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PubMed2004

Evidence for through-space electron transfer in the distance dependence of normal and inverted electron transfer in oligoproline arrays.

Four new helical oligoproline assemblies containing 16, 17, 18, and 19 proline residues and ordered arrays of a Ru(II)-bipyridyl chromophore and a phenothiazine electron-transfer donor have been synthesized in a modular fashion by solid-phase peptide synthesis. These arrays are illustrated and abbreviated as CH(3)CO-Pro(6)-Pra(PTZ)-Pro(n)()-Pra(Ru(II)b(2)m)(2+)-Pro(6)-NH(2), where PTZ is 3-(10H-phenothiazine-10)propanoyl and (Ru(II)b'(2)m)(2+) is bis(4,4'-diethylamide-2,2'-bipyridine)(4-methyl,4'-carboxylate,2,2'-bipyridine)ruthenium(II) dication with n = 2 (2), 3 (3), 4 (4), and 5 (5). They contain PTZ as an electron-transfer donor and (Ru(II)b'(2)m)(2+) as a metal-to-ligand charge transfer (MLCT) light absorber and are separated by proline-to-proline through-space distances ranging from 0 (n = 2) to 12.9 A (n = 5) relative to the n = 2 case. They exist in the proline-II helix form in water, as shown by circular dichroism measurements. Following laser flash Ru(II) --> b'(2)m MLCT excitation at 460 nm in water, excited-state PTZ --> Ru(2+) quenching (k(2)) occurs by reductive electron transfer, followed by Ru(+) --> PTZ(+) back electron transfer (k(3)), as shown by transient absorption and emission measurements in water at 25 degrees C. Quenching with DeltaG degrees = -0.1 eV is an activated process, while back electron transfer occurs in the inverted region, DeltaG degrees = -1.8 eV, and is activationless, as shown by temperature dependence measurements. Coincidentally, both reactions have comparable distance dependences, with k(2)( )()varying from = 1.9 x 10(9) (n = 2) to 2.2 x 10(6) s(-)(1) (n = 4) and k(3) from approximately 2.0 x 10(9) (n = 2) to 2.2 x 10(6) s(-)(1) (n = 4). For both series there is a rate constant enhancement of approximately 10 for n = 5 compared to n = 4 and a linear decrease in ln k with the through-space separation distance, pointing to a significant and probably dominant through-space component to intrahelical electron transfer.

Electrons↗
PubMed2006

DFT/B3LYP study of the substituent effect on the reaction enthalpies of the individual steps of single electron transfer-proton transfer and sequential proton loss electron transfer mechanisms of phenols antioxidant action.

The reaction enthalpies related to the individual steps of two phenolic antioxidants action mechanisms, single electron transfer-proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET), for 30 meta and para-substituted phenols (ArOH) were calculated using DFT/B3LYP method. These mechanisms represent the alternative ways to the extensively studied hydrogen atom transfer (HAT) mechanism. Except the comparison of calculated reaction enthalpies with available experimental and/or theoretical values, obtained enthalpies were correlated with Hammett constants. We have found that electron-donating substituents induce the rise in the enthalpy of proton dissociation (PDE) from ArOH+* radical cation (second step in SET-PT) and in the proton affinities of phenoxide ions ArO- (reaction enthalpy of the first step in SPLET). Electron-withdrawing groups cause the increase in the reaction enthalpies of the processes where electron is abstracted, i.e., in the ionization potentials of ArOH (first step in SET-PT) and in the enthalpy of electron transfer from ArO- (second step in SPLET). Found results indicate that all dependences of reaction enthalpies on Hammett constants of the substituents are linear. The calculations of liquid-phase reaction enthalpies for several para-substituted phenols indicate that found trends hold also in water, although substituent effects are weaker. From the thermodynamic point of view, entering SPLET mechanism represents the most probable process in water.

Antioxidants↗
PubMed2005

Estimates of electronic coupling for excess electron transfer in DNA.

