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PubMed1997

Selectivity of electron-donor- and electron-acceptor-bonded silica packing materials for hydrophobic environmental contaminants in polar and non-polar eluents.

Electron-acceptor-bonded stationary phases, 2-(nitrophenyl)ethylsilyl (NPE) and 3-(p-nitrophenoxy)propylsilyl (NPO), and electron-donor-bonded phases, 3-(N-carbazolyl)propylsilyl (CZP), 2-(1-pyrenyl)ethylsilyl (PYE), and 5-coronenylpentylsilyl (COP), were prepared from silica particles and their selectivities were examined in both polar and non-polar solvents for specific isomers of polychlorodibenzo-p-dioxins (PCDDs), hexachloronaphthalenes (HxCNs) and planar and non-planar polychlorobiphenyl (PCB) congeners. Although no single stationary phase was able to separate all the isomer pairs that are coproduced during the synthesis of the PCDDs and HxCNs, pairs can be separated by selecting a suitable stationary phase and solvent. The separation of mixtures of PCDD isomers were found to be most successful with PYE and NPO phases, which yielded the opposite elution orders for each isomer pair that is produced as a mixture. Similar results were obtained for the HxCN isomers that were separated on PYE and CZP phases. The COP phase provided easier separation of non-ortho-substituted and mono-ortho-substituted PCBs from the other PCBs based on the planarity than PYE phase.

Chromatography, High Pressure Liquid↗
PubMed2002

Comparing electron ionization high-resolution and electron capture low-resolution mass spectrometric determination of polybrominated diphenyl ethers in plasma, serum and milk.

Gas chromatography coupled to low-resolution mass spectrometry with electron capture negative ionization as detection mode (GC-LRMS (ECNI)) has been compared to gas chromatography coupled to high-resolution mass spectrometry using electron ionization as detection mode (GC-HRMS (EI)) for determination of polybrominated diphenyl ethers (PBDEs) in biological samples. Extracts of 5.0 g plasma, serum and milk samples were analyzed using both methods. The GC-LRMS (ECNI) and GC-HRMS (EI) systems were found to be equally well suited for determination of PBDEs in the biological samples, as well as in standard solutions, with respect to response, detection limits and repeatability at the pg-level. The estimated limits of detection (LOD) in milk extracts ranged from 0.3-0.6 pg PBDE/g milk and 0.4-0.7 pg PBDE/g milk, for the GC-LRMS (ECNI) and GC-HRMS (EI) systems, respectively. The method repeatability including sample preparation was in the range 4.7-8.4% and 0.6-10% relative standard deviation (RSD) for the GC-LRMS (ECNI) and GC-HRMS (EI) systems, respectively.

Calibration↗
PubMed1989

Transmission electron microscopy and immunocytochemical studies of yeast: analysis of HMG-CoA reductase overproduction by electron microscopy.

The results and anecdotes presented here are intended only as a general guide to other would-be immunocytochemists, because other proteins will undoubtedly respond at least somewhat differently than does HMG-CoA reductase. Nevertheless, based on these experiences, we offer the following suggestions: 1. Antiserum of high specificity should be raised and affinity-purified. Using this antiserum, immunofluorescence microscopy should be attempted before resorting to electron microscopic localization. In the absence of immunolocalization at the light-microscope level, it may be a waste of time to pursue the problem to higher levels of resolution. 2. Cells should be prefixed in 1% formaldehyde-1% glutaraldehyde. Direct fixation of the growing culture and use of phosphate buffer are recommended. The prefixed sample can then be divided into two or three aliquots. One aliquot should receive no postfixation (for optimal immunoreactivity), while the others can be postfixed in osmium-potassium ferricyanide (for possible immunolocalization) or permanganate (for ultrastructural analysis). Because of its ease of use, Spurr's resin should be tried initially. If immunocytochemistry is successful, no further preparations are necessary. If unsuccessful, LR White resin is recommended, but the sample must be treated to remove the cell wall. Electron microscopy and immunocytochemistry offer views into the molecular arrangement of individual cells, a view not easily obtained by other means. It is satisfying and often enlightening to be able to see the extremes as well as the average. In studies of the organization of karmellae, for example, ultrastructural analysis easily revealed the asymmetric segregation pattern, while immunoblots and cell fractionation could not even demonstrate the existence of this membrane organization. The richness of the information available to those who can avert reductionist tendencies, even for a short time, is remarkable.

