Beam-energy Dependence Of Charge Separation Along The Magnetic Field In Au+au Collisions At Rhic.

Local parity-odd domains are theorized to form inside a quark-gluon plasma which has been produced in high-energy heavy-ion collisions. The local parity-odd domains manifest themselves as charge separation along the magnetic field axis via the chiral magnetic effect. The experimental observation of charge separation has previously been reported for heavy-ion collisions at the top RHIC energies. In this Letter, we present the results of the beam-energy dependence of the charge correlations in Au+Au collisions at midrapidity for center-of-mass energies of 7.7, 11.5, 19.6, 27, 39, and 62.4 GeV from the STAR experiment. After background subtraction, the signal gradually reduces with decreased beam energy and tends to vanish by 7.7 GeV. This implies the dominance of hadronic interactions over partonic ones at lower collision energies.

The effect of the generation and handling in the acquired electrostatic charge in airborne particles

The measurement of the charge distribution in laboratory generated aerosols particles was carried out. Four cases of electrostatic charge acquisition by aerosol particles were evaluated. In two of these cases. the charges acquired by the particles were naturally derived from the aerosol generation procedure itself, without using any additional charging method. Ill the other two cases, a corona charger and an impact charger were utilized as Supplementary methods for charge generation. Two types of aerosol generators were used in the dispersion of particles in the gas Stream: the vibrating orifice generator TSI model 3450 and the rotating plate generator TSI model 3433. In the vibrating orifice generator. a Solution of methylene blue Was used and the generated particles were mono-dispersed. Different mono-aerosols were generated with particle diameters varying from 6.0 x 10(-6) m to 1.4 x 10(-5) m. In the rotating plate generator, a poly-dispersed phosphate rock concentrate with Stokes mean diameter of 1.30 x 10(-6) m and size range between 1.5 x 10(-7) m and 8.0 x 10(-6) m Was utilized as powder material in all tests. In the tests performed with the mono-dispersed particles. the median charges of the particles varied between -3.0 x 10-(16) C and -5.0 x 10(-18) degrees C and a weak dependence between particle size and charge was observed. The particles were predominantly negatively charged. In the tests with the poly-dispersed particles the median charges varied fairly linearly with the particle diameter and were negative. The order of magnitude of the results obtained is in accordance with data reported in the literature. The charge distribution, in this case, was wider, so that an appreciable amount of particles were positively charged. The relative spread of the distribution varied with the charging method. It was also noticed that the corona charger acted very effectively in charging the particles. (C) 2008 Elsevier BY. All rights reserved.

Facile photoinduced charge separation through a cyanoacetylide bridge in a heterobimetallic Fe(II)-Re(I) complex

Photoinduced Fe-to-bpy charge transfer in [{Cp(dppe)Fe}-(mu-C CC N){Re(CO)(3)(bpy)}]PF6 has been observed by ps-TRIR spectroscopy, supported by UV-Vis/IR spectroelectrochemistry and DFT calculations.

Ultrafast charge separation in a photoreactive rhenium-appended porphyrin assembly monitored by picosecond transient infrared spectroscopy

Rhenium(bipyridine)(tricarbonyl)(picoline) units have been linked covalently to tetraphenylmetalloporphyrins of magnesium and zinc via an amide bond between the bipyridine and one phenyl substituent of the porphyrin. The resulting complexes, abbreviated as [Re(CO)(3)(Pic)Bpy-MgTPP][OTf] and [Re(CO)(3)(Pic)Bpy-ZnTPP][OTf], exhibit no signs of electronic interaction between the Re(CO)(3)(bpy) units and the metalloporphyrin units in their ground states. However, emission spectroscopy reveals solvent-dependent quenching of porphyrin emission on irradiation into the long-wavelength absorption bands localized on the porphyrin. The characteristics of the excited states have been probed by picosecond time-resolved absorption (TRVIS) spectroscopy and time-resolved infrared (TRIR) spectroscopy in nitrile solvents. The presence of the charge-separated state involving electron transfer from MgTPP or ZnTPP to Re(bpy) is signaled in the TRIR spectra by a low-frequency shift in the nu(CO) bands of the Re(CO)(3) moiety similar to that observed by spectroelectrochemical reduction. Long-wavelength excitation of [Re(CO)(3)(Pic)Bpy-MTPP][OTf] results in characteristic TRVIS spectra of the S-1 state of the porphyrin that decay with a time constant of 17 ps (M = Mg) or 24 ps (M = Zn). The IR bands of the CS state appear on a time scale of less than 1 ps (Mg) or ca. 5 ps (Zn) and decay giving way to a vibrationally excited (i.e., hot) ground state via back electron transfer. The IR bands of the precursors recover with a time constant of 35 ps (Mg) or 55 ps (Zn). The short lifetimes of the charge-transfer states carry implications for the mechanism of reaction in the presence of triethylamine.

