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PubMed2004

Reduction of the thylakoid electron transport chain by stromal reductants--evidence for activation of cyclic electron transport upon dark adaptation or under drought.

The reduction of P700(+), the primary electron donor of photosystem I (PSI), following a saturating flash of white light in the presence of the photosystem II (PSII) inhibitor 3-(3.4-dichlorophenyl)-1,1-dimethylurea (DCMU), was examined in barley plants exposed to a variety of conditions. The decay kinetic fitted to a double exponential decay curve, implying the presence of two distinct pools of PSI. A fast component, with a rate constant for decay of around 0.03-0.04 ms(-1) was observed to be sensitive to the duration of illumination. This rate constant was slower than, but comparable to, that observed in non-inhibited samples (i.e. where linear flow was active). It was substantially faster than values typically reported for experiments where PSII activity is inhibited. The magnitude of this component rose in leaves that were dark-adapted or exposed to drought. This component was assigned to PSI centres involved in cyclic electron transport. The remaining slowly decaying P700(+) population (rate constant of around 0.001-0.002 ms(-1)) was assigned to centres normally involved in linear electron transport (but inhibited here because of the presence of DCMU), or inactivated centres involved in the cyclic pathway. Processes that might regulate the relative flux through cyclic electron transport are discussed.

Adaptation, Physiological↗

PubMed2000

Electronic structure contributions to electron transfer in blue Cu and Cu(A).

The experimentally determined electronic structures of mononuclear blue Cu and binuclear Cu(A) centers are summarized and their relation to intra- and inter-protein electron transfer (ET) kinetics are described. Specific contributions of the electronic structures of these two broad classes of Cu ET proteins to H(AB), lambda, and deltaE degrees are discussed. Also, the role of the protein structure in determining key geometric features which define the electronic structures of the metal sites in these proteins is considered.

Bacterial Proteins↗

PubMed1988

Kinetics of electron transfer between cytochrome c and iron hexacyanides. Evidence for two electron-transfer sites.

The electron-transfer reaction between ferrocytochrome c and ferricyanide has been studied by the method of photoexcitation. The observed transfer rate shows saturation behaviour at high ferricyanide concentration. Data analysis indicates that there are two binding sites of vastly different affinities at which electron transfer occurs. The binding constant for the strong binding site decreases from 1600 M-1 to 80 M-1 as the ionic strength increases from 15 mM to 140 mM. At 20 degrees C, the intramolecular electron-transfer rate for this site is 4.65 X 10(4) s-1, which gives an electron-transfer distance of approx. 9.7 A according to Hopfield's model.

Animals↗

PubMed1991

Fibrinogen structure in projection at 18 A resolution. Electron density by co-ordinated cryo-electron microscopy and X-ray crystallography.

Electron microscope images of frozen-hydrated crystals of a proteolytically modified fibrinogen show excellent preservation of the structure. An electron density map of the key centric projection of the crystal at 18 A resolution has been obtained by combining the phases derived from cryo-electron microscopy with X-ray amplitudes. Simulation methods developed in earlier studies have been used to interpret the map. In contrast to the earlier images, the map allows us to visualize the coiled-coil region of the molecule and possible substructure in the beta domains. The map also shows that there is a marked difference in density in the two regions corresponding to the molecular ends where the gamma domains interact. A possible interpretation of this finding is provided by assuming substructure in the gamma domains and the breaking of molecular symmetry where these domains interact. Some additional constraints useful for the determination of the three-dimensional structure were obtained from cryo-electron micrographs of a perpendicular view at 25 A resolution. Implications of this working model for the molecular length and contacts in the filaments in both the crystal and fibrin are described. The data used here will be valuable as a starting point for obtaining the three-dimensional structure.

Computer Simulation↗

PubMed1990

Identification of precipitable Ca2+ by electron spectroscopic imaging and electron energy loss spectroscopy in the organ of Corti of the guinea pig.

Ca2+ cations were precipitated with potassium antimonate in the cochlea of the guinea pig, and the formed precipitates were localized by electron microscopy using either elastically or inelastically scattered electrons. The elemental composition of the precipitates was determined by electron spectroscopic imaging (ESI) and electron energy loss spectroscopy (EELS). It was found that calcium, antimony and oxygen were the dominating elements in the precipitates formed in different cell types in the inner ear.