Histocytochemistry↗
PubMed1985

Scanning electron microscopy and transmission electron microscopy of the ciliated cells of the trachea of the rabbit treated with misonidazole alone and in combination with ionizing radiation.

The trachea is often located in the treatment volume when irradiating malignant tumours in the thorax. In order to evaluate possible synergism between misonidazole and irradiation on this tissue, the following studies were made. Fifty rabbits were treated with daily injections of 100 mg misonidazole given i.p. on consecutive days from 1 to 10 days. Morphological investigations of the trachea were made with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and light microscopy (LM). Physiological examinations were performed with recording of the ciliary beat frequency. The results were compared with those from a group of 100 rabbits given misonidazole in a similar manner and exposed to irradiation (2 Gy) 15-30 min after each injection. Ten rabbits were used as controls. The results are compared to the effect of fractionated irradiation alone with 2 Gy/day. Fractionated irradiation of the ciliary epithelium in the trachea of the rabbit has shown dose-dependent physiological and morphological effects. Misonidazole potentiates these effects of radiation with a more pronounced change of the ciliary beat frequency and an increased metabolic activity as could be visualized on TEM. The combination of drug and irradiation also induced a hyperplasia of the ciliary epithelium. Misonidazole itself had no effect on the ciliary beat frequency, but caused a hypoplasia of the ciliary epithelium.

Animals↗
PubMed2002

Measurements of electron beam damage for organic crystals in a high voltage electron microscope with image plates.

The critical dose for extinction of the diffraction pattern from behenic acid monolayer crystals increased with increasing accelerating voltages. The mean values at 114, 500 and 1000kV were 0.6, 1.8 and 2.2 electrons/A(2), respectively. The critical dose at 500-1000kV is three or more times as large as that at 114kV. Considering this with the recent measured value of the detective quantum efficiency of the image plate, 0.5 at 500kV and 0.4 at 1000kV, 1.5 times as much information can be collected from a crystal at 500-1000kV as at 114kV. Therefore, the combined use of high voltage electron microscopy and image plate detectors offers a significant improvement over conventional electron microscopy approaches for the study of radiation sensitive organic molecule crystals.

Chemistry, Organic↗
PubMed2003

Observations of the intestinal mucosa using environmental scanning electron microscopy (ESEM); comparison with conventional scanning electron microscopy (CSEM).

In order to evaluate the potential use of environmental scanning electron microscopy (ESEM) in biology, structural changes of the jejunal villi of rats were studied after periods of fasting and refeeding, using a conventional scanning electron microscope (CSEM) and ESEM. While observation using the CSEM, involves chemical fixation, drying and coating, observation of fresh, unprepared materials can be directly realized with the ESEM. Environmental microscopy provides a relatively new technology for imaging hydrated materials without specimen preparation and conductive coating. Direct observation of biological samples in their native state is therefore possible with an ESEM. After fasting, the jejunal mucosa is dramatically reduced in size, splits and holes appearing at the tip of the villi. These changes were observed whatever the type of technique used. Artifacts due to the sample preparation for CSEM observation (drying, coating) can therefore be excluded. However, CSEM and ESEM must be used jointly. While, CSEM must be preferred for surface analysis involving high magnifications, ESEM observation, on the other hand, can prove valuable for determining the living aspect of the samples.

Animals↗
PubMed2002

Ultrasoft magnetic films investigated with Lorentz tranmission electron microscopy and electron holography.

