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Electron Distribution (electron + distribution)
Selected AbstractsInterflavin electron transfer in human cytochrome P450 reductase is enhanced by coenzyme bindingFEBS JOURNAL, Issue 12 2003Relaxation kinetic studies with coenzyme analogues The role of coenzyme binding in regulating interflavin electron transfer in human cytochrome P450 reductase (CPR) has been studied using temperature-jump spectroscopy. Previous studies [Gutierrez, A., Paine, M., Wolf, C.R., Scrutton, N.S., & Roberts, G.C.K. Biochemistry (2002) 41, 4626,4637] have shown that the observed rate, 1/,, of interflavin electron transfer (FADsq , FMNsq,FADox , FMNhq) in CPR reduced at the two-electron level with NADPH is 55 ± 2 s,1, whereas with dithionite-reduced enzyme the observed rate is 11 ± 0.5 s,1, suggesting that NADPH (or NADP+) binding has an important role in controlling the rate of internal electron transfer. In relaxation experiments performed with CPR reduced at the two-electron level with NADH, the observed rate of internal electron transfer (1/, = 18 ± 0.7 s,1) is intermediate in value between those seen with dithionite-reduced and NADPH-reduced enzyme, indicating that the presence of the 2,-phosphate is important for enhancing internal electron transfer. To investigate this further, temperature jump experiments were performed with dithionite-reduced enzyme in the presence of 2,,5,-ADP and 2,-AMP. These two ligands increase the observed rate of interflavin electron transfer in two-electron reduced CPR from 1/, = 11 s,1 to 35 ± 0.2 s,1 and 32 ± 0.6 s,1, respectively. Reduction of CPR at the two-electron level by NADPH, NADH or dithionite generates the same spectral species, consistent with an electron distribution that is equivalent regardless of reductant at the initiation of the temperature jump. Spectroelectrochemical experiments establish that the redox potentials of the flavins of CPR are unchanged on binding 2,,5,-ADP, supporting the view that enhanced rates of interdomain electron transfer have their origin in a conformational change produced by binding NADPH or its fragments. Addition of 2,,5,-ADP either to the isolated FAD-domain or to full-length CPR (in their oxidized and reduced forms) leads to perturbation of the optical spectra of both the flavins, consistent with a conformational change that alters the environment of these redox cofactors. The binding of 2,,5,-ADP eliminates the unusual dependence of the observed flavin reduction rate on NADPH concentration (i.e. enhanced at low coenzyme concentration) observed in stopped-flow studies. The data are discussed in the context of previous kinetic studies and of the crystallographic structure of rat CPR. [source] Regulation of expression of terminal oxidases in Paracoccus denitrificansFEBS JOURNAL, Issue 8 2001Marijke F. Otten In order to study the induction of terminal oxidases in Paracoccus denitrificans, their promoters were fused to the lacZ reporter gene and analysed in the wild-type strain, in an FnrP-negative mutant, in a cytochrome bc1 -negative mutant, and in six single or double oxidase-negative mutant strains. The strains were grown under aerobic, semi-aerobic, and denitrifying conditions. The oxygen-sensing transcriptional-regulatory protein FnrP negatively regulated the activity of the qox promoter, which controls expression of the ba3 -type quinol oxidase, while it positively regulated the activity of the cco promoter, which controls expression of the cbb3 -type cytochrome c oxidase. The ctaDII and ctaC promoters, which control the expression of the aa3 -type cytochrome c oxidase subunits I and II, respectively, were not regulated by FnrP. The activities of the latter two promoters, however, did decrease with decreasing oxygen concentrations in the growth medium, suggesting that an additional oxygen-sensing mechanism exists that regulates transcription of ctaDII and ctaC. Apparently, the intracellular oxygen concentration (as sensed by FnrP) was not the only signal to which the oxidase promoters responded. At given extracellular oxygen status, both the qox and the cco promoters responded to mutations in terminal oxidase genes, whereas the ctaDII and ctaC promoters did not. The change of electron distribution through the respiratory network, resulting from elimination of one or more oxidase genes, may have changed intracellular signals that affect the activities of the qox and cco promoters. On the other hand, the re-routing of electron distribution in the respiratory mutants hardly affected the oxygen consumption rate as compared to that of the wild-type. This suggests that the mutants adapted their respiratory network in such a way that they were able to consume oxygen at a rate similar to that of the wild-type strain. [source] V-Shaped Thiophene-Based Oligomers with Improved Electroluminescence Properties,ADVANCED FUNCTIONAL MATERIALS, Issue 4 2005G. Barbarella Abstract The synthesis via the Stille coupling of a new family of oligomers derived from benzo[b]thiophene is reported. Owing to their branched molecular structure lacking any symmetry element, these compounds display a low tendency to crystallization and better film-forming properties than their linear counterparts. Spin-coated films show photoluminescence efficiencies up to 50,%. Light-emitting diodes with spin-coated films as the active layers display markedly improved performance with respect to similar