cation of zinc tetrapentylporphyrin is dimerized with an excess of the neutral counterpart to form the dimer radical cation in which the unpaired electron is delocalized over both porphyrin rings. The dimeric radical cation exhibits an NIR absorption spectrum characteristic of weak pi-bond formation between the porphyrin rings. When cofacial porphyrin dimers, linked by different spacers, are oxidized such pi-bond formation between the porphyri…
Single amino acids in the lumenal loop domain influence the stability of the major light-harvesting chlorophyll a/b complex.
2004
The major light-harvesting complex of photosystem II (LHCIIb) is one of the most abundant integral membrane proteins. It greatly enhances the efficiency of photosynthesis in green plants by binding a large number of accessory pigments that absorb light energy and conduct it toward the photosynthetic reaction centers. Most of these pigments are associated with the three transmembrane and one amphiphilic alpha helices of the protein. Less is known about the significance of the loop domains connecting the alpha helices for pigment binding. Therefore, we randomly exchanged single amino acids in the lumenal loop domain of the bacterially expressed apoprotein Lhcb1 and then reconstituted the muta…
Picosecond time-resolved study on the nature of high-energy-state quenching in isolated pea thylakoids different localization of zeaxanthin dependent…
1996
Abstract The influence of the transthylakoid proton gradient on the kinetics of picosecond fluorescence decay was examined using isolated pea thylakoids having high or low zeaxanthin contents. Fluorescence lifetime measurements were performed with open (Fo) and closed (Fm) PS II reaction centers. Zeaxanthin formation in membrane energized isolated thylakoids led to a marked decrease of the average fluorescence lifetime at both Fm and Fo. In contrast, when zeaxanthin synthesis was blocked by the inhibitor DTT, the fluorescence lifetime decrease was less pronounced in the Fm state and totally missing in the Fo state. Samples containing the uncoupler ammonium chloride did not exhinit any zeaxa…
Myoglobin embedded in saccharide amorphous matrices: water-dependent domains evidenced by small angle X-ray scattering
2010
We report Small Angle X-ray Scattering (SAXS) measurements performed on samples of carboxy-myoglobin (MbCO) embedded in low-water trehalose glasses. Results showed that, in such samples, "low-protein" trehalose-water domains are present, surrounded by a protein-trehalose-water background; such finding is supported by Infrared Spectroscopy (FTIR) measurements. These domains, which do not appear in the absence of the protein and in analogous sucrose systems, preferentially incorporate the incoming water at the onset of rehydration, and disappear following large hydration. This observation suggests that, in organisms under anhydrobiosis, analogous domains could play a buffering role against th…
Quantum Chemical Simulations of Excited-State Absorption Spectra of Photosynthetic Bacterial Reaction Center and Antenna Complexes
2011
The semiempirical ZINDO/S CIS configuration interaction method has been used to study the ground- and excited-state absorption spectra of wild type and heterodimer M202HL reaction centers from purple bacterium Rhodobacter sphaeroides as well as of peripheral LH2 and LH3 light harvesting complexes from purple bacterium Rhodopseudomonas acidophila. The calculations well reproduce the experimentally observed excited-state absorption spectra between 1000 and 17,000 cm(-1), despite the necessarily limited number of chromophores and protein subunits involved in the calculations. The electron density analysis reveals that the charge transfer between adjacent chromophores dominates the excited-stat…
Symmetry Breaking and Establishment of Dorsal/Ventral Polarity in the Early Sea Urchin Embryo
2015
The mechanisms imposing the Dorsal/Ventral (DV) polarity of the early sea urchin embryo consist of a combination of inherited maternal information and inductive interactions among blastomeres. Old and recent studies suggest that a key molecular landmark of DV polarization is the expression of nodal on the future ventral side, in apparent contrast with other metazoan embryos, where nodal is expressed dorsally. A subtle maternally-inherited redox anisotropy, plus some maternal factors such as SoxB1, Univin, and p38-MAPK have been identified as inputs driving the spatially asymmetric transcription of nodal. However, all the mentioned factors are broadly distributed in the embryo as early as no…
Shape coexistence in Hg-178
2019
Lifetime measurements of excited states in Hg-178 have been performed using the Rh-103(Kr-78, p2n) reaction at a beam energy of 354 MeV. The recoil-decay tagging (RDT) technique was applied to select the Hg-178 nuclei and associate the prompt gamma rays with the correlated characteristic ground-state alpha decay. Lifetimes of the four lowest yrast states of Hg-178 have been determined using the recoil distance Doppler-shift (RDDS) method. The experimental data are compared to theoretical predictions with focus on shape coexistence. The results confirm the shift of the deformed prolate structures to higher lying states but also indicate their increasing deformation with decreasing neutron nu…