SEARCH · PubMed Health

Results for “ELECTRONICS”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3Linked to original sources

PubMed1989

Ester and related derivatives of ring N-pentafluorobenzylated 5-hydroxymethyluracil. Hydrolytic stability, mass spectral properties, and trace detection by gas chromatography-electron-capture detection, gas chromatography-electron-capture negative ion mass spectrometry, and moving-belt liquid chromatography-electron-capture negative ion mass spectrometry.

One consequence of radiation damage to DNA is the conversion of thymine to 5-hydroxymethyluracil (HMU). In order to sensitively detect this DNA adduct by gas chromatography (GC) or high-performance liquid chromatography (HPLC) with electron-capture detection techniques, it is necessary to derivatize it. This study was designed to select an optimum ester derivative of the aliphatic hydroxyl group on HMU. N1, N3-Bis(pentafluorobenzyl)-HMU was formed as a parent derivative, and from this a series of esters. Also O-pentafluorobenzyl and O-tetrafluorobenzyl ether derivatives were prepared. Of the esters the pivalyl derivative was the best choice because it formed easily, was relatively stable to aqueous hydrolysis (t 1/2 = 9.8 days at pH 11.5, 24 degrees C) and gave a response at fmol levels by GC and LC comparable to that of the ethers. Unanticipated was a good response as well for the parent derivative, a free hydroxyl compound, by GC and LC at this level. The work also demonstrates a high performance by LC-electron-capture negative ion mass spectrometry with a belt interface for the trace detection of derivatives of this type.

Chemical Phenomena↗

PubMed2000

Differential coupling through Val-344 and Tyr-442 of trimethylamine dehydrogenase in electron transfer reactions with ferricenium ions and electron transferring flavoprotein.

Modeling studies of the trimethylamine dehydrogenase-electron transferring flavoprotein (TMADH-ETF) electron transfer complex have suggested potential roles for Val-344 and Tyr-442, found on the surface of TMADH, in electronic coupling between the 4Fe-4S center of TMADH and the FAD of ETF. The importance of these residues in electron transfer, both to ETF and to the artificial electron acceptor, ferricenium (Fc(+)), has been studied by site-directed mutagenesis and stopped-flow spectroscopy. Reduction of the 6-(S)-cysteinyl FMN in TMADH is not affected by mutation of either Tyr-442 or Val-344 to a variety of alternate side chains, although there are modest changes in the rate of internal electron transfer from the 6-(S)-cysteinyl FMN to the 4Fe-4S center. The kinetics of electron transfer from the 4Fe-4S center to Fc(+) are sensitive to mutations at position 344. The introduction of smaller side chains (Ala-344, Cys-344, and Gly-344) leads to enhanced rates of electron transfer, and likely reflects shortened electron transfer "pathways" from the 4Fe-4S center to Fc(+). The introduction of larger side chains (Ile-344 and Tyr-344) reduces substantially the rate of electron transfer to Fc(+). Electron transfer to ETF is not affected, to any large extent, by mutation of Val-344. In contrast, mutation of Tyr-442 to Phe, Leu, Cys, and Gly leads to major reductions in the rate of electron transfer to ETF, but not to Fc(+). The data indicate that electron transfer to Fc(+) is via the shortest pathway from the 4Fe-4S center of TMADH to the surface of the enzyme. Val-344 is located at the end of this pathway at the bottom of a small groove on the surface of TMADH, and Fc(+) can penetrate this groove to facilitate good electronic coupling with the 4Fe-4S center. With ETF as an electron acceptor, the observed rate of electron transfer is substantially reduced on mutation of Tyr-442, but not Val-344. We conclude that the flavin of ETF does not penetrate fully the groove on the surface of TMADH, and that electron transfer from the 4Fe-4S center to ETF may involve a longer pathway involving Tyr-442. Mutation of Tyr-442 likely disrupts electron transfer by perturbing the interaction geometry of TMADH and ETF in the productive electron transfer complex, leading to less efficient coupling between the redox centers.

Cations, Divalent↗

PubMed2002

Manipulating the production and recombination of electrons during electron transfer: Femtosecond control of the charge-transfer-to-solvent (CTTS) dynamics of the sodium anion.