Electronic coupling V(da) is one of the key parameters that determine the rate of charge transfer through DNA. While there have been several computational studies of V(da) for hole transfer, estimates of electronic couplings for excess electron transfer (ET) in DNA remain unavailable. In the paper, an efficient strategy is established for calculating the ET matrix elements between base pairs in a pi stack. Two approaches are considered. First, we employ the diabatic-state (DS) method in which donor and acceptor are represented with radical anions of the canonical base pairs adenine-thymine (AT) and guanine-cytosine (GC). In this approach, similar values of V(da) are obtained with the standard 6-31G(*) and extended 6-31+ +G(**) basis sets. Second, the electronic couplings are derived from lowest unoccupied molecular orbitals (LUMOs) of neutral systems by using the generalized Mulliken-Hush or fragment charge methods. Because the radical-anion states of AT and GC are well reproduced by LUMOs of the neutral base pairs calculated without diffuse functions, the estimated values of V(da) are in good agreement with the couplings obtained for radical-anion states using the DS method. However, when the calculation of a neutral stack is carried out with diffuse functions, LUMOs of the system exhibit the dipole-bound character and cannot be used for estimating electronic couplings. Our calculations suggest that the ET matrix elements V(da) for models containing intrastrand thymine and cytosine bases are essentially larger than the couplings in complexes with interstrand pyrimidine bases. The matrix elements for excess electron transfer are found to be considerably smaller than the corresponding values for hole transfer and to be very responsive to structural changes in a DNA stack.

Base Pairing↗
PubMed2007

Electron photoemission from charged films: absolute cross section for trapping 0-5 eV electrons in condensed CO2.

The electron trapping or attachment cross section of carbon dioxide (CO2) condensed as thin films on a spacer of Ar is obtained using a simple model for electron trapping in a molecular film and then charge releasing from the same film by photon absorption. The measurements are presented for different electron exposures and impact energies, film thicknesses, and probing photon energies. The cross section for trapping an electron of incident energy between 0 and 5 eV reveals three different attachment processes characterized by a maximum at about 0.75 eV, a structured feature around 2.25 eV, and a shoulder around 3.75 eV. From the measurement of their dependence with the probing photon energy, the two lowest processes produce traps having a vertical electron binding energy of approximately 3.5 eV, whereas the highest one yields a slightly higher value of approximately 3.7 eV. The 0.75 eV maximum corresponds to the formation of vibrational Feshbach resonances in (CO2)n- anion clusters. The 2.25 eV feature is attributed to the formation of a vibrationally excited 2Piu anion in (CO2)n- clusters, followed by fast decay into its vibrational ground state without undergoing autodetachment. Finally, 3.75 eV shoulder is assigned to the well-known dissociative electron attachment process from 2Piu anion state producing the O- anion in the gas phase and the (CO2)nO- anions in clusters.

Carbon Dioxide↗
PubMed2005

Experimental determination of electron source parameters for accurate Monte Carlo calculation of large field electron therapy.

Extensive work has been performed to validate Monte Carlo models for both photon and electron beams under standard conditions. However, for large field electron beam therapy, Monte Carlo simulations have not been able to provide good agreement when compared to the measured dose distributions. Since the accuracy of the calculation relies heavily on the geometry parameters of the linear accelerator and the characteristics of the incident electron beam, it is crucial to have a complete comprehension of these independent factors. In this work, the electron focal spot size for a CL21EX linac with various energies (6, 9, 12 and 16 MeV) was measured with a slit camera composed of alternating lead and paper sheets. For all the energies investigated, the electron focal spot is found to be elliptical and has a full width at half maximum (FWHM) ranging from 1.69 mm to 2.24 mm. A shift with respect to the crosshair was associated with each measured focal spot. In addition, we present an improved result for the large field in-air profile by utilizing a proposed divergent beam model in conjunction with the experimental focal spot dimension. This model can potentially provide a solution to the Monte Carlo validation of large field electron beam therapy.

Electrons↗
PubMed1977

1-Methoxy-5-methylphenazinium methyl sulfate. A photochemically stable electron mediator between NADH and various electron acceptors.