As a tribute to the scientific work of Professor Gareth Thomas in the field of structure-property relationships this paper delineates a new possibility of Lorentz transmission electron microscopy (LTEM) to study the magnetic properties of soft magnetic films. We show that in contrast to the traditional point of view, not only does the direction of the magnetization vector in nano-crystalline films make a correlated small-angle wiggling, but also the magnitude of the magnetization modulus fluctuates. This fluctuation produces a rapid modulation in the LTEM image. A novel analysis of the ripple structure in nano-crystalline Fe-Zr-N film corresponds to an amplitude of the transversal component of the magnetization deltaMy of 23 mT and a longitudinal fluctuation of the magnetization of the order of deltaMx = 30 mT. The nano-crystalline (Fe99Zr1)1-xNx films have been prepared by DC magnetron reactive sputtering with a thickness between 50 and 1000 nm. The grain size decreased monotonically with N content from typically 100 nm in the case of N-free films to less than 10 nm for films containing 8 at%. The specimens were examined with a JEOL 2010F 200 kV transmission electron microscope equipped with a post column energy filter (GIF 2000 Gatan Imaging Filter). For holography, the microscope is mounted with a biprism (JEOL biprism with a 0.6 microm diameter platinum wire).

Alloys↗
PubMed1992

Protein-tyrosyl radical interactions in photosystem II studied by electron spin resonance and electron nuclear double resonance spectroscopy: comparison with ribonucleotide reductase and in vitro tyrosine.

The stable tyrosine radical in photosystem II, YD*, has been studied by ESR and ENDOR spectroscopies to obtain proton hyperfine coupling constants from which the electron spin density distribution can be deduced. Simulations of six previously published ESR spectra of PSII (one at Q band; five at X band, of which two were after specific deuteration and two others were of oriented membranes) can be achieved by using a single set of magnetic parameters that includes anisotropic proton hyperfine tensors, an anisotropic g tensor, and noncoincident axis systems for the g and A tensors. From the spectral simulation of the oriented samples, the orientation of the phenol head group of YD* with respect to the membrane plane has been determined. A similar orientation for YZ*, the redox-active tyrosine in PSII that mediates electron transfer between P680 and the oxygen-evolving complex, is expected. ENDOR spectra of YD* in PSII preparations from spinach and Synechocystis support the set of hyperfine coupling constants but indicate that small differences between the two species exist. Comparison with the results of spectral simulations for tyrosyl radicals in ribonucleotide reductase from prokaryotes or eukaryotes and with in vitro radicals indicates that the spin density distribution remains that of an odd-alternant radical but that interactions with the protein can shift spin density within this basic pattern. The largest changes in spin density occur at the tyrosine phenol oxygen and at the ring carbon para to the oxygen, which indicates that mechanisms exist in the protein environment for fine-tuning the chemical and redox properties of the radical species.

Cyanobacteria↗
PubMed1984

An electron-electron double-resonance study of interactions between [14N]- and [15N]stearic acid spin-label pairs: lateral diffusion and vertical fluctuations in dimyristoylphosphatidylcholine.

Vertical fluctuations of the terminal methyl groups of stearic acid acyl chains toward the surface of dimyristoylphosphatidylcholine (DMPC) bilayers have been investigated by using spin-label electron-electron double-resonance ( ELDOR ) methodology. Spin-label pairs consisting of two populations of stearic acid spin-labels were employed, each at 0.25 mol% concentration, where the nitroxides of the first population were 15N substituted and the nitroxides of the second contained 14N. Various combinations of labels with the nitroxide moieties located at carbons 5, 12, or 16 (C5, C12, C16) were used. ELDOR permits measurement of collision frequencies between the two constituents of the pair, for example, between 15N spin-labels at C5 and 14N labels at C16. Intramolecular contributions to the ELDOR effect including nitrogen nuclear relaxation are eliminated by the use of spin-label pairs. Above the main phase transition temperature, bimolecular collisions between C5 and C16 occur with about half the frequency of C16:C16 collisions. It is concluded that vertical fluctuations are very pronounced. A dependence of these fluctuations on temperature and pH has been observed. Lateral diffusion constants calculated from the bimolecular collision frequencies of C16:C16 pairs are 4.56 X 10(-8), 5.77 X 10(-8), and 8.09 X 10(-8) cm2/s at 27, 37, and 47 degrees C. These values are in good agreement with previous measurements of lipid diffusion in DMPC.

Diffusion↗
PubMed2005

Anabaena flavodoxin as an electron carrier from photosystem I to ferredoxin-NADP+ reductase. Role of flavodoxin residues in protein-protein interaction and electron transfer.

