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At least 181 records · Page 10Linked to original sources

PubMed1998

Site-directed mutagenesis of Phe 97 to Glu in amicyanin alters the electronic coupling for interprotein electron transfer from quinol methylamine dehydrogenase.

Conversion by site-directed mutagenesis of Phe 97 of amicyanin to Glu significantly decreases the rate constant for the electron-transfer reaction from the quinol form of methylamine dehydrogenase to amicyanin. It is shown that the DeltaG degrees and reorganizational energy (lambda) associated with the electron-transfer reaction are unaffected by the mutation and that the decrease in the electron-transfer rate is attributable completely to a decrease in the electronic coupling matrix element (HAB). Phe 97 is not a part of the predicted pathway for electron-transfer from the tryptophan tryptophylquinone cofactor of MADH to the copper of amicyanin. The most likely explanation for these results is that the mutation of this residue at the protein-protein interface causes an increase in the interprotein distance within the protein complex. The change in distance necessary to cause the observed reduction of HAB is calculated assuming a range of beta values, and assuming either solely a direct distance dependence or a pathway dependence, for the long-range electron-transfer reaction. Thermodynamic analysis of the association constants for complex formation reveal that the reaction with the mutant amicyanin exhibits a large positive change in heat capacity whereas this is not observed in the reaction with the wild-type. This may be explained by the replacement of a hydrophobic residue with a polar residue at what is normally a hydrophobic protein-protein interface. The impact of these results on possible explanations for the relatively large reorganizational energy associated with this interprotein electron-transfer reaction is also discussed.

Bacterial Proteins↗

PubMed2003

Effects of hydrogen bonding on metal ion-promoted intramolecular electron transfer and photoinduced electron transfer in a ferrocene-quinone dyad with a rigid amide spacer.

A ferrocene-quinone dyad (Fc-Q) with a rigid amide spacer and Fc-(Me)Q dyad, in which the amide proton acting as a hydrogen-bonding acceptor is replaced by the methyl group, are employed to examine the effects of hydrogen bonding on both the thermal and the photoinduced electron-transfer reactions. The hydrogen bonding of the semiquinone radical anion with the amide proton in Fc-Q(.-) produced by the electron-transfer reduction of Fc-Q is indicated by the significant positive shift of the one-electron reduction potential of Fc-Q. The hyperfine coupling constants of Fc-Q(.-) also indicate the existence of hydrogen bonding, agreeing with those predicted by the density functional calculation. The hydrogen-bonding dynamics in the photoinduced electron transfer from the ferrocene (Fc) to the quinone moiety (Q) in Fc-Q have been successfully detected in the femtosecond laser flash photolysis experiments. Thermal intramolecular electron transfer from Fc to Q in Fc-Q and Fc-(Me)Q also occurs efficiently in the presence of metal ions in acetonitrile at 298 K. The hydrogen bond formed between the semiquinone radical anion and the amide proton in Fc-Q results in remarkable acceleration of the rate of metal ion-promoted electron transfer as compared to the rate of Fc-(Me)Q in which hydrogen bonding is prohibited. The metal ion-promoted electron-transfer rates are well correlated with the binding energies of superoxide ion-metal ion complexes, which are derived from the g(zz) values of the ESR spectra.

Amides↗

PubMed2003

Density functional theory studies of electron interaction with DNA: can zero eV electrons induce strand breaks?

The discovery of DNA strand breaks induced by low energy secondary electrons sparks a necessity to elucidate the mechanism. Through theoretical studies based on a sugar-phosphate-sugar model that mimics a backbone section of the DNA strand, it is found that bond cleavages at 3' or 5'C-O sites after addition of an electron are possible with a ca. 10 kcal/mol activation barrier. Moreover, the potential energy surfaces show that dissociation at both sites is highly favorable thermodynamically. Although the phosphate group in DNA is not a favored site for electron attachment because of competitive electron transfer to the bases, any electrons which attach to phosphates on first encounter may induce strand breaks even when the electron energy is near zero eV. These findings have profound implication as low energy secondary electrons are abundantly generated in all types of ionization radiation.