The scavenging of a solvated electron represents the simplest possible electron-transfer (ET) reaction. In this work, we show how a sequence of femtosecond laser pulses can be used to manipulate an ET reaction that has only electronic degrees of freedom: the scavenging of a solvated electron by a single atom in solution. Solvated electrons in tetrahydrofuran are created via photodetachment using the charge-transfer-to-solvent (CTTS) transition of sodide (Na(-)). The CTTS process ejects electrons to well-defined distances, leading to three possible initial geometries for the back ET reaction between the solvated electrons and their geminate sodium atom partners (Na(0)). Electrons that are ejected within the same solvent cavity as the sodium atom (immediate contact pairs) undergo back ET in approximately 1 ps. Electrons ejected one solvent shell away from the Na(0) (solvent-separated contact pairs) take hundreds of picoseconds to undergo back ET. Electrons ejected more than one solvent shell from the sodium atom (free solvated electrons) do not recombine on subnanosecond time scales. We manipulate the back ET reaction for each of these geometries by applying a "re-excitation" pulse to promote the localized solvated electron ground state into a highly delocalized excited-state wave function in the fluid's conduction band. We find that re-excitation of electrons in immediate contact pairs suppresses the back ET reaction. The kinetics at different probe wavelengths and in different solvents suggest that the recombination is suppressed because the excited electrons can relocalize into different solvent cavities upon relaxation to the ground state. Roughly one-third of the re-excited electrons do not collapse back into their original solvent cavities, and of these, the majority relocalize into a cavity one solvent shell away. In contrast to the behavior of the immediate pair electrons, re-excitation of electrons in solvent-separated contact pairs leads to an early time enhancement of the back ET reaction, followed by a longer-time recombination suppression. The recombination enhancement results from the improved overlap between the electron and the Na(0) one solvent shell away due to the delocalization of the wave function upon re-excitation. Once the excited state decays, however, the enhanced back ET is shut off, and some of the re-excited electrons relocalize even farther from their geminate partners, leading to a long-time suppression of the recombination; the rates for recombination enhancement and relocalization are comparable. Enhanced recombination is still observed even when the re-excitation pulse is applied hundreds of picoseconds after the initial CTTS photodetachment, verifying that solvent-separated contact pairs are long-lived, metastable entities. Taken together, all these results, combined with the simplicity and convenient spectroscopy of the sodide CTTS system, allow for an unprecedented degree of control that is a significant step toward building a full molecular-level picture of condensed-phase ET reactions.

Journal Article↗

PubMed2003

Electronic structure of linear thiophenolate-bridged heteronuclear complexes [LFeMFeL](n)(+) (M = Cr, Co, Fe; n = 1-3): a combination of kinetic exchange interaction and electron delocalization.

The electronic properties of the isostructural series of heterotrinuclear thiophenolate-bridged complexes of the general formula [LFeMFeL](n)(+) with M = Cr, Co and Fe where L represents the trianionic form of the ligand 1,4,7-tris(4-tertbutyl-2-mercaptobenzyl)-1,4,7-triazacyclononane, synthesized and investigated by a number of experimental techniques in the previous work(1), are subjected now to a theoretical analysis. The low-lying electronic excitations in these compounds are described within a minimal model supported by experiment and quantum chemistry calculations. It was found indeed that various experimental data concerning the magnetism and electron delocalization in the lowest states of all seven compounds are completely reproduced within a model which includes the electron transfer between magnetic orbitals at different metal centers and the electron repulsion in these orbitals (the Hubbard model). Moreover, due to the trigonal symmetry of the complexes, only the electron transfer between nondegenerate orbital, a(1), originating from the t(2g) shell of each metal ion in a pseudo-octahedral coordination, is relevant for the lowest states. An essential feature resulting from quantum chemistry calculations, allowing to explain the unusual magnetic properties of these compounds, is the surprisingly large value and, especially, the negative sign of the electron transfer between terminal iron ions, beta'. According to their electronic properties the series of complexes can be divided as follows: (1). The complexes [LFeFeFeL](3+) and [LFeCrFeL](3+) show localized valences in the ground electronic configuration. The strong antiferromagnetic exchange interaction and the resulting spin 1/2 of the ground-state arise from large values of the transfer parameters. (2). In the complex [LFeCrFeL](+), due to a higher energy of the magnetic orbital on the central Cr ion than on the terminal Fe ones, the spin 3/2 and the single unpaired a(1) electron are almost localized at the chromium center in the ground state. (3). The complex [LFeCoFeL](3+) has one ground electronic configuration in which two unpaired electrons are localized at terminal iron ions. The ground-state spin S = 1 arises from a kinetic mechanism involving the electron transfer between terminal iron ions as one of the steps. Such a mechanism, leading to a strong ferromagnetic interaction between distant spins, apparently has not been discussed before. (4). The complex [LFeFeFeL](2+) is characterized by both spin and charge degrees of freedom in the ground manifold. The stabilization of the total spin zero or one of the itinerant electrons depends on beta', i.e., corresponds to the observed S = 1 for its negative sign. This behavior does not fit into the double exchange model. (5). In [LFeCrFeL](2+) the delocalization of two itinerant holes in a(1) orbitals takes place over the magnetic core of chromium ion. Although the origin of the ground-state spin S = 2 is the spin dependent delocalization, the spectrum of the low-lying electronic states is again not of a double exchange type. (6). Finally, the complex [LFeCoFeL](2+) has the ground configuration corresponding to the electron delocalization between terminal iron atoms. The estimated magnitude of the corresponding electron transfer is smaller than the relaxation energy of the nuclear distortions induced by the electron localization at one of the centers, leading to vibronic valence trapping observed in this compound.

