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PubMed2000

Electron boltzmann kinetic equation averaged over fast electron bouncing and pitch-angle scattering for fast modeling of electron cyclotron resonance discharge

The electron distribution function (EDF) in an electron cyclotron resonance (ECR) discharge is far from Maxwellian. The self-consistent simulation of ECR discharges requires a calculation of the EDF on every magnetic line for various ion density profiles. The straightforward self-consistent simulation of ECR discharges using the Monte Carlo technique for the EDF calculation is very computer time expensive, since the electron and ion time scales are very different. An electron Boltzmann kinetic equation averaged over the fast electron bouncing and pitch-angle scattering was derived in order to develop an effective and operative tool for the fast modeling (FM) of low-pressure ECR discharges. An analytical solution for the EDF in a loss cone was derived. To check the validity of the FM, one-dimensional (in coordinate) and two-dimensional (in velocity) Monte Carlo simulation codes were developed. The validity of the fast modeling method is proved by comparison with the Monte Carlo simulations. The complete system of equations for FM is presented and ready for use in a comprehensive study of ECR discharges. The variations of plasma density and of wall and sheath potentials are analyzed by solving a self-consistent set of equations for the EDF.

Journal Article↗
PubMed1984

Assessment of electron irradiation damage to biomolecules by electron diffraction and electron energy-loss spectroscopy.

Electron radiation damage is one of the most severe problems in high resolution electron microscopy by biomolecules. The techniques of electron diffraction and electron energy-loss spectroscopy were applied to gain a better understanding of radiation damage in amino acids and nucleic acid bases. The results when compared with G-values for the release of ammonia and hydrogen sulphide from amino acids seem to indicate that bond scission is an important cause of radiation damage at moderate doses of irradiation. High resolution structural disorder in nucleic acid bases was found to involve loss of atoms peripheral to the main ring structure.

Adenine↗
PubMed1982

Electron-electron double resonance and saturation-recovery studies of nitroxide electron and nuclear spin-lattice relaxation times and Heisenberg exchange rates: lateral diffusion in dimyristoyl phosphatidylcholine.

Lateral diffusion constants of the stearic acid nitroxide radical spin label 2-(14-carboxytetradecyl)-2-ethyl-4,4-dimethyl-3-oxazolindinyl oxide in dispersions of dimyristoyl phosphatidylcholine have been measured. Electron-electron double resonance methods were used to determine the product of the bimolecular collision frequency and T1e, the electron spin-lattice relaxation time. T1e in turn was measured by the technique of saturation recovery. The theoretical model of Träuble and Sackmann was then used to relate the bimolecular collision frequencies to the diffusion constants. Results are in agreement with other methods. Lower spin-label concentrations than were used in previous electron paramagnetic resonance studies are needed (label-to-lipid ratio less than 0.5 mol%). Analysis of the data also yields values of the nitrogen nuclear spin-lattice relaxation time of the nitroxide moiety. These values are indicative of membrane fluidity.

Cyclic N-Oxides↗
PubMed2002

Simulations of the (1)H electron spin echo-electron nuclear double resonance and (2)H electron spin echo envelope modulation spectra of exchangeable hydrogen nuclei coupled to the S(2)-state photosystem II manganese cluster.

The pulsed EPR methods of electron spin echo envelope modulation (ESEEM) and electron spin echo-electron nuclear double resonance (ESE-ENDOR) are used to investigate the proximity of exchangeable hydrogens around the paramagnetic S(2)-state Mn cluster of the photosystem II oxygen-evolving complex. Although ESEEM and ESE-ENDOR are both pulsed electron paramagnetic resonance techniques, the specific mechanisms by which nuclear spin transitions are observed are quite different. We are able to generate good simulations of both (1)H ESE-ENDOR and (2)H ESEEM signatures of exchangeable hydrogens at the S(2)-state cluster. The convergence of simulation parameters for both methods provides a high degree of confidence in the simulations. Several exchangeable protons-deuterons with strong dipolar couplings are observed. In the simulations, two of the close ( approximately 2.5 A) hydrogen nuclei exhibit strong isotropic couplings and are therefore most probably associated with direct substrate ligation to paramagnetic Mn. Another two of the close ( approximately 2.7 A) hydrogen nuclei show no isotropic couplings and are therefore most probably not contained in Mn ligands. We suggest that these proximal hydrogens may be associated with a Ca(2+)-bound substrate, as indicated in recent mechanistic proposals for O(2) formation.