DNA↗

PubMed2006

The direct electron transfer of myoglobin based on the electron tunneling in proteins.

The electron tunneling of the protein-polypeptide interactions was observed in the study of direct electron transfer of the myoglobin (Mb) on the electrode surface. The Mb was selected as a redox active protein and gelatine was selected to couple with Mb to form an electron tunneling. The electrochemical results indicated the presence of the electron tunneling and the direct electron transfer. The circular dichroism spectra suggested that the beta-sheet chain of gelatine could interact with alpha-helical chain to form an electron tunneling to promote the protein direct electrochemistry. The SDS-PAGE results proved that the electron tunneling between Mb and gelatine was noncovalent hydrogen bonds. The immobilized Mb showed a couple of quasi-reversible redox peaks with a formal potential of -0.37V (vs SCE) in 0.1 M pH 7.0 PBS. The modified electrodes displayed a rapid amperometric response to the reduction of oxygen, H2O2, and nitrite.

Circular Dichroism↗

PubMed2000

Recruitment of a foreign quinone into the A(1) site of photosystem I. II. Structural and functional characterization of phylloquinone biosynthetic pathway mutants by electron paramagnetic resonance and electron-nuclear double resonance spectroscopy.

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance studies of the photosystem (PS) I quinone acceptor, A(1), in phylloquinone biosynthetic pathway mutants are described. Room temperature continuous wave EPR measurements at X-band of whole cells of menA and menB interruption mutants show a transient reduction and oxidation of an organic radical with a g-value and anisotropy characteristic of a quinone. In PS I complexes, the continuous wave EPR spectrum of the photoaccumulated Q(-) radical, measured at Q-band, and the electron spin-polarized transient EPR spectra of the radical pair P700(+) Q(-), measured at X-, Q-, and W-bands, show three prominent features: (i) Q(-) has a larger g-anisotropy than native phylloquinone, (ii) Q(-) does not display the prominent methyl hyperfine couplings attributed to the 2-methyl group of phylloquinone, and (iii) the orientation of Q(-) in the A(1) site as derived from the spin polarization is that of native phylloquinone in the wild type. Electron spin echo modulation experiments on P700(+) Q(-) show that the dipolar coupling in the radical pair is the same as in native PS I, i.e. the distance between P700(+) and Q(-) (25.3 +/- 0.3 A) is the same as between P700(+) and A(1)(-) in the wild type. Pulsed electron-nuclear double resonance studies show two sets of resolved spectral features with nearly axially symmetric hyperfine couplings. They are tentatively assigned to the two methyl groups of the recruited plastoquinone-9, and their difference indicates a strong inequivalence among the two groups when in the A(1) site. These results show that Q (i) functions in accepting an electron from A(0)(-) and in passing the electron forward to the iron-sulfur clusters, (ii) occupies the A(1) site with an orientation similar to that of phylloquinone in the wild type, and (iii) has spectroscopic properties consistent with its identity as plastoquinone-9.

Alkyl and Aryl Transferases↗

PubMed1978

Calculated yields and slowing-down spectra for electrons in liquid water: implications for electron and photon RBE.

Detailed Monte Carlo calculations have been carried out of slowing-down spectra and yields for a number of end-points for electrons in liquid water. These investigations were made to study differences in physical effects of different low-LET radiations and implications for RBE. Initial electron energies from 1 keV to 1 MeV were used, and all secondary electrons were followed in the computations unitl their energies fell below 10 eV. Though there are substantial differences in the slowing-down spectra at energies near and above the K-shell ionisation potential of oxygen, the energy spectrum of electrons at lower energies is found to be essentially independent of the initial energy of the primary electron. The number of events per unit energy deposited is also essentially independent of the primary electron energy. Based on these calculations, there appears to be little basis for ascribing differences in RBE for low-LET radiations to differences in physical effects produced by secondary electrons of low energy (less than or equal to 1 keV).

Electrons↗

PubMed2002

Comparison of Monte Carlo calculated electron slowing-down spectra generated by 60Co gamma-rays, electrons, protons and light ions.