Journal Article↗

PubMed2002

Mimicking biological electron transport in sol-gel glass: photoinduced electron transfer from zinc cytochrome C to plastocyanin or cytochrome C mediated by mobile inorganic complexes.

Biomimetic studies of electron-transport chains are important for establishing the molecular mechanisms of long-range communications between proteins. We mimic these biological assemblies by encapsulating metalloproteins in sol-gel silica glass and letting mobile inorganic complexes shuttle electrons between the immobilized proteins. We present two examples of such rudimentary electron-transport chains. In both of them the immobilized electron donor is the zinc-substituted cytochrome c, Zncyt; the immobilized electron acceptor is either cupriplastocyanin, pc(II), or ferricytochrome c, cyt(III); and the mobile charge carrier Q/Q(-) is the redox couple FeEDTA(-)(/2)(-) or Ru(NH(3))(6)(3+/2+). The redox processes are photoinduced: Zncyt is excited by the laser pulse and converted to the triplet state, (3)Zncyt, which is a strong reducing agent. Visible absorption, circular dichroism, and electron paramagnetic resonance spectra of the metalloproteins show that encapsulation in sol-gel glass does not affect their intrinsic redox properties. The rigid silica glass spatially separates the proteins from each other. In this matrix, the electron-transfer reactions between (3)Zncyt and pc(II) and between (3)Zncyt and cyt(III), which occur fast in solution, are completely suppressed in the absence of a charge carrier Q/Q(-). The reactivity of FeEDTA(-) and Ru(NH(3))(6)(3+) (as quenchers Q of (3)Zncyt) is minimally affected by the interior of the sol-gel glass. In the glass, the second-order rate constants for the excited-state electron transfer, from (3)Zncyt to Q, are (8.9 +/- 0.6) x 10(6) and (8.0 +/- 2.4) x 10(6) M(-)(1) s(-)(1) for FeEDTA(-) and Ru(NH(3))(6)(3+), respectively. This reaction is followed by the ground-state back electron transfer, from Q(-) to Zncyt(+). In the "monoprotein" glasses Zncyt/Q, the respective second-order rate constants for this back electron-transfer reaction are (4.9 +/- 0.2) x 10(7) and (7.8 +/- 2.7) x 10(7) M(-)(1) s(-)(1). In the "diprotein" glasses Zncyt/Q/pc(II) and Zncyt/Q/cyt(III), containing also the acceptor protein pc(II) or cyt(III), Zncyt(+) decays on two time scales. The faster and major component of this decay is analogous to the only mode of the decay in the Zncyt/Q glasses and is a second-order process. Between 25 and 40% of the initially formed Zncyt(+), however, lives longer (k(slow) =1.1 +/- 0.2 s(-)(1)) and decays by a first-order process. We attribute the lengthening of the Zncyt(+) lifetime to a partial escape of the photogenerated Q(-) into the glass pores, where it reacts with the immobilized pc(II) or cyt(III). Indeed, the visible absorption spectra show the photoinduced reduction of pc(II) and cyt(III). Evidently, the small inorganic complexes, FeEDTA(-)(/2)(-) and Ru(NH(3))(6)(3+/2+), move through the glass pores, react with the encapsulated metalloproteins, and establish the interprotein electron transfer. Each interprotein reaction now occurs in two steps: a mobile charge carrier Q receives an electron from (3)Zncyt, and Q(-) then delivers an electron to pc(II) or cyt(III). Ultimately, the energy of visible light is converted to reducing equivalents for plastocyanin and cytochrome c. The sequential electron transfer described here resembles the events in a rudimentary electron-transport chain. Our findings demonstrate the promise of integrating proteins, with their optimally adjusted redox sites, in photocatalytic materials.