Animals↗

PubMed1994

A comparison of three electron planning algorithms for a 16 MeV electron beam.

PURPOSE: We report results of a comparison of three electron planning algorithms, an Age-Diffusion Pencil beam algorithm and two (2-D) and three dimensional (3-D) Hogstrom pencil beam algorithms, using simple 2 x 2 cm air and hard bone inhomogeneities and a complex anthropomorphic head and neck phantom. METHODS AND MATERIALS: The simple inhomogeneities have variable dimensions outside the plane of calculation to test the effects of out of plane scattering on 2-D algorithms, compared with dose measured by film below the inhomogeneity in the dose fall-off range. Comparisons are also made of a parotid treatment field for 16 MeV electrons, and the dose measured by high sensitivity thermoluminescent dosimeters in the head and neck phantom. RESULTS: Behind the simple inhomogeneities, the electron algorithms are found to underestimate the dose behind the air cavity by up to 40% and overestimated the dose behind bone by up to 30%. In the head phantom, the presence of inhomogeneities also presents problems for the algorithms, with overestimations of dose of up to 20% found behind bone-tissue interfaces, apparently due to shielding by high density bone. Overestimations of up to 17% are also found beside interfaces parallel to the beam. Underestimations of dose of up to 10% are found on the beam-side of interfaces, due to under-prediction of backscattered electrons. All three investigated algorithms underestimate the dose by up to 20% behind extreme surface curvature. One algorithm is found to underestimate the dose in the falloff region while another overestimates the dose around the 90% isodose. CONCLUSION: Clinicians should be aware of the limitations of their planning systems.

Algorithms↗

PubMed2005

Modulation of photosynthetic electron transport in the absence of terminal electron acceptors: characterization of the rbcL deletion mutant of tobacco.

Tobacco rbcL deletion mutant, which lacks the key enzyme Rubisco for photosynthetic carbon assimilation, was characterized with respect to thylakoid functional properties and protein composition. The Delta rbcL plants showed an enhanced capacity for dissipation of light energy by non-photochemical quenching which was accompanied by low photochemical quenching and low overall photosynthetic electron transport rate. Flash-induced fluorescence relaxation and thermoluminescence measurements revealed a slow electron transfer and decreased redox gap between Q(A) and Q(B), whereas the donor side function of the Photosystem II (PSII) complex was not affected. The 77 K fluorescence emission spectrum of Delta rbcL plant thylakoids implied a presence of free light harvesting complexes. Mutant plants also had a low amount of photooxidisible P700 and an increased ratio of PSII to Photosystem I (PSI). On the other hand, an elevated level of plastid terminal oxidase and the lack of F0 'dark rise' in fluorescence measurements suggest an enhanced plastid terminal oxidase-mediated electron flow to O2 in Delta rbcL thylakoids. Modified electron transfer routes together with flexible dissipation of excitation energy through PSII probably have a crucial role in protection of PSI from irreversible protein damage in the Delta rbcL mutant under growth conditions. This protective capacity was rapidly exceeded in Delta rbcL mutant when the light level was elevated resulting in severe degradation of PSI complexes.

Darkness↗

PubMed2006

Distance determination in human ubiquitin by pulsed double electron-electron resonance and double quantum coherence ESR methods.

Recently, distance measurements by pulsed ESR (electron spin resonance) have been obtained using pulsed DEER (double electron-electron resonance) and DQC (double quantum coherence) in SDSL (site directed spin labeling) proteins. These methods can observe long range dipole interactions (15-80A). We applied these methods to human ubiquitin proteins. The distance between the 20th and the 35th cysteine was estimated in doubly spin labeled human ubiquitin. Pulsed DEER requires two microwave sources. However, a phase cycle is not usually required in this method. On the other hand, DQC-ESR at X-band ( approximately 9GHz) can acquire a large echo signal by using pulses of short duration and high power, but this method has an ESEEM (electron spin echo envelope modulation) problem. We used a commercial pulsed ESR spectrometer and compared these two methods.

Algorithms↗

PubMed1998

Electron transfer in the heliobacterial reaction center: evidence against a quinone-type electron acceptor functioning analogous to A1 in photosystem I.