devices based on linear oligothiophenes, with luminance values up to more than 10,000,cd,m,2. Semiempirical PM3 and ZINDO/S calculations provide insight into the molecular geometries and electron distribution of the frontier orbitals of the new compounds. Cyclic voltammetry data indicates that the transformation of the thienyl sulfur of benzo[b]thiophene to the corresponding thienyl- S,S -dioxide leads an increase in electron affinity by 0.5,0.7,V, analogous to that of the corresponding linear oligomers. [source] Reaction mechanisms between methylamine and a few Schiff bases: Ab initio potential energy surfaces of a catalytic step in semicarbazide sensitive amino oxidases (SSAO)INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2001Giuliano Alagona Abstract The potential energy surfaces for the transamination reaction catalyzed by SSAO were explored for some of the possible reactants considered in a preliminary investigation (Comput Chem 2000, 24, 311). The proton transfer to methylamine (as a model of the catalytic base belonging to the enzyme active site),either from the keto or enol form of the reactant Schiff bases with one of the possible cofactors, pyridoxal phosphate, PLP (using as a model the pyridoxal ring protonated at N),was investigated. The enol form seems to be preferred in the region of the neutral intermediate, because even the keto form undergoes a spontaneous rearrangement to the enol form once the C, proton is delivered to methylamine, producing methylammonium. When the proton is returned back to the Schiff base (on C1), the adduct is about 1.4 kcal/mol more stable than the reactants, while a canonical electron distribution is obtainable only for the enol form. The proton transfer to methylamine was also studied in the presence of the other possible cofactor (para or ortho) topaquinone, TQ. A steep uphill pathway, similar to the keto-pyridoxal Schiff base one, is obtained using the Schiff base with pTQ, which requires a rearrangement to the final intermediate. On the contrary, using the oTQ structures with the quinonoid O on the same side of methylamine, the proton abstracted from the Schiff base goes spontaneously onto the other quinonoid oxygen. The effect on the barrier heights produced by the presence of a variety of functional groups in the vicinity of the pyridoxal ring nitrogen was also examined. © 2001 John Wiley & Sons, Inc. Int J Quant Chem, 2001 [source] Amplification of nonlinear currents generation at harmonics frequencies of submillimeter radiationLASER PHYSICS LETTERS, Issue 9 2006G. Ferrante Abstract The conditions are found when rapid heating of the electrons of a degenerate semiconductor in the presence of two electric fields, one constant and the other variable, is accompanied by the formation of a distribution function significantly departing from the Fermi one. It is also shown that the newly found modification of the electron distribution yields a relative amplification of nonlinear currents generation. (© 2006 by Astro, Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] Hard electron energy distribution in the relativistic shocks of gamma-ray burst afterglowsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008L. Resmi ABSTRACT Particle acceleration in relativistic shocks is not a very well understood subject. Owing to that difficulty, radiation spectra from relativistic shocks, such as those in gamma-ray burst (GRB) afterglows, have been often modelled by making assumptions about the underlying electron distribution. One such assumption is a relatively soft distribution of the particle energy, which need not be true always, as is obvious from observations of several GRB afterglows. In this paper, we describe modifications to the afterglow standard model to accommodate energy spectra which are ,hard'. We calculate the overall evolution of the synchrotron and Compton flux arising from such a distribution. We also model two afterglows, GRB010222 and GRB020813, under this assumption and estimate the physical parameters. [source] The X-ray/,-ray spectrum of XTE J1550,564 in the very high stateMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2003Marek Gierli ABSTRACT We fit the broad-band X-ray/,-ray spectrum (0.8,1000 keV) of the accreting black hole XTE J1550,564 in the very high state. The quasi-simultaneous data from ASCA, RXTE and OSSE (on CGRO) show that the disc is strongly Comptonized, with a high-energy tail extending out to several hundred keV. However, inverse Compton scattering by a purely thermal or purely power-law electron distribution cannot explain the observed spectrum. Instead, the data require a hybrid distribution, with both thermal and non-thermal electrons scattering the disc photons. This is very similar to the electron distribution inferred for other high and very high state black hole binaries, showing that it is a generic feature of high mass accretion rate black holes. [source] Ab initio determination of the valence electron distribution in the average structure of the incommensurately modulated calaverite AuTe2ACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2001Razvan Caracas The valence-electron density distribution of the average structure of incommensurately modulated calaverite, AuTe2, has been computed using density-functional theory. High-density regions, centered around the Au and Te atoms, are not spheric, but present charge concentrations along the Au,Te and Te,Te