Biomimetic Materials↗

PubMed1976

Scanning electron microscopy of hepatic ultrastructure: secondary, backscattered, and transmitted electron imaging.

Several methods of tissue preparation and different modes of operation of the scanning electron microscope were used to study the ultrastructure of rat liver. Rat livers were perfusion fixed with buffered 2 per cent paraformaldehyde or a mixture of 1.5 per cent paraformaldehyde and 1 per cent glutaraldehyde and processed as follows. Tissue blocks were postfixed in buffered 2 per cent osmium tetroxide followed sequentially by the ligand-mediated osmium binding technique, dehydration and cryofracture in ethanol, and critical point drying. They were then examined without metal coating in the scanning electron microscope operating in the secondary electron and backscattered electron modes. Fifty-micrometer sections were cut with a tissue sectioner, stained with lead citrate, postfixed with osmium, dehydrated, critical point dried, and examined in the secondary electron and back-scattered electron modes. Frozen sections (0.25 to 0.75 mum. thick) were cut by the method of Tokuyasu (Toluyasu KT: J Cell Biol 57:551, 1973) and their scanning transmission electron microscope images were examined either with a scanning transmission electron microscope detector or with a conversion stub using the secondary electron detector. Secondary electron images of the liver prepared by ligand-mediated osmium binding and subsequent cryofracture revealed such intracellular structures as cisternae of the endoplasmic reticulum, lysosomes, mitochondria, lipid droplets, nucleolus and nuclear chromatin, as well as the usual surface morphology, Lipocytes in the perisinusoidal space were readily identified. Backscattered electron images. Unembedded frozen sections had little drying artifact and were virtually free of freezing damage. The scanning transmission electron microscope image revealed those organelles visualized by the secondary electron mode in the ligand-mediated osmium binding-treated tissue.

Animals↗

PubMed2000

Electron fluence perturbation correction factors for solid state detectors irradiated in megavoltage electron beams.

The perturbation correction factor gamma(p) is defined as the deviation of the absorbed dose in the medium from that predicted by the Spencer-Attix extension of the Bragg-Gray cavity theory where the medium occupies exactly the same volume as the solid state cavity and the electron fluence energy spectrum in the cavity is identical in shape, but not necessarily in magnitude, to that in the medium. The value of gamma(p) has been examined for TL detectors irradiated in megavoltage electron beams (5-20 MeV) using the EGS4 Monte Carlo code. LiF and CaF2 solid state detectors simulated were standard size discs of thickness 1 mm and diameter 3.61 mm irradiated in a water phantom with their centres at d(max) or close to it. Values of gamma(p) for LiF ranged from 0.998 +/- 0.005 to 0.994 +/- 0.005 for electron beams with initial energies of 5 and 20 MeV respectively. For CaF2 the corresponding values were 0.956 +/- 0.006 to 0.989 +/- 0.006 for the same size cavities irradiated at the same depth. EGS4 Monte Carlo simulations demonstrate that the total electron fluence (primary electrons and delta-rays) in these solid state detector materials is significantly different from that in water for the same incident electron energy and depth of irradiation. Thus the Spencer-Attix assumption that the electron fluence energy spectrum in the cavity is identical in shape to that in the medium is violated. Differences in the total electron fluence give rise to electron fluence perturbation correction factors which were up to 5% less than unity for CaF2, indicating a strong violation in this case, but were generally less than 1% for LiF. It is the density of the cavity which perturbs the electron fluence, but it is actually the atomic number differences between the medium and cavity that are responsible for the large electron fluence perturbation correction factors for detectors irradiated close to d(max) because the atomic number affects the change in stopping power with energy. When correction is made for the difference between the electron fluence spectrum in the uniform water phantom and the solid state cavity, the Spencer-Attix cavity equation predicts the dose to water within 0.3% in both clinical and monoenergetic electron beams. Harder's formulation for computing the average mass collision stopping power of water to calcium fluoride, surprisingly, requires perturbation correction factors that are closer to unity than those determined using the Spencer-Attix integrals at depths close to d(max).

Electrons↗

PubMed1975

Flavocytochrome b2: kinetic studies by absorbance and electron-paramagnetic-resonance spectroscopy of electron distribution among prosthetic groups.