This paper describes the properties and application of 1-methoxy-5-methylphenazinium methyl sulfate (1-methoxyPMS), which is a photochemically stable, versatile electron carrier. Like 5-methylphenazinium methyl sulfate (PMS), it mediates electron transfer between NADH and various electron acceptors such as tetrazolium dyes or the electrode of an enzymic electric cell, and yet it does not deteriorate upon storage under scattered light in normal laboratories. The rate of reduction of 1-methoxyPMS coupled to the reoxidation of NADH produced by the lactate dehydrogenase reaction, was even faster than that of PMS. It was also successfully employed as an electron mediator in the enzymic electric cell method for the assay of NAD-linked dehydrogenases. 1-MethoxyPMS solution is rosy pink, and its standard redox potential (E0') is approximately +0.063 V. The use of 1-methoxyPMS will be beneficial in biochemistry as well as medical technology, where PMS has been used as an electron mediator in various electron transfer systems.

Electron Transport↗
PubMed2003

Recent advances in electron imaging, image interpretation and applications: environmental scanning electron microscopy.

One of the latest developments in electron microscopy is the environmental scanning electron microscope (ESEM), which enables soft, moist and/or electrically insulating materials to be viewed without pre-treatment, unlike conventional scanning electron microscopy, in which specimens must be solid, dry and usually electrically conductive. Such an advance has significant implications for studies of the 'native' surfaces of specimens including rocks and minerals, polymers, biological tissues and cells, food and pharmaceutical products, precious artefacts and forensic material, for example. Previous types of electron microscopes made scientists think carefully about the physics of electron-beam interactions with specimens and, hence, the interpretation of images. We now face additional factors influencing the emission and detection of electron signals, unique to the imaging of specimens in the partial vacuum of an ESEM. Just as importantly, we must consider the thermodynamic and kinetic stability of specimens, as appropriate, and explore the possibilities for new applications, particularly those of a dynamic nature. This paper briefly describes some of the issues involved and reviews the current state of understanding.

Environmental Monitoring↗
PubMed2002

A Monte Carlo investigation of fluence profiles collimated by an electron specific MLC during beam delivery for modulated electron radiation therapy.

Modulated electron radiation therapy (MERT) is able to deliver conformal dose to shallow tumors while significantly reducing dose to distal structures and surrounding tissues. An electron specific multileaf collimator (eMLC) has been proposed and constructed as an effective means of delivering electron beams for MERT. The aim of this work is to apply the Monte Carlo method to investigate the fluence profiles collimated by the eMLC in order to achieve accurate beam delivery for MERT. In this work, the EGS4/BEAM code was used to simulate the eMLC collimated electron beams of 6-20 MeV generated from a Varian Clinac 2100C linear accelerator. An attempt was made to describe the fluence profiles with an analytic Sigmoid function. The function parameters were determined by the fittings of the Monte Carlo simulated fluence profiles. How the function parameters depend on the eMLC aperture size, the off-axis location, and the electron beam energy has been investigated. It has been found that the eMLC collimated fluence profiles are dependent on beam energy, while almost independent of leaf location or dimension of MLC aperture. There is little difference in the fluence profiles collimated by the leaf side and the leaf end for the straight-edged leaves. It is possible that these energy-dependent Sigmoid functions can serve as operators to account for the energy dependence of the eMLC collimated fluence profiles. These operators can be incorporated into the inverse planning algorithm to derive desired dose distributions using a set of electron beams of variable energy and field size suitable for delivery by the eMLC.

Computer Simulation↗
PubMed1994

Spatial distribution of bremsstrahlung in a dual electron beam used in total skin electron treatments: errors due to ionization chamber cable irradiation.