bonds. The electronic band structure and its corresponding density of states reveal the presence of three electronic band groups, constituted mainly by Te 5s, Au 5d and hybrids of Te 6p + Au 6s + Au 5d orbitals. The electrons belonging to the last block are responsible for the chemical bonds. [source] Electron Fluctuation in Pericyclic and Pseudopericyclic ReactionsCHEMPHYSCHEM, Issue 1 2006Eduard Matito Borderline cases: Differentiating between pericyclic and pseudopericyclic reactions is controversial. The authors analyze the electron distribution of the transition state of some known and some controversial reactions by means of the electron localization function (ELF) to elucidate their mechanism (see figure). This analysis provides a definitive criterion to distinguish between both electrocyclic processes. [source] Cyclotron Maser Radiation in Space and Laboratory PlasmasCONTRIBUTIONS TO PLASMA PHYSICS, Issue 5-6 2004R. Bingham Abstract One of the best known coherent radio emission mechanisms is the electron cyclotron maser instability. In this article we will demonstrate that electron cyclotron maser emission is directly associated with particular types of charged particle acceleration and propagation in space and laboratory plasmas. These include electron ring distributions, horseshoe or crescent shaped electron distribution functions. Planetary and stellarmagnetospheres are examples of where horseshoe or crescent shaped electron distributions can be found and astrophysical shocks produce ring shaped electron distribution functions. In the laboratory horseshoe or crescent shaped distributions are produced whenever an electron beam propagates into a stronger magnetic field region. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electron structure of polysilanes.INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2001Are these polymers one-dimensional systems? Abstract Electron structures of polysilane, 1-methyl polysilane, 1,1-dimethyl polysilane, 1-phenyl polysilane, 1,1-diphenyl polysilane, and 1-methyl 1-phenyl polysilane in solid state were calculated using the cyclic cluster orbital method (CCO), based on a Hartree,Fock approach using a quasirelativistic INDO Hamiltonian applied to cyclic clusters. Effect of the redistribution of electron density on the silicon backbone chains after the substitution of hydrogen by methyl and phenyl groups has been investigated. Going from one- to three-dimensional models, significant changes appear in calculated electron distributions, as well as in the band structure topologies of the corresponding polymers. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 157,168, 2001 [source] Theoretical studies on high-valent manganese porphyrins: Toward a deeper understanding of the energetics, electron distributions, and structural features of the reactive intermediates of enzymatic and synthetic manganese-catalyzed oxidative processesISRAEL JOURNAL OF CHEMISTRY, Issue 1 2000Abhik Ghosh We present here a relatively comprehensive theoretical study, based on nonlocal density functional theory calculations, of the energetics, electron distributions, and structural features of the low-lying electronic states of various high-valent intermediates of manganese porphyrins. Two classes of molecules have been examined: (a) compounds with the general formula [(P)MnX2]0 (P = porphyrin; X = F, Cl, PF6) and (b) high-valent manganese-oxo species. For [(P)Mn(PF6)2]0, the calculations reveal a number of nearly equienergetic quartet and sextet states as the lowest states, consistent with experimental results on a comparable species, [(TMP)Mn(ClO4)2]0 (TMP = tetramesitylporphyrin). In contrast, [(P)MnCl2]0 and [(P)MnF2]0 have a single well-defined S = 3/2 Mn(IV) ground state, again in agreement with experiment, with the three unpaired spins largely concentrated (>90%) on the manganese atom. Manganese(IV)-oxo porphyrins have an S = 3/2 ground state, with the three unpaired spins distributed approximately 2.3:0.7 between the manganese and oxygen atoms. The metal-to-oxygen spin delocalization, as measured by the oxygen spin population, for MnIV = O porphyrins is less than, but still qualitatively similar to, that in analogous iron(IV)-oxo intermediates, indicating that the MnIV = O bond is significantly weaker than the FeIV = O bond in an analogous molecule. Thus, the optimized metal,oxygen bond distances are 1.654 and 1.674 Å for (P)FeIV(O)(Py) and (P)MnIV(O)(Py), respectively (Py = pyridine). This is consistent with the experimental observation that MnIV = O stretching frequencies are over 10% lower than FeIV = O stretching frequencies for analogous compounds. For [(P)Mn(O)(PF6)]0, [(P)Mn(O)(Py)]+, and [(P)Mn(O)(F)]0, the ground states clearly correspond to a (dxy)2 Mn(V) configuration and the short Mn,O distances of 1.541, 1.546, and 1.561 Å for the three compounds, respectively, reflect the formal triple bond character of the Mn,O interaction. Interestingly, the corresponding Mn(IV)-oxo porphyrin cation radical states are calculated to be a few tenths of an electrovolt higher than the Mn(V) ground states, suggesting that the Mn(IV)-oxo porphyrin cation radicals are not likely to exist as ground-state species. [source] Erratum: Effects of non-thermal tails in Maxwellian electron distributions on synchrotron and Compton processesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2001Grzegorz Wardzi First page of article [source] | |||||||||||||