The reduction by L-lactate of the prosthetic groups of flavocytochrome b2 (L-lactate cytochrome c oxidoreductase from aerobic yeast, a tetrameric molecule containing one haem and one flavin mononucleotide per protomer) was reinvestigated. It was confirmed that the enzyme ultimately takes up 3 electrons per protomer from this 2-electron donor. Stopped-flow absorbance data at an haem isosbestic point to follow the oxidized flavin and in a haem band indicate that, under the conditions used, haem and flavin reduction time courses are indistinguishable, both being biphasic (phases I and II). Comparison with electron paramagnetic resonance data (Fe3+ haem and flavosemiquinone signals) led to a complete description at 24 degrees C of the time courses of the various reduction states of the prosthetic groups. It has been previously demonstrated (Morton and Sturtevant, 1964) that, after the formation of the enzyme-substrate complex, the electron transfer to the enzyme takes place as the first and rate-limiting step of the turnover. In the present study, an initial burst of fully reduced flavin, of small amplitude, is detected at the very beginning of phase I (before 6 ms). The redox forms which accumulate thereafter till the end of phase I (30-35 ms) are the reduced haem (up to 80%), the flavin semiquinone (up to 50%) and the fully reduced flavin (from 25% up to 35%); the total of electrons distributed at the end of phase I is about 2 per protomer meaning that, in this phase, each enzyme site acts as a 2-electron and not a 3-electron acceptor. A 2-electron flow as the limiting step during phase I with the rate constant kI accounts for the steady-state electron flow during catalysis. Phase I is followed by the much slower phase II which corresponds to the entry of the third electron and cannot be involved in the turnover. The interpretation of the results are given as a scheme, with the proper rate constants, allowing a satisfactory fitting of experimental data by simulation. Among the elementary steps required are a rapid distribution of one electron from reduced flavin to the haem, a rapid interprotomers dismutation between couples of flavin semiquinone regenerating two oxidized flavin per tetramer. The very low reactivity of the latter for the entry of the third electron per protomer is tentatively explained by the occurrence of a slow additional step limiting the final reduction reaction. It was observed that, over phase I and the beginning of phase II, from 15 to 200 ms, all the redox species remain apparently under equilibrium conditions. Parallel studies (titrations of flavocytochrome b2 by L-lactate) showed that the set of equilibrium parameters relative to haem and flavin species is significantly different in the "final" equilibrium (after 30 s) from that in the time interval 15-200 ms. Such an anomaly suggests a conformation change takes place very slowly in the molecule after the acceptance of the first two electrons per protomer.

Cytochromes↗

PubMed2001

Electron transfer and conformational change in complexes of trimethylamine dehydrogenase and electron transferring flavoprotein.

The trimethylamine dehydrogenase-electron transferring flavoprotein (TMADH.ETF) electron transfer complex has been studied by fluorescence and absorption spectroscopies. These studies indicate that a series of conformational changes occur during the assembly of the TMADH.ETF electron transfer complex and that the kinetics of assembly observed with mutant TMADH (Y442F/L/G) or ETF (alpha R237A) complexes are much slower than are the corresponding rates of electron transfer in these complexes. This suggests that electron transfer does not occur in the thermodynamically most favorable state (which takes too long to form), but that one or more metastable states (which are formed more rapidly) are competent in transferring electrons from TMADH to ETF. Additionally, fluorescence spectroscopy studies of the TMADH.ETF complex indicate that ETF undergoes a stable conformational change (termed structural imprinting) when it interacts transiently with TMADH to form a second, distinct, structural form. The mutant complexes compromise imprinting of ETF, indicating a dependence on the native interactions present in the wild-type complex. The imprinted form of semiquinone ETF exhibits an enhanced rate of electron transfer to the artificial electron acceptor, ferricenium. Overall molecular conformations as probed by small-angle x-ray scattering studies are indistinguishable for imprinted and non-imprinted ETF, suggesting that changes in structure likely involve confined reorganizations within the vicinity of the FAD. Our results indicate a series of conformational events occur during the assembly of the TMADH.ETF electron transfer complex, and that the properties of electron transfer proteins can be affected lastingly by transient interaction with their physiological redox partners. This may have significant implications for our understanding of biological electron transfer reactions in vivo, because ETF encounters TMADH at all times in the cell. Our studies suggest that caution needs to be exercised in extrapolating the properties of in vitro interprotein electron transfer reactions to those occurring in vivo.

Acyl-CoA Dehydrogenase↗

PubMed1969