Redox control of light-induced charge separation in a transition metal cluster: photochemistry of a methyl viologen-substituted [Os-3(CO)10(a-diimine)] cluster

Sub)picosecond transient absorption (TA) and time-resolved infrared (TRIR) spectra of the cluster [OS3(CO)(10-) (AcPy-MV)](2+) (the clication AcPy-MV = Acpy-MV2+ = [2-pyridylacetimine-N-(2-(1'-methyl-4,4'-bipyridine-1,1'-diium-1-yl) ethyl)] (PF6)(2)) (1(2+)) reveal that photoinduced electron transfer to the electron-accepting 4,4'-bipyridine-1,1'diium (MV2+) moiety competes with the fast relaxation of the initially populated sigmapi* excited state of the cluster to the ground state and/or cleavage of an Os-Os bond. The TA spectra of cluster 12 in acetone, obtained by irradiation into its lowest-energy absorption band, show the characteristic absorptions of the one-electron-reduced MV*(+) unit at 400 and 615 nm, in accordance with population of a charge-separated (CS) state in which a cluster-core electron has been transferred to the lowest pi* orbital of the remote MV2+ unit. This assignment is confirmed by picosecond TRIR spectra that show a large shift of the pilot highest-frequency nu(CO) band of 1(2+) by ca. +40 cm(-1), reflecting the photooxidation of the cluster core. The CS state is populated via fast (4.2 x 10(11) s(-1)) and efficient (88%) oxidative quenching of the optically populated sigmapi* excited state and decays biexponentially with lifetimes of 38 and 166 ps (1:2:1 ratio) with a complete regeneration of the parent cluster. About 12% of the cluster molecules in the sigmapi* excited state form long-lived open-core biradicals. In strongly coordinating acetonitrile, however, the cluster core-to-MV2+ electron transfer in cluster 12+ results in the irreversible formation of secondary photoproducts with a photooxidized cluster core. The photochemical behavior of the [Os-3(CO)(10)(alpha-diimine-MV)](2+) (donor-acceptor) dyad can be controlled by an externally applied electronic bias. Electrochemical one-electron reduction of the MV2+ moiety prior to the irradiation reduces its electron-accepting character to such an extent that the photoinduced electron transfer to MV*+ is no longer feasible. Instead, the irradiation of reduced cluster 1(.)+ results in the reversible formation of an open-core zwitterion, the ultimate photoproduct also observed upon irradiation of related nonsubstituted clusters [Os-3(CO)(10)(alpha-diimine)] in strongly coordinating solvents such as acetonitrile.

Photoinduced hole-transfer in semiconducting polymer/low-bandgap cyanine dye blends: evidence for unit charge separation quantum yield

Power-conversion efficiencies of organic heterojunction solar cells can be increased by using semiconducting donor-acceptor materials with complementary absorption spectra extending to the near-infrared region. Here, we used continuous wave fluorescence and absorption, as well as nanosecond transient absorption spectroscopy to study the initial charge transfer step for blends of a donor poly(p-phenylenevinylene) derivative and low-band gap cyanine dyes serving as electron acceptors. Electron transfer is the dominant relaxation process after photoexcitation of the donor. Hole transfer after cyanine photoexcitation occurs with an efficiency close to unity up to dye concentrations of similar to 30 wt%. Cyanines present an efficient self-quenching mechanism of their fluorescence, and for higher dye loadings in the blend, or pure cyanine films, this process effectively reduces the hole transfer. Comparison between dye emission in an inert polystyrene matrix and the donor matrix allowed us to separate the influence of self-quenching and charge transfer mechanisms. Favorable photovoltaic bilayer performance, including high open-circuit voltages of similar to 1 V confirmed the results from optical experiments. The characteristics of solar cells using different dyes also highlighted the need for balanced adjustment of the energy levels and their offsets at the heterojunction when using low-bandgap materials, and accentuated important effects of interface interactions and solid-state packing on charge generation and transport.

Fundamental Limits on Wavelength, Efficiency and Yield of the Charge Separation Triad

In an attempt to optimize a high yield, high efficiency artificial photosynthetic protein we have discovered unique energy and spatial architecture limits which apply to all light-activated photosynthetic systems. We have generated an analytical solution for the time behavior of the core three cofactor charge separation element in photosynthesis, the photosynthetic cofactor triad, and explored the functional consequences of its makeup including its architecture, the reduction potentials of its components, and the absorption energy of the light absorbing primary-donor cofactor. Our primary findings are two: First, that a high efficiency, high yield triad will have an absorption frequency more than twice the reorganization energy of the first electron transfer, and second, that the relative distance of the acceptor and the donor from the primary-donor plays an important role in determining the yields, with the highest efficiency, highest yield architecture having the light absorbing cofactor closest to the acceptor. Surprisingly, despite the increased complexity found in natural solar energy conversion proteins, we find that the construction of this central triad in natural systems matches these predictions. Our analysis thus not only suggests explanations for some aspects of the makeup of natural photosynthetic systems, it also provides specific design criteria necessary to create high efficiency, high yield artificial protein-based triads.