Binding Sites↗
PubMed2004

Amyloidosis of Alzheimer's Abeta peptides: solid-state nuclear magnetic resonance, electron paramagnetic resonance, transmission electron microscopy, scanning transmission electron microscopy and atomic force microscopy studies.

Aggregation cascade for Alzheimer's amyloid-beta peptides, its relevance to neurotoxicity in the course of Alzheimer's disease and experimental methods useful for these studies are discussed. Details of the solid-phase peptide synthesis and sample preparation procedures for Alzheimer's beta-amyloid fibrils are given. Recent progress in obtaining structural constraints on Abeta-fibrils from solid-state NMR and scanning transmission electron microscopy (STEM) data is discussed. Polymorphism of amyloid fibrils and oligomers of the 'Arctic' mutant of Abeta(1-40) was studied by (1)H,(13)C solid-state NMR, transmission electron microscopy (TEM) and atomic force microscopy (AFM), and a real-time aggregation of different polymorphs of the peptide was observed with the aid of in situ AFM. Recent results on binding of Cu(II) ions and Al-citrate and Al-ATP complexes to amyloid fibrils, as studied by electron paramagnetic resonance (EPR) and solid-state (27)Al NMR techniques, are also presented.

Alzheimer Disease↗
PubMed1984

Measurement of the oxidation-reduction potentials for one-electron and two-electron reduction of electron-transfer flavoprotein from pig liver.

Potentiometric titrations of pig liver electron-transfer flavoprotein (ETF) were performed at pH 7.5 and 4 degrees C, both in the reductive and oxidative directions. Reduction of ETF to the hydroquinone form required a total of two reducing equivalents/mol of ETF with the formation of sub-stoichiometric amounts of anionic semiquinone as an intermediate. The oxidation-reduction potentials for the two one-electron couples, oxidized ETF/ETF semiquinone and ETF semiquinone/fully reduced ETF, are +4 mV and -50 mV respectively. The overall midpoint potential for the two-electron couple (oxidized ETF/fully reduced ETF) is -23 mV.

Animals↗
PubMed1995

Electron spin resonance and electron nuclear double resonance studies of flavoproteins involved in the photosynthetic electron transport in the cyanobacterium Anabaena sp. PCC 7119.

The flavins of ferredoxin-NADP+ reductase (FNR) and flavodoxin from the cyanobacterium Anabaena PCC 7119 were obtained in their semiquinone states at pH 7 by photoreduction of the pure proteins in the presence of EDTA and 5-deazariboflavin. For FNR, the ESR signal of the FAD semiquinone was centred at g = 2.005 with linewidths 2.0 mT in H2O and 1.48 mT in D2O. These data are in agreement with those reported for other neutral flavin semiquinones. The linewidths were the same when measured either at X-band (9.35 GHz) or at S-band (4 GHz), indicating that line broadening is due to unresolved nuclear hyperfine couplings, caused in part by exchangeable protons. When the substrate, NADP+, was added to the semiquinone form of the protein no changes in the linewidth or shape of the spectra were detected, but a decrease in the ESR signal due to the FNR semiquinone was observed, consistent with the reduction of NADP+ to NADPH by reduced FNR and, subsequent displacement of the equilibrium. No changes in the shape or linewidth of the FNR ESR signals were observed when photoreduction of FNR was performed in the presence of either flavodoxin or ferredoxin. Electron nuclear double resonance (ENDOR) spectroscopy of FNR semiquinone from Anabaena PCC 7119 provided further information about the interactions of the flavin radical with protons. A group of signals, with couplings of 5-9.5 MHz, is attributed to protons on C6 and on 8-CH3 of the flavin ring. No change in these hyperfine couplings was detected when the protein was studied in D2O, but the coupling Aiso attributed to protons on 8-CH3 decreased from 8.12 MHz to 7.72 MHz in the presence of NADP+. The decrease in the electron spin density distribution on this part of the flavin ring system was attributed to binding of the substrate, polarising the electron density distribution of the flavin towards the pyrimidine ring. A second group of signals was observed, with hyperfine couplings less than 3 MHz, some of which disappeared when the protein was transferred into D2O. Effects of NADP+ binding to the protein were also observed in these weak couplings. These signals are attributed to displaced water protons, or to exchangeable protons from amino acid residues on the protein near the flavin-binding site, involved in substrate stabilization.(ABSTRACT TRUNCATED AT 400 WORDS)

Anabaena↗
PubMed1980

Scanning electron microscope histochemistry: the use of backscattered electrons to identify epidermal Langerhans cells in the scanning electron microscope.