The spatial distribution of the bremsstrahlung dose in total skin electron therapy and large electron fields has been assumed to be symmetric along the match line; however, the measured data in the entire plane have not been available. Das et al. ["Spatial distribution of bremsstrahlung in total skin electron irradiation," Med. Phys. 19, 810 (1992)] reported the ion-chamber measured bremsstrahlung data in the Stanford six-field technique and found that the bremsstrahlung dose profile was 2.5 times greater in the caudal plane (beam facing toward floor) as compared to the cephalic plane. This observation was recently quoted by Klevenhagen [Physics and Dosimetry of Therapy Electron Beams (Medical Physics, Madison, WI, 1993)]. Further study using thermoluminescent dosimeters and careful measurements using ion chambers showed that the bremsstrahlung radiation was indeed symmetric at the match line as theoretically predicted and was not a function of the beam angle or the type of floor. The earlier observations of Das et al. were found to be due to cable effects. Ionization chamber cable response in a large electron field and the associated errors in the bremsstrahlung distribution are presented. It is suggested that a cable-free dosimeter should be used for measuring low levels of bremsstrahlung radiation in large electron fields and at extended distances.

Biophysical Phenomena↗
PubMed1999

A rationalised virological electron microscope specimen testing policy. PHLS North West Viral Gastroenteritis and Electron Microscopy Subcommittee.

The aim of this project was to produce guidance for a rationalised virological electron microscopy specimen testing policy for PHLS North West, to facilitate centralisation of a groupwide diagnostic electron microscopy service on a single site. Careful specimen selection to limit numbers and the groupwide use of commercially available enzyme immunoassays has allowed PHLS North West to reduce the number of specimens prepared for electron microscopy. The rationalised virological electron microscopy specimen testing policy has enabled a diagnostic electron microscopy service to be provided from a single site with a manageable workload. Implementation of this specimen testing policy by PHLS North West has been successful and may be applicable to other laboratories (or groups of laboratories) to maximise the use of expensive electron microscopy facilities.

Aged↗
PubMed1995

Using laser scanning confocal microscopy as a guide for electron microscopic study: a simple method for correlation of light and electron microscopy.

Anatomic study of synaptic connections in the nervous system is laborious and difficult, especially when neurons are large or have fine branches embedded among many other processes. Although electron microscopy provides a powerful tool for such study, the correlation of light microscopic appearance and electron microscopic detail is very time-consuming. We report here a simple method combining laser scanning confocal microscopy and electron microscopy for study of the synaptic relationships of the neurons in the antennal lobe, the first central neuropil in the olfactory pathway, of the moth Manduca sexta. Neurons were labeled intracellularly with neurobiotin or biocytin, two widely used stains. The tissue was then sectioned on a vibratome and processed with both streptavidin-nanogold (for electron microscopic study) and streptavidin-Cy3 (for confocal microscopic study) and embedded in epon/araldite. Interesting areas of the labeled neuron were imaged in the epon/araldite blocks with laser scanning confocal microscopy and then thin-sectioned at the indicated depth for electron microscopic study. This method provides an easy, reliable way to correlate three-dimensional light microscopic information with electron microscopic detail, and can be very useful in studies of synaptic connections.

Animals↗
PubMed1987

Scanning electron microscopy in the backscattered electron imaging (BEI) mode: applications to clinical hematology.

Elements of high atomic number backscatter electrons that carry information of cytochemical and of immunocytochemical significance in high resolution scanning electron microscopy (SEM). Reaction products of enzyme cytochemistry containing lead or osmium have been used to localize the sites of phosphatase and of endogenous peroxidase, respectively. Particles of colloidal gold, ranging in diameter from 40 to 13 nm, have been successfully used to label cell surface antigens specifically identified by murine monoclonal antibodies. The diagnostic potential of the scanning electron microscope in clinical hematology appears to be considerably enhanced by the use of the backscattered electron imaging (BEI) mode. Quantitation of the number of gold-labeled epitopes on cell surfaces cannot be achieved in the conventional secondary electron mode of the SEM, but is an attractive possibility in the backscattered electron imaging mode.

Antigens, Surface↗
PubMed1984

Identification of cells by backscattered electron imaging of silver stained bulk tissues in scanning electron microscopy.