When analysing the factors affecting the relative biological effectiveness (RBE) of different radiation qualities, it is essential to consider particularly the low-energy slowing-down electrons (around 100 eV to 1 keV) since they have the potential of inflicting severe damage to the DNA. We present a modified and extended version of the Monte Carlo code PENELOPE that enables scoring of slowing-down spectra. mean local energy imparted spectra and average intra-track nearest-neighbour energy deposition distances of the secondary electrons generated by different radiation qualities, such as electrons, photons, protons and light ions in general. The resulting spectra show that the low-linear energy transfer (LET) beams, 60Co gamma-rays and electrons with initial energies of 0.1 MeV and higher, have as expected approximately the same electron slowing-down fluence per unit dose in the biologically important low-energy interval. Consistent with the general behaviour of the RBE of low-energy electrons, protons and light ions, the low-energy electron slowing-down fluence per unit dose is larger than for low-LET beams, and it increases with decreasing initial projectile energy.

Cobalt Radioisotopes↗

PubMed2003

Alteration of protein constituents induced by low-energy (<35 eV) electrons: II. Dissociative electron attachment to amino acids containing cyclic groups.

We report measurements of the desorption of anions from thin condensed films of tryptophan (Trp), histidine (His) and proline (Pro) stimulated by 5-35 eV electron impact. H-, O-, OH- and CN- desorb from Trp, His and Pro, whereas CH2- is observed only from Pro fragmentation. Below 12 eV, the anion yield functions exhibit resonant structures indicative of dissociative electron attachment. For all three amino acids, this process is likely to be initiated by the resonant capture of the incident electron at the NH3(+)-CH-.....-COO- and/or NH2-CH-.....-COOH group of the molecule. Temporary electron attachment to the ring leads to anion desorption only for tryptophan and proline. The energy-averaged yields measured at the detector of the mass spectrometer are (4.9, 0.3 and 54.0) x 10(-8) H-/incident electron and (3.4, 2.9, 1.8) x 10(-11) O-/incident electron, respectively, from Trp, His and Pro dissociation. Fragmentation of amino acids is found to be as intense as that of the nucleic acid bases. These results are discussed within the context of radiobiological damage induced by secondary electrons.

Amino Acids↗

PubMed2002

[Measurement of the mean energy of the off-axis incident electron without the beam guide: using a scintillating fiber beam-energy monitor for electron beam therapy].

In the case of total skin electron therapy without the beam guide, the electron beam is scattered just outside the gantry exit, dose uniformity in the field is broken, and dose is spread outside the light field. The aims of this study were to measure the mean energy of the off-axis incident electron beam without the beam guide and to establish a reference for the clinical situation. For the measurement, a 4 MeV electron beam was selected among several energies from the linear accelerator. A scintillating fiber beam energy monitor measured the mean energy of the incident electron beam. This energy monitor is a small, light-weight piece of equipment composed of a wedge absorber, scintillation fiber, and photodiode. We found the relationship between electron energy and the indicated value of the energy monitor by means of the estimation of correction factors for five different kinds of electron energy. The preferable linear correlation of 0.997 of the coefficient of determination (R(2)) was obtained. From the results of measurement at each point, those variations due to the off-axial distance were about 5% within the measured area. It was assumed that the energy did not change rapidly beyond the light field. Clinically, this amount of variation in energy may not cause any problem.

Electrons↗

PubMed1991

Backscattered electron imaging for high resolution surface scanning electron microscopy with a new type YAG-detector.