Membrane fragments from Heliobacillus mobilis were characterized using time resolved optical spectroscopy and photovoltage measurements in order to detect a possible participation of menaquinone (MQ), functioning analogous to the phylloquinone A1 in photosystem I, as intermediate in electron transfer from the primary acceptor A0 to the iron-sulfur cluster FX in the photosynthetic reaction center. The spectroscopic data obtained exclude that electron transfer from a semiquinone anion MQ- to FX occurred in the time window from 2 ns to 4 micros, where it would be expected in analogy to photosystem I. In the case of a prereduction of FX, only the primary pair P798+A0- was formed. The photovoltage data yielded a single kinetic phase with a time constant of 700 ps for the transmembrane electron transfer beyond A0; the relative amplitude of this phase suggests that it reflects electron transfer from A0- to FX.

Bacteria↗

PubMed2004

Interresidual distance determination by four-pulse double electron-electron resonance in an integral membrane protein: the Na+/proline transporter PutP of Escherichia coli.

Proximity relationships within three doubly spin-labeled variants of the Na+/proline transporter PutP of Escherichia coli were studied by means of four-pulse double electron-electron resonance spectroscopy. The large value of 4.8 nm for the interspin distance determined between positions 107 in loop 4 and 223 in loop 7 strongly supports the idea of these positions being located on opposite sides of the membrane. Significant smaller values of between 1.8 and 2.5 nm were found for the average interspin distances between spin labels attached to the cytoplasmic loops 2 and 4 (position 37 and 107) and loops 2 and 6 (position 37 and 187). The large distance distribution widths visible in the pair correlation functions reveal a high flexibility of the studied loop regions. An increase of the distance between positions 37 and 187 upon Na+ binding suggests ligand-induced structural alterations of PutP. The results demonstrate that four-pulse double electron-electron resonance spectroscopy is a powerful means to investigate the structure and conformational changes of integral membrane proteins reconstituted in proteoliposomes.

Amino Acid Transport Systems, Neutral↗

PubMed1980

Calcium deposition in human pituitary adenomas studied by histology, electron microscopy, electron diffraction and X-ray spectrometry.

The incidence and morphogenesis of calcification were investigated by histology, electron microscopy, electron diffraction and X-ray spectrometry in 312 surgically removed human pituitary adenomas. By light microscopy the incidence of calcification was 9%. The calcium deposits were evident in several adenoma types, but were most frequent and extensive in prolactin-producing adenomas. Histologically calcium deposits represented amorphous precipitates and psammoma bodies staining with hematoxylin, the von Kossa technique and alizarin red. By electron microscopy, calcified deposits were noted in the cytoplasm of adenoma cells and extracellular space. Electron diffraction and X-ray spectrometry of intracellular precipitates as well as psammoma bodies showed the presence of phosphate, calcium and organically bound sulphur with a ratio of 3.5:5:2, indicating that they contained calcium hydroxyapatite. Present findings provide evidence that mitochondria with microcrystal deposition of calcium represent the primary sites of calcification.

Adenoma↗

PubMed1987

Electron microscopic and immunolabeling studies of the lesional and normal skin of patients with mycosis fungoides treated by total body electron beam irradiation.

Biopsy specimens were taken from lesional and normal skin of nine patients with mycosis fungoides before and after total body electron beam therapy. By electron microscopy, lesional skin had one and one-half to ten times as many epidermal Langerhans cells and indeterminate cells as did the normal skin. In successfully treated lesional skin 1 month after the end of electron beam therapy, the density of epidermal Langerhans cells and indeterminate cells had decreased markedly. In incompletely resolved lesions, Langerhans cells and indeterminate cells were still at pretreatment levels. Epidermal T6 and Ia antigens showed the same pattern of response. Epidermal cell suspensions from lesional and normal skin before and after electron beam therapy were assayed for epidermal thymocyte activating factor. The values of production of this factor did not correlate with the source of the epidermal cells, response to therapy, or the patient's disease course. Skin lesions resembling xerosis and parapsoriasis and histologically lacking the criteria for mycosis fungoides appeared during clinical remissions. These nonspecific skin lesions had densities of epidermal Langerhans cells, indeterminate cells, and T6-positive and Ia-positive cells comparable to levels found in pretreatment lesional skin.