Anuran tadpole tail muscle was stained en bloc by a modified light microscope silver stain for light microscopy and freeze-fractured in liquid nitrogen after partial dehydration with ethanol. The fractured specimens were observed in both secondary electron and backscattered electron modes in a scanning electron microscope. Since the cell nuclei specifically stained with silver provided high contrast against the unstained background due to atomic number contrast of backscattered electron image, various cells were easily identified by a comparison of secondary electron images and compositional images of backscattered electron signals.

Animals↗
PubMed1993

Absence of electron transport (Rho 0 state) restores growth of a manganese-superoxide dismutase-deficient Saccharomyces cerevisiae in hyperoxia. Evidence for electron transport as a major source of superoxide generation in vivo.

To address the possibility that electron transport is a biologically significant source of superoxide anion (O2-.) during exposure to hyperoxia in vivo, we constructed Saccharomyces cerevisiae strains with selective disruptions in the gene encoding the mitochondrial manganese-containing superoxide dismutase (Mn-SOD) and/or genes encoding proteins critical for complexes in electron transport. We hypothesized that complete absence of electron transport would restore growth in hyperoxia to a Mn-SOD-deficient yeast. We found that yeast deficient in Mn-SOD activity failed to grow normally in hyperoxia (95% O2, 5% CO2). In contrast, Mn-SOD-deficient yeast with complete absence of electron transport (the Rho 0 state) grew normally in hyperoxia. By comparison, Mn-SOD-deficient yeast which were deficient only in cytochrome-c-oxidase, the terminal step in electron transport, had only partially restored growth in hyperoxia. Our results indicate that electron transport is a major source of O2-. in vivo, and that the principal site of this O2-. production is proximal to the cytochrome-c-oxidase complex.

Electron Transport↗
PubMed1987

Electron paramagnetic resonance and magnetic circular dichroism studies of electron-transfer flavoprotein-ubiquinone oxidoreductase from pig liver.

Pig liver electron-transfer flavoprotein-ubiquinone oxidoreductase has been investigated by room temperature UV-visible, low-temperature electron paramagnetic resonance and low-temperature magnetic circular dichroism spectroscopies. The results provide unambiguous evidence for the presence of a single [4Fe-4S] cluster that is diamagnetic in the isolated enzyme and becomes paramagnetic with an S = 1/2 ground state on reduction with dithionite or enzymatically with the physiological electron donor. The EPR data for samples at pH 7.8 indicate that FAD is reduced by one electron to the anionic semiquinone form in the enzymatically reduced enzyme, and by two electrons to the hydroquinone form by excess dithionite. The possibility of weak spin-spin interaction between the FAD semiquinone and the [4Fe-4S]1+ center is discussed in the light of the observation of a small increase in the linewidth of the Fe-S EPR in enzymatically reduced samples.

Animals↗
PubMed2005

Stabilization of non-productive conformations underpins rapid electron transfer to electron-transferring flavoprotein.

Crystal structures of protein complexes with electron-transferring flavoprotein (ETF) have revealed a dual protein-protein interface with one region serving as anchor while the ETF FAD domain samples available space within the complex. We show that mutation of the conserved Glu-165beta in human ETF leads to drastically modulated rates of interprotein electron transfer with both medium chain acyl-CoA dehydrogenase and dimethylglycine dehydrogenase. The crystal structure of free E165betaA ETF is essentially identical to that of wild-type ETF, but the crystal structure of the E165betaA ETF.medium chain acyl-CoA dehydrogenase complex reveals clear electron density for the FAD domain in a position optimal for fast interprotein electron transfer. Based on our observations, we present a dynamic multistate model for conformational sampling that for the wild-type ETF. medium chain acyl-CoA dehydrogenase complex involves random motion between three distinct positions for the ETF FAD domain. ETF Glu-165beta plays a key role in stabilizing positions incompatible with fast interprotein electron transfer, thus ensuring high rates of complex dissociation.

Acyl-CoA Dehydrogenase↗