Biochemical and structural studies indicate that electrostatic and hydrophobic interactions are critical in the formation of optimal complexes for efficient electron transfer (ET) between ferredoxin-NADP(+) reductase (FNR) and ferredoxin (Fd). Moreover, it has been shown that several charged and hydrophobic residues on the FNR surface are also critical for the interaction with flavodoxin (Fld), although, so far, no key residue on the Fld surface has been found to be the counterpart of such FNR side chains. In this study, negatively charged side chains on the Fld surface have been individually modified, either by the introduction of positive charges or by their neutralization. Our results indicate that although Glu16, Glu20, Glu61, Asp65, and Asp96 contribute to the orientation and optimization of the Fld interaction, either with FNR or with photosystem I (PSI) (presumably through the formation of salt bridges), for efficient ET, none of these side chains is involved in the formation of crucial salt bridges for optimal interaction with FNR. These data support the idea that the FNR-Fld interaction is less specific than the FNR-Fd interaction. However, analysis of the reactivity of these mutated Flds toward the membrane-anchored PSI complex indicated that all mutants, except Glu16Gln, lack the ability to form a stable complex with PSI. Thr12, Thr56, Asn58, and Asn97 are present in the close environment of the isoalloxazine ring of FMN in Anabaena Fld. Their roles in the interaction with and ET to FNR and PSI have also been studied. Mutants at these Fld positions indicate that residues in the close environment of the isoalloxazine ring modulate the ability of Fld to bind to and to exchange electrons with its physiological counterparts.

Amino Acid Sequence↗
PubMed1996

Observation of a flavin semiquinone in the resting state of monoamine oxidase B by electron paramagnetic resonance and electron nuclear double resonance spectroscopy.

Monoamine oxidase (MAO) plays an essential role in the regulation of various neurotransmitter and xenobiotic amines. Inhibitors of MAO have been employed in the treatment of depression and as adjuncts in Parkinson's disease therapy. X-Band and Q-band electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopic techniques are employed to characterize a signal assigned as a stable red anionic semiquinone radical in the resting state of MAO B. It is shown that the radical signal is not affected during substrate (either benzylamine or phenylethylamine) turnover, by anaerobic incubation with substrate, or by covalent modification of the active site flavin cofactor in the catalytically active dimer. Upon denaturation, however, the semiquinone absorbances and EPR signals are lost. Photoreduction of the native enzyme in the presence of ethylenediaminetetraacetate generates an EPR signal that is not the same as that obtained in the resting state and shows different proton ENDOR signals. These results suggest that the two flavin prosthetic groups that exist in catalytically active monoamine oxidase B are physically distinct.

Animals↗
PubMed1997

A new non-heme iron environment in Paracoccus denitrificans adenylate kinase studied by electron paramagnetic resonance and electron spin echo envelope modulation spectroscopy.

Adenylate kinase from the Gram-negative bacterium Paracoccus denitrificans (AKden) has structural features highly similar to those of the enzyme from Gram-positive organisms. Atomic absorption spectroscopy of the recombinant protein, which is a dimer, revealed the presence of two metals, zinc and iron, each binding most probably to one monomer. Under oxidizing conditions, the electron paramagnetic resonance (EPR) spectrum of AKden at 4.2 K consists of features at g = 9.23, 4.34, 4.21, and 3.68. These features are absent in the ascorbate-reduced protein and are characteristic of a S = 5/2 spin system in a rhombic environment with E/D = 0.24 and are assigned to a non-heme Fe3+ (S = 5/2) center. The zero-field splitting parameter D (D = 1.4 +/- 0.2 cm-1) was estimated from the temperature dependence of the EPR spectra. These EPR characteristic as well as the difference absorption spectrum (oxidized minus reduced) of AKden are similar to those reported for the non-heme iron protein rubredoxin. Nevertheless, the redox potential of the Fe2+/Fe3+ couple in AKden was measured at +230 +/- 30 mV, which is more positive than the redox potential of the non-heme iron in rubredoxin. Binding of cyanide converts the iron from the high-spin (S = 5/2) to the low-spin (S = 1/2) spin state. The EPR spectrum of the non-heme Fe3+(S = 1/2) in the presence of cyanide has g values of 2.45, 2.18, and 1.92 and spin-Hamiltonian parameters R/lambda = 7. 4 and R/mu = 0.56. The conversion of the non-heme iron to the low-spin (S = 1/2) state allowed the study of its local environment by electron spin echo envelope modulation spectroscopy (ESEEM). The ESEEM data revealed the existence of 14N or 15N nuclei coupled to the low-spin iron after addition of KC14N or KC15N respectively. This demonstrated that iron in AKden has at least one labile coordination position that can be easily occupied by cyanide. Other possible magnetic interactions with nitrogen(s) from the protein are discussed.