Epidermal sheets reacted for ATPase were examined in the scanning electron microscope and images based on atomic number contrast formed by collecting the backscattered electrons. Langerhans cells lying within the tissue were revealed and could be related to surface structures by reference to conventional secondary electron images.

Adenosine Triphosphatases↗
PubMed1979

Electron-paramagnetic-resonance measurements of the electron-transfer components of the reaction centre of Rhodopseudomonas viridis. Oxidation--reduction potentials and interactions of the electron acceptors.

Oxidation-reduction potentiometry was carried out on Rhodopseudomonas viridis chromatophores. Measurements of e.p.r. signals of the semiquinone-iron type at g=1.82 have revealed a more complex situation than previously reported. The presence of three different components is indicated. The midpoint potential (E(m)) of the primary acceptor quinone/semiquinone couple was found to be approx. -165mV at pH10, with a pK being reached at around pH7.5. The primary acceptor also accepts a second electron with an E(m) of -525mV, but this redox transition exhibits a hysteresis effect. Interaction effects indicate the presence of another component with E(m) values at pH10 of approx. -165mV (pK reached at around pH7.5) for single reduction and -350mV (pK at pH10 or greater) for double reduction. It is suggested that this component is the secondary acceptor. Another semiquinone-iron-type component which gives a g=1.82 signal is also present. This component is distinguishable from the primary acceptor by its e.p.r. spectrum, which shows a double peak at g=1.82 and a g(x) line at g=1.76. This component has E(m) values at pH10 for single and double reduction of -15mV and approx. -150mV respectively. Both of these E(m) values are pH-dependent. The presence of an interaction between this component and the photoreduced primary acceptor indicates the close proximity of these components. However, the midpoint potential of this component indicates a function as a secondary electron-transport component rather than an electron acceptor in the reaction centre. The dependence of the bacteriopheophytin intermediate (I) doublet e.p.r. signal on the presence of the semiquinone-iron form of the primary acceptor is demonstrated. The midpoint potential of the I/I(-) couple is estimated to be lower than -600mV.

Bacterial Chromatophores↗
PubMed1991

Reduction of the Bremsstrahlung component of clinical electron beams: implications for electron arc therapy and total skin electron irradiation.

The dose due to Bremsstrahlung in stationary electron beams of nominal energies in the range 6-20 MeV is typically between 1-7% of the maximum dose and is usually not clinically significant. However, in treatments using rotational or multiple electron beams where the x-ray dose from several beams is added the x-ray dose will reach much higher proportions and will be of clinical significance. Moreover, this dose often is located in normal tissue beyond the target volume. Reduction of this x-ray dose is therefore desirable. In the present study a reduction of the x-ray component of electron beams produced by a Clinac 2100C accelerator by a change of the transmission ion chamber and scattering foils is reported. A reduction in Bremsstrahlung of up to 50% can be achieved.

Electrons↗
PubMed2003

Characterization of Co25Ag75 and (Co90Al10)28Ag72 granular films by electron diffraction, high-resolution transmission electron microscopy and electron spectroscopic imaging.

Series of sputter-deposited Co25Ag75 and (Co90Al10)28Ag72 giant-magnetoresistance granular films were characterized by electron diffraction, high-resolution transmission electron microscopy and electron spectroscopic imaging. Crystalline particles of fcc silver and hcp cobalt were detected in both thin-film systems before annealing. Annealing of (Co90Al10)28Ag72 films at 773 and 823 K yielded mixtures of fcc and hcp cobalt clusters and notably enlarged silver particles. In addition, crystallites of bcc Ag3Al were detected in the sample annealed at 823 K. The mesoscopic structure of the as-deposited films was investigated by dark-field imaging showing columnar growth-domains for silver. The columns were preserved during thermal treatment up to 773 K, whereas annealing at 823 K destroyed these domains.

Alloys↗
PubMed1978

[Contribution to deep electron pendulous therapy. VIII. Communication: concerning the problem of diverging contours in telecentric electron pendulous irradiation using the electron energies 10 MeV and 20 MeV (author's transl)].