Double Layer Coating for backscattered electron imaging is a coating and imaging method especially suitable for high resolution scanning electron microscopy (SEM) of large biological samples. Since the backscattered electron (BSE) signal from thin metal coating layers is quite low, field emission SEM's and very sensitive BSE-detectors are required for this method. In this study an improved BSE-detector of the YAG type was used with an in-lens type field emission SEM. Two samples were investigated in order to demonstrate and to improve the potential of this new approach: (1) cryo-prepared cultured kidney cells were shadowed by electron beam evaporation with platinum-carbon (unidirectionally at a fixed angle of 45 degrees) and then coated with an additional 10 nm carbon layer; and (2) cryo-prepared trichocyst matrixes (paracrystalline structures contained in secretory granules, the trichocysts, found in Paramecium) were coated by ion beam sputtering with about 1 nm of platinum. This sample was rotated and tumbled during coating in order to obtain as uniform a metal layer as possible and then an additional 10 nm carbon layer was evaporated over the metal. When these samples were viewed at a primary beam accelerating voltage (Vo) of 10 kV or higher, contrast was good on the unidirectionally coated cell culture samples. However, trichocyst matrixes with the thinner and more uniform coating showed very poor contrast because most of the BSE detected represented beam-specimen interactions from the bulk of the sample and not in the thin platinum layer. The situation was improved by using low Vo (4 kV). Under these conditions the penetration depth of the electrons is reduced and a greater proportion of the BSE electrons are scattered by the platinum layer. The results were compared with freeze-fracture and deep-etch transmission electron microscope studies of the trichocyst matrixes from the literature: Almost similar resolution is achieved on the biological structures but a better impression of the three dimensional arrangement of the whole trichocyst matrix is obtained with the SEM. The globular particles form disc-like structures that are connected with each other by thin fibers.

Animals↗

PubMed1983

[Concept of the mean stopping power and the mean electron energy in electron dosimetry].

In consideration of the variations of the values E,Er,EH and Ew of the energy spectrum of electrons at the measuring point in case of a given energy on the surface Eo and in the depth t of a phantom with a low Z-number, the authors present the practical realisation of a conception allowing to determine by one single conversion curve Sw,1 (Er) the energy-dose of rapid electrons in water Dw, at least within the spectrum of Eo = 5 to 42 MeV and Er = 2.2 to 42 MeV, from the cavity ion dose Jc measured absolutely or by means of a calibrated electron chamber. Sw,1 (Er) is the relative unrestricted mass collision stopping power as a function of the mean rest energy Er at the measuring point. Er can be calculated in a simple manner from Eo and t and coincides almost perfectly with the most probable energy Ew of measured electron spectra. The authors present an algorithm for the calculation of the true mean electron energy E from Eo and t with a good coincidence with the values of measured electron spectra. It is shown that, contrary to the curve with one parameter Sw,1 (Er), a relation of Sw,1 to E leads to a series of curves with two parameters Sw,1 (Eo, t). The method using a relation of Sw,1 to Er which offers the advantage of a simple practical application and good precision is presented as "conception of the mean stopping power in electron dosimetry".

Electrons↗

PubMed1980

Studies on electron transfer from general acyl-CoA dehydrogenase to electron transfer flavoprotein.

General acyl coenzyme A dehydrogenase from pig liver mitochondria, which was prepared as a complex with C8CoA and mixed with electron-transfer flavoprotein, rapidly reduces the electron-transfer flavoprotein to a 1-electron-reduced form (anionic semiquinone). A second electron is transferred more slowly to form the fully reduced electron-transfer flavoprotein. Transfer of the first electron is faster than turnover in the dichlorophenolindophenol reduction assay. These observations show that the acyl-CoA dehydrogenase-electron-transfer flavoprotein system utilizes this semiquinone catalytically. A concomitant appearance of semiquinone from the general acyl-CoA dehydrogenase could not be detected under similar conditions.

Acyl-CoA Dehydrogenases↗

PubMed2001

Nanocrystalline Domain Identification in Gold Films, by Backscattered Electron Imaging and Energy-Filtered Transmission Electron Microscopy.

Gold nanocrystallites dispersed in an inhomogeneous gold matrix are detected by high-resolution scanning electron microscopy using a field emission source and backscattered electron detection in the composition mode, as well as by energy-filtered transmission electron microscopy in the plasmon energy region. The identity of the nanocrystalline domains was established by observing the same evaporated gold film samples but using bright-field, dark-field, electron diffractogram, and electron energy loss spectroscopy images in the transmission electron microscope. Comparison of these images shows that backscattered electron and plasmon energy detection can be used to identify crystalline domains in an otherwise chemically uniform sample. Copyright 2001 Academic Press.