Adenylate Kinase↗
PubMed1998

Electron transfer from flavin to iron in the Pseudomonas oleovorans rubredoxin reductase-rubredoxin electron transfer complex.

Rubredoxin reductase (RR) and rubredoxin form a soluble and physiological eT complex. The complex provides reducing equivalents for a membrane-bound omega-hydroxylase, required for the hydroxylation of alkanes and related compounds. The gene (alkT) encoding RR has been overexpressed and the enzyme purified in amounts suitable for studies of eT by stopped-flow spectroscopy. The eT reactions from NADH to the flavin of RR and from reduced RR to the 1Fe and 2Fe forms of rubredoxin have been characterized by transient kinetic and thermodynamic analysis. The reductive half-reaction proceeds in a one-step reaction involving oxidized enzyme and a two-electron-reduced enzyme-NAD+ charge-transfer complex. Flavin reduction is observed at 450 nm and charge-transfer formation at 750 nm; both steps are hyperbolically dependent on NADH concentration. The limiting flavin reduction rate (180 +/- 4 s-1) is comparable to the limiting rate for charge-transfer formation (189 +/- 7 s-1) and analysis at 450 and 750 nm yielded enzyme-NADH dissociation constants of 36 +/- 2 and 43 +/- 5 microM, respectively. Thermodynamic analysis of the reductive half-reaction yielded values for changes in entropy (DeltaS = -65.8 +/- 2.2 J mol-1 K-1), enthalpy (DeltaH = 37.8 +/- 0.6 kJ mol-1) and Gibbs free energy (DeltaG = 57.5 +/- 0.7 kJ mol-1 at 298 K) during hydride ion transfer to the flavin N5 atom. Spectral analysis of mixtures of 1Fe or 2Fe rubredoxin and RR suggest that conformational changes accompany eT complex assembly. Both the 1Fe (nonphysiological) and 2Fe (physiological) forms of rubredoxin were found to oxidize two electron-reduced rubredoxin reductase with approximately equal facility. Rates for the reduction of rubredoxin are hyperbolically dependent on rubredoxin concentration and the limiting rates are 72. 7 +/- 0.6 and 55.2 +/- 0.3 s-1 for the 1Fe and 2Fe forms, respectively. Analysis of the temperature dependence of eT to rubredoxin using eT theory revealed that the reaction is not adequately described as a nonadiabatic eT reaction (HAB >> 80 cm-1). eT to both the 1Fe and 2Fe forms of rubredoxin is therefore gated by an adiabatic process that precedes the eT reaction from flavin to iron. Possible origins of this adiabatic event are discussed.

Electron Transport↗
PubMed2000

Electron paramagnetic resonance and electron nuclear double resonance spectroscopic identification and characterization of the tyrosyl radicals in prostaglandin H synthase 1.

The tyrosyl radicals generated in reactions of ethyl hydrogen peroxide with both native and indomethacin-pretreated prostaglandin H synthase 1 (PGHS-1) were examined by low-temperature electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopies. In the reaction of peroxide with the native enzyme at 0 degrees C, the tyrosyl radical EPR signal underwent a continuous reduction in line width and lost intensity as the incubation time increased, changing from an initial, 35-G wide doublet to a wide singlet of slightly smaller line width and finally to a 25-G narrow singlet. The 25-G narrow singlet produced by self-inactivation was distinctly broader than the 22-G narrow singlet obtained by indomethacin treatment. Analysis of the narrow singlet EPR spectra of self-inactivated and indomethacin-pretreated enzymes suggests that they reflect conformationally distinct tyrosyl radicals. ENDOR spectroscopy allowed more detailed characterization by providing hyperfine couplings for ring and methylene protons. These results establish that the wide doublet and the 22-G narrow singlet EPR signals arise from tyrosyl radicals with different side-chain conformations. The wide-singlet ENDOR spectrum, however, is best accounted for as a mixture of native wide-doublet and self-inactivated 25-G narrow-singlet species, consistent with an earlier EPR study [DeGray et al. (1992) J. Biol. Chem. 267, 23583-23588]. We conclude that a tyrosyl residue other than the catalytically essential Y385 species is most likely responsible for the indomethacin-inhibited, narrow-singlet spectrum. Thus, this inhibitor may function by redirecting radical formation to a catalytically inactive side chain. Either radical migration or conformational relaxation at Y385 produces the 25-G narrow singlet during self-inactivation. Our ENDOR data also indicate that the catalytically active, wide-doublet species is not hydrogen bonded, which may enhance its reactivity toward the fatty-acid substrate bound nearby.