Antibodies, Monoclonal↗

PubMedDate not supplied

High-resolution electron microscopy and electron tomography: resolution versus precision.

The performance of high-resolution electron microscopy and electron tomography is usually discussed in terms of two-point resolution, expressing the possibility of perceiving separately two image points of an object. However, the concept resolution obtains another meaning if one uses prior knowledge about the object and the imaging procedure in the form of a parametric model describing the expectations of the observations. The unknown parameters, such as the positions of the components in an object, can be measured quantitatively by fitting this model to the observations. Due to the statistical nature of the experiment, the resulting solutions for the positions of the components and therefore for the distance between the components will never be exact. An alternative to resolution is then the precision with which the distance can be measured. In the present paper, it is shown that the precision depends on the size of the components, the distance between the components, the resolution of the instrument, and the number of electron counts. For electron tomography, it also depends on the orientation of the object with respect to the rotation axis.

Image Processing, Computer-Assisted↗

PubMed1982

Electron uptake and delivery sites on plastocyanin in its reactions with the photosynthetic electron transport system.

French bean plastocyanin is stoichiometrically and specifically labeled upon reduction by Cr(II)aq ions, yielding a substitution-inert (Cr(III) adduct at the protein surface. The effect of the modification on the activity of plastocyanin in electron transfer between photosystems II and I has been investigated. The photoreduction and photooxidation by chloroplasts or by photosystem I reaction centers, respectively, chloroplasts or by photosystem I reaction centers, respectively, of native and Cr(III)-labeled plastocyanin have been compared. It was found that whereas the photoreduction rates of native and Cr-labeled plastocyanin were indistinguishable, the rates of photooxidation of the modified protein were markedly attenuated relative to those of the native one. This difference in reactivity clearly reflects the perturbation of the electron transfer pathway to P700. These findings, in conjunction with the structure of plastocyanin and the locus of CR(III) binding on its surface, lead to the following interpretation: (a) There are most probably two physiologically significant, electron transfer sites on plastocyanin. (b) The site involved in the electron transfer to P700 is most likely in the region of tyrosine-83 and the negatively charged patch proximal to it. By elimination we assume that the second site is centered at the hydrophobic region of histidine-87.

Chloroplasts↗

PubMed1999

Calculated protein and proton motions coupled to electron transfer: electron transfer from QA- to QB in bacterial photosynthetic reaction centers.

Reaction centers from Rhodobacter sphaeroides were subjected to Monte Carlo sampling to determine the Boltzmann distribution of side-chain ionization states and positions and buried water orientation and site occupancy. Changing the oxidation states of the bacteriochlorophyll dimer electron donor (P) and primary (QA) and secondary (QB) quinone electron acceptors allows preparation of the ground (all neutral), P+QA-, P+QB-, P0QA-, and P0QB- states. The calculated proton binding going from ground to other oxidation states and the free energy of electron transfer from QA-QB to form QAQB- (DeltaGAB) compare well with experiment from pH 5 to pH 11. At pH 7 DeltaGAB is measured as -65 meV and calculated to be -80 meV. With fixed protein positions as in standard electrostatic calculations, DeltaGAB is +170 meV. At pH 7 approximately 0.2 H+/protein is bound on QA reduction. On electron transfer to QB there is little additional proton uptake, but shifts in side chain protonation and position occur throughout the protein. Waters in channels leading from QB to the surface change site occupancy and orientation. A cluster of acids (GluL212, AspL210, and L213) and SerL223 near QB play important roles. A simplified view shows this cluster with a single negative charge (on AspL213 with a hydrogen bond to SerL233) in the ground state. In the QB- state the cluster still has one negative charge, now on the more distant AspL210. AspL213 and SerL223 move so SerL223 can hydrogen bond to QB-. These rearrangements plus other changes throughout the protein make the reaction energetically favorable.

Benzoquinones↗

PubMed2000

Diethyl pyrocarbonate modification abolishes fast electron accepting ability of cytochrome b561 from ascorbate but does not influence electron donation to monodehydroascorbate radical: identification of the modification sites by mass spectrometric analysis.