Journal Article↗

PubMed1995

Electronic quality of life questionnaires: a comparison of pen-based electronic questionnaires with conventional paper in a gastrointestinal study.

The use of pen-based electronic questionnaires and conventional paper questionnaires was compared in a randomized crossover study. Forty-six patients, aged 17-81 years, suffering from gastro-intestinal disorders, initially filled in a paper quality of life questionnaire for familiarization purposes, then on two subsequent visits completed electronic and paper questionnaires in randomized order. At the last visit they completed a preference survey. The results showed a high degree of acceptability of the electronic questionnaire, with 57% of patient preferring electronic and 13% preferring paper, while the remaining 30% expressed no preference. Neither age, gender nor familiarity with technology showed any marked association with patients' preferences. All patients found both paper and electronic questionnaires easy to use. Data were more complete on the electronic questionnaire (100%) than on the paper (99.1%). Data handling procedures were greatly simplified. These results show that major benefits in completeness of data, speed of data flow, and data handling workload can be obtained from the use of pen-based electronic questionnaires.

Adolescent↗

PubMed2006

Effect of two intermediate electron donors, NADPH and FADH(2), on Spirulina Delta (6)-desaturase co-expressed with two different immediate electron donors, cytochrome b (5) and ferredoxin, in Escherichia coli.

When the gene desD encoding Spirulina Delta(6)-desaturase was heterologously expressed in E. coli, the enzyme was expressed without the ability to function. However, when this enzyme was co-expressed with an immediate electron donor, i.e. the cytochrome b (5) domain from Mucor rouxii, the results showed the production of GLA (gamma-linolenic acid), the product of the reaction catalyzed by Delta(6)-desaturase. The results revealed that in E. coli cells, where cytochrome b (5) is absent and ferredoxin, a natural electron donor of Delta(6)-desaturase, is present at a very low level, the cytochrome b (5) domain can complement for the function of ferredoxin in the host cells. In the present study, the Spirulina-ferredoxin gene was cloned and co-expressed with the Delta(6)-desaturase in E. coli. In comparison to the co-expression of cytochrome b ( 5 ) with the Delta(6)-desaturase, the co-expression with ferredoxin did not cause any differences in the GLA level. Moreover, the cultures containing the Delta(6)-desaturase co-expressed with cytochrome b (5) and ferredoxin were exogenously supplied with the intermediate electron donors, NADPH (nicotinamide adenine dinucleotide phosphate, reduced form) and FADH(2) (flavin adenine dinucleotide, reduced form), respectively. The GLA level in these host cells increased drastically, by approximately 50%, compared to the cells without the intermediate electron donors. The data indicated that besides the level of immediate electron donors, the level of intermediate electron donors is also critical for GLA production. Therefore, if the pools of the immediate and intermediate electron donors in the cells are manipulated, the GLA production in the heterologous host will be affected.

Base Sequence↗

PubMed2006

Probing the electronic structure of ZnO nanowires by valence electron energy loss spectroscopy.

Valence electron energy loss spectroscopy in a transmission electron microscope is employed to investigate the electronic structure of ZnO nanowires with diameter ranging from 20 to 100 nm. Its excellent spatial resolution enables this technique to explore the electronic states of a single nanowire. We found that all of the basic electronic structure characteristics of the ZnO nanowires, including the 3.3 eV band gap, the single electron interband transitions at approximately = 9.5, approximately = 13.5,and approximately = 21.8 eV, and the bulk plasmon oscillation at approximately 18.8 eV, resemble those of the bulk ZnO. Momentum transfer resolved energy loss spectra suggest that the 13.5 eV excitation is actually consisted of two weak excitations at approximately = 12.8 and approximately = 14.8 eV, which originate from transitions of two groups of the Zn 3d electrons to the empty density of states in the conduction band, with a dipole-forbidden nature. The energy loss spectra taken from single nanowires of different diameters show several size-dependent features, including an increase in the oscillator strength of the surface plasmon resonance at approximately = 11.5 eV, a broadening of the bulk plasmon peak, and splitting of the O 2s transition at approximately = 21.8 eV into two peaks, which coincides with a redshift of the bulk plasmon peak, when the nanowire diameter decreases. All these observations can be well explained by the increased surface/volume ratio in nanowires of small diameter.