The mode of correction of the isodose curves from telecentric electron pendulous technique using a constant patient radius rp = 30 cm (Isodosenatlas, Siemens, 1973) is represented with regard to its application in patients with diverging surface contours. Correction is possible by two different methods: 1st by experimental determination of an air gap factor for the shift of isodoses, and 2nd by two factors depending on the focus-skin distance and on the angle of incidence of the electron beam. Determination of the factors is performed either by means of fixed fields measured by vertical and at oblique incidence of the beam and a depth dose distribution measured at the central axis, with oblique incidence of the electrons.

Electrons↗
PubMed1978

[Characterization of heavy metal secretory granules in the islet tissue of adult white rats by means of the light and electron microscopy, including the high voltage electron miscroscopy and the electron microanalysis (author's transl)].

The metal secretory granules of Langerhans' islets of white rats were examined after histochemical detection by sulfide silver staining method as well as by conventional electron microscopy and high voltage electron microscopy. Zinc and calcium were determined with electron probe x-ray-microanalysis. The metals are located at the inside of cytoplasmatic vacuoles, a result, we discuss in connection with the release of pancreatic hormones.

Animals↗
PubMed2001

Definitive support by transmission electron microscopy, electron diffraction, and electron density maps for the formation of a BCC lattice from poly[N-[3,4,5-tris(n-dodecan-1-yloxy)benzoyl]ethyleneimine].

Transmission electron microscopy (TEM), electron diffraction (ED), and electron density maps (EDM) experiments were carried out on a poly[N-[3,4,5-tris(n-dodecan-1-yloxy)benzoyl]ethyleneimine] [poly[(3,4,5)12G1-Oxz]] with a degree of polymerization (DP) of 20. All experiments confirmed the thermotropic body-centered cubic (BCC) Im3m lattice suggested previously by X-ray diffraction (XRD) experiments. The unit cell parameter determined by ED at 23 degrees C is a = 42.4 A, in good agreement with XRD results which show a = 42.6 A after quenching from 70 degrees C. EDM of the XRD results confirm that the supramolecular minidendrimer obtained from poly[(3,4,5)12G1-Oxz] adopts a spherical "inverse micellar-like" structure, with the polyethyleneimine backbone and the aromatic groups microsegregated and concentrated in the corners and in the center of the cubic unit cell. A space-filling continuum is realized by the n-alkyl groups that radiate out of the aromatic core of the spherical dendrimer. This manuscript is only the second example of complete structural analysis of a lattice generated from supramolecular objects and complements the previous example reported from our laboratory on the Pm3n lattice.

Journal Article↗
PubMed1976

Pulmonary mineral dust. A study of ninety patients by electron microscopy, electron microanalysis, and electron microdiffraction.

The results of a study of 90 patients are presented. Intrapulmonary mineral deposits were characterized by electron diffraction and electron probe microanalysis. Using this method, pneumoconioses may be distinguidhed from other pneumopathies. In cases of pneumoconiosis, there exists a specific relationship between the etiology of the dust exposure and the crystallographic characteristics of the intrapulmonary deposits. The nature of the deposits may be indicative of a specific type of pneumoconiosis. This method is particularly useful in differentiating between asbestos bodies and ferruginous bodies. The value of the method in general and its importance in the study of pneumoconiosis are discussed.

Asbestos↗
PubMed1985

Electron spin relaxation of synthetic melanin and melanin-containing human tissues as studied by electron spin echo and electron spin resonance.

Electron spin lattice relaxation times (T1) and the phase memory times (Tm) were obtained for the synthetic melanin system from 3-hydroxytyrosine (dopa) by means of electron spin echo spectroscopy at 77 degrees K. Saturation behavior of the ESR spectra of melanins in melanin-containing tissue and of the synthetic melanin was also determined at the same temperature. The spin lattice relaxation time and the spectral diffusion time of the synthetic melanin are very long (4.3 ms and 101 microseconds, respectively, in the solid state), and the ESR signal saturates readily at low microwave powers. On the other hand, ESR spectra of natural melanins from the tissues chosen for this study, as well as those of synthetic melanins which contain Fe3+ of g = 4.3 and Mn2+ of g = 2, are relatively difficult to saturate compared with samples without such metal ions. These results show clearly that a large part of those two metal ions in sites responsible for the ESR spectral components with these particular g values are coordinated to melanin in melanin-containing tissue, and modify the magnetic relaxation behavior of the melanin. Accumulations of these metal ions in melanins are different from system to system, and they increase in the order: hair (black), retina and choroid (brown), malignant melanoma of eye and skin, and lentigo and nevus of skin.

Copper↗