Animals↗
PubMed2001

Electron spin resonance and electronic structure of vanadyl-porphyrin in heavy crude oils.

A study of vanadyl-porphyrin by electron spin resonance (ESR) was carried out looking for answers about the role that the central V=O ion plays when these kinds of molecules are present in heavy crude oils. The eigenvalues of the linear combination of atomic orbitals (LCAO) were obtained from the experimental values of g and parameters (ESR). The contributions to the molecular orbitals that describe the various energy levels of vanadyl ion were also obtained for the porphyrin species. The trends of the degree of covalent character of the metal-ligand bonds and the length of the vanadium-oxygen chemical bond are discussed. It is interesting to note that the Fermi contact term, K(eff), is essentially constant for all samples investigated, and it was found to be independent of the calculated electron delocalization (1 - delta(2)) and shows only little variation among the three different samples of oil. The orbital energies derived from our ESR study qualitatively agree with those predicted from MO theory for synthetic vanadyl-porphyrins.

Algorithms↗
PubMed2006

Electronic effects of electron-donating and -withdrawing groups in model complexes for iron-tyrosine-containing metalloenzymes.

Three new iron(III) complexes with the ligand N,N'-bis(2-hydroxybenzyl)-N,N'-bis(pyridin-2-ylmethyl)ethylenediamine, H2bbpen, containing electron-donating and -withdrawing groups (Me, Br, NO2) in the 5-position of the phenol rings were synthesized and fully characterized by IR spectroscopy, ESI mass spectrometry, and CHN elemental analyses. X-ray structures of the iron(III) complexes containing NO2 and Me groups were determined. The effects of the substituents on the electronic properties of the complexes were detected by UV-visible spectroscopy, cyclic voltammetry, and X-ray crystallography. Linear correlations between the Hammett parameter for the substituents (sigma(p)) and the Fe(III)/Fe(II) redox potentials or ligand-metal charge-transfer (LMCT) processes of the complexes were obtained. A theoretical study using DFT is presented and shows good agreement between the experimental and calculated data.

Crystallography, X-Ray↗
PubMed2001

Single two-electron transfers vs successive one-electron transfers in polyconjugated systems illustrated by the electrochemical oxidation and reduction of carotenoids.

Examination of cyclic voltammetric responses reveals that inversion of the standard potentials of the first and second electron transfers occurs in the oxidation of beta-carotene and 15,15'-didehydro-beta-carotene (but not in their reduction) as well as in the reduction of canthaxanthin (but not in its oxidation). The factors that control potential inversion in these systems, and more generally in symmetrical molecules containing conjugated long chains, are investigated by quantum chemical calculations. Two main interconnected effects emerge. One is the localization of the charges in the di-ion toward the ends of the molecule at a large distance from one another, thus minimizing Coulombic repulsion. The same effect favors the solvation of the di-ion providing additional stabilization. In contrast, the charge in the ion radical is delocalized over the whole molecular framework, thus disfavoring its stabilization by interaction with the solvent. The combination of the two solvation effects allows potential inversion to occur as opposed to the case where the two electrophores are linked by a saturated bridge where potential inversion cannot occur. Localization of the charges in the di-ion, and thus potential inversion, is favored by the presence of electron-accepting terminal groups for reductions (as the two carbonyl groups in canthaxanthin) and of hole-accepting terminal groups for oxidations (as in beta-carotene).

Carotenoids↗