Cytochrome b(561) from bovine adrenal chromaffin vesicles contains two heme B prosthetic groups and transports electron equivalents across the vesicle membranes to convert intravesicular monodehydroascorbate radical to ascorbate. To elucidate the mechanism of the transmembrane electron transfer, effects of the treatment of purified cytochrome b(561) with diethyl pyrocarbonate, a reagent specific for histidyl residues, were examined. We found that when ascorbate was added to the oxidized form of diethyl pyrocarbonate-treated cytochrome b(561), less than half of the heme iron was reduced but with a very slow rate. In contrast, radiolytically generated monodehydroascorbate radical was oxidized rapidly by the reduced form of diethyl pyrocarbonate-modified cytochrome b(561), as observed for untreated cytochrome b(561). These results indicate that the heme center specific for the electron acceptance from ascorbate was perturbed by the modification of amino acid residues nearby. We identified the major modification sites by mass spectrometry as Lys85, His88, and His161, all of which are fully conserved and located on the extravesicular side of cytochrome b(561) in the membranes. We suggest that specific N-carbethoxylation of the histidyl ligands of the heme b at extravesicular side abolishes the electron-accepting ability from ascorbate.

Amino Acid Sequence↗

PubMed2002

Intramolecular electron transfer in heterosubstituted benzene derivatives as probed by dissociative electron attachment.

The electron transmission and dissociative electron attachment spectra of the 1-chloroalkyl benzene derivatives, C(6)H(5)(CH(2))(3)Cl and C(6)H(5)(CH(2))(4)Cl, and of the sulfur and silicon derivatives, C(6)H(5)SCH(2)Cl, C(6)H(5)Si(CH(3))(2)CH(2)Cl and C(6)H(5)CH(2)Si(CH(3))(2)CH(2)Cl, are presented for the first time. The relative Cl(-) fragment anion currents generated by electron attachment to the benzene pi* LUMO are measured in the series C(6)H(5)(CH(2))(n)Cl, with n = 1-4, and in the heteroatomic compounds. The Cl(-) yield reflects the rate of intramolecular electron transfer between the pi-system and the remote chlorine atom, which in turn depends on the extent of through-bond coupling between the localized pi* and sigma*(Cl-C) orbitals. In compounds C(6)H(5)(CH(2))(n)Cl the Cl(-) current rapidly decreases with increasing length of the saturated chain. This decrease is significantly attenuated when a carbon atom of the alkyl skeleton is replaced with a third-row heteroatom. This greater ability to promote through-bond coupling between the pi* and sigma*(Cl-C) orbitals is attributed to the sizably lower energy of the empty sigma*(S-C) and sigma*(Si-C) orbitals with respect to the sigma*(C-C) orbitals. In the sulfur derivative the increase of the Cl(-) current is larger than in the silicon analogue. In this case, however, other negative fragments are observed, due to dissociation of the S-C bonds.

Benzene Derivatives↗

PubMed2003

The electronic structure of the photoexcited triplet state of free-base (tetraphenyl)porphyrin by time-resolved electron-nuclear double resonance and density functional theory.

The photoexcited triplet states of free-base porphyrin (H(2)P) and free-base tetraphenylporphyrin (H(2)TPP) have been investigated by time-resolved electron paramagnetic resonance and electron-nuclear double resonance in a toluene glass at 80 K. Both the zero-field splitting parameters, D and E, and the proton A(zz) hyperfine coupling tensor components could be determined. D is about 13% larger in H(2)P than in H(2)TPP. In contrast, however, the A(zz) hyperfine coupling tensor components showed differences of less than 2%. To aid the understanding of these results, the electronic structures of H(2)P and H(2)TPP have been modeled using density functional theory. The geometrical structures of both molecules in their lowest triplet states were calculated using the Becke3 Lee-Yang-Parr composite exchange correlation functional and the 6-31G* basis set. Hyperfine couplings for these structures were calculated using the same functional but with the extended EPR-II basis set. These allow unambiguous assignment of the experimentally determined couplings. The theoretical values for H(2)P and H(2)TPP agree with the experimental values in that the presence of the phenyl groups has only a small effect on the unpaired electron spin-density distribution. The difference in sensitivity of the zero-field splitting parameters and the hyperfine couplings to mesophenyl substitution is discussed in terms of the wave functions of the four frontier orbitals of porphyrins introduced by Gouterman.

Electron Spin Resonance Spectroscopy↗