Journal Article↗

PubMed1997

Electron transfer and electronic energy relaxation under high hydrostatic pressure.

The following question has been addressed in the present work. How external high (up to 8 kbar) hydrostatic pressure acts on photoinduced intramolecular electron transfer and on exciton relaxation processes? Unlike phenomena, as they are, have been studied in different systems: electron transfer in an artificial Zn-porphyrin-pyromellitimide (ZnP-PM) supramolecular electron donor-acceptor complex dissolved in toluene measured at room temperature; exciton relaxation in a natural photosynthetic antenna protein called FMO protein measured at low temperatures, between 4 and 100 K. Spectrally selective picosecond time-resolved emission technique has been used to detect pressure-induced changes in the systems. The following conclusions have been drawn from the electron transfer study: (i) External pressure may serve as a potential and sensitive tool not only to study, but also to control and tune elementary chemical reactions in solvents; (ii) Depending on the system parameters, pressure can both accelerate and inhibit electron transfer reactions; (iii) If competing pathways of the reaction are available, pressure can probably change the branching ratio between the pathways; (iv) The classical nonadiabatic electron transfer theory describes well the phenomena in the ZnP-PM complex, assuming that the driving force or/and reorganisation energy depend linearly on pressure; (v) A decrease in the ZnP-PM donor-acceptor distance under pressure exerts a minor effect on the electron transfer rate. The effect of pressure on the FMO protein exciton relaxation dynamics at low temperatures has been found marginal. This may probably be explained by a unique structure of the protein [D.E. Trondrud, M.F. Schmid, B.W. Matthews, J. Mol. Biol. 188 (1986) p. 443; Y.-F. Li, W. Zhou, E. Blankenship, J.P. Allen, J. Mol. Biol., submitted]. A barrel made of low compressibility beta-sheets may, like a diving bell, effectively screen internal bacteriochlorophyll a molecules from external influence of high pressure. The origin of the observed slow pico = and subnanosecond dynamics of the excitons at the exciton band bottom remains open. The phenomenon may be due to weak coupling of phonons to the exciton states or/and to low density of the relevant low-frequency ( approximately 50 cm(-1)) phonons. Exciton solvation in the surrounding protein and water-glycerol matrix may also contribute to this effect. Drastic changes of spectral, kinetic and dynamic properties have been observed due to protein denaturation, if the protein was compressed at room temperature and then cooled down, as compared to the samples, first cooled and then pressurised.

Journal Article↗

PubMed2001

How far do electrons move? A semiempirical investigation of thermal electron-transfer distances in cationic bis(hydrazine) and bis(hydrazyl) mixed-valence compounds.

A computational approach for estimating thermal electron-transfer reaction distances in symmetrical mixed-valence compounds is described and applied to a series of bis(hydrazine) and bis(hydrazyl) radical cations and derivatives, some of which have been investigated experimentally by Nelsen and co-workers. Ground-state semiempirical charge distributions are obtained by using optimized reactant geometries. Advantage is then taken of the approximate C(2) symmetry, or the approximate mirror symmetry, of each of the targeted compounds, and the inherent degeneracy of the corresponding electron-transfer reactions, such that the change in dipole moment (Delta-mu) upon charge transfer can be estimated from an appropriately distance-weighted sum of charge differences between approximately symmetry-equivalent atoms found on the donor and acceptor sides of the molecule. Delta-mu can then be related directly to the effective one-electron-transfer distance. We find that calculated adiabatic electron-transfer distances can differ appreciably from the geometric donor-site/acceptor-site separation distances. Furthermore, for a fixed geometric separation distance, the effective electron-transfer distance can vary considerably, depending on chemical substituent composition and/or isomeric configuration. Further advantage is taken of the approximate donor-site/acceptor-site symmetry, in the context of a Newton-Cave type analysis, to establish the relative importance of electronic delocalization effects versus self-polarization and inductive effects in diminishing or enhancing effective one-electron-transfer distances.

Journal Article↗