Charge separation and recombination in novel polymeric absorber materials for organic solar cells : a photophysical study

In dieser Dissertation wird die Ladungsträgergeneration und -rekombination in neuen polymeren Absorbermaterialien für organische Solarzellen untersucht. Das Verständnis dieser Prozesse ist wesentlich für die Entwicklung neuer photoaktiver Materialsysteme, die hohe Effizienzen erzielen und organische Solarzellen konkurrenzfähig im Bereich der erneuerbaren Energien machen. Experimentell verwendet diese Arbeit hauptsächlich die Methode der transienten Absorptionsspektroskopie, die sich für die Untersuchung photophysikalischer Prozesse auf einer Zeitskala von 100 fs bis 1 ms als sehr leistungsfähig erweist. Des Weiteren wird eine soft-modeling Methode vorgestellt, die es ermöglicht, photophysikalische Prozesse aus einer gemessenen transienten Absorptions-Datenmatrix zu bestimmen, wenn wenig a priori Kenntnisse der Reaktionskinetiken vorhanden sind. Drei unterschiedliche Donor:Akzeptor-Systeme werden untersucht; jedes dieser Systeme stellt eine andere Herangehensweise zur Optimierung der Materialien dar in Bezug auf Lichtabsorption über einen breiten Wellenlängenbereich, effiziente Ladungstrennung und schnellen Ladungstransport. Zuerst wird ein Terpolymer untersucht, das aus unterschiedlichen Einheiten für die Lichtabsorption und den Ladungstransport besteht. Es wird gezeigt, dass es möglich ist, den Fluss angeregter Zustände vom Chromophor auf die Transporteinheit zu leiten. Im zweiten Teil wird der Einfluss von Kristallinität auf die freie Ladungsträgergeneration mit einer Folge von ternären Mischungen, die unterschiedliche Anteile an amorphem und semi-kristallinem Polymer enthalten, untersucht. Dabei zeigt es sich, dass mit steigendem amorphen Polymeranteil sowohl der Anteil der geminalen Ladungsträgerrekombination erhöht als auch die nicht-geminale Rekombination schneller ist. Schlussendlich wird ein System untersucht, in dem sowohl Donor als auch Akzeptor Polymere sind, was zu verbesserten Absorptionseigenschaften führt. Die Rekombination von Ladungstransferzuständen auf der unter 100 ps Zeitskala stellt hier den hauptsächliche Verlustkanal dar, da freie Ladungsträger nur an Grenzflächen erzeugt werden können, an denen Donor und Akzeptor face-to-face zueinander orientiert sind. Darüber hinaus wird festgestellt, dass weitere 40-50% der Ladungsträger durch die Rekombination von Grenzflächenzuständen verloren gehen, die aus mobilen Ladungsträgern geminal gebildet werden.

The roles of electrostatic charge pairing in the interaction between cytochrome P-450 and NADPH-cytochrome P-450 reductase

The cytochrome P450 enzyme catalysis requires two electrons transferred from NADPH-cytochrome P450 reductase (reductase) to P450. Electrostatic charge-pairing has been proposed to be one of the major forces in the interaction between P450 and reductase. In order to obtain further insight into the molecular basis for the protein interaction, I used two methods, chemical modification and specific anti-peptide antibodies, to study the involvement and importance of charged amino acid residues. Acetylation of lysine residues of P450c and P450b by acetic anhydride dramatically inhibited the reductase-supported P450c-dependent ethoxycoumarin hydroxylation activity, but P450 activity supported by cumene hydroperoxide is relatively unchanged. The modification of lysine residues of P450c and P450b did not grossly disturb the protein conformation as revealed by several spectral studies. This differential effect of lysine modification on the P450 activity in the system reconstituted with reductase versus the system supported by cumene hydroperoxide suggested an important role for P450 lysine residues in the interaction with reductase. Using $\rm\sp{14}C$-acetic anhydride, P450 lysine residues were labelled and further identified on P450c and P450b. Those lysine residues are at position 97, 271, 279, and 407 for P450c, and 251, 384, 422, 433, and 473 for P450b. Alignment of those identified lysine residues on P450c and P450b with amino acid residues identified in other studies indicated those residues reside in three major sequence areas. Modification of arginine residues of P450b by phenylglyoxal and 2, 3-butanedione have no significant effect on P450 activity either supported by NADPH and reductase or supported by cumene hydroperoxide. Further studies using $\rm\sp{14}C$-phenylglyoxal reveals that no incorporation of phenylglyoxal into P450b was found. These results demonstrated a predominant role of lysine residues of P450 in the electrostatic interaction with reductase. To understand the protein binding sites on each of P450 and reductase, I generated three anti-peptide antibodies against regions on reductase and five anti-peptide antibodies against five putative reductase binding sites on P450c. These anti-peptide antibodies were affinity purified and characterized on ELISA and by Western blot analysis. Inhibition experiments using these antibodies demonstrated that regions 109-120 and 204-220 of reductase are probably the two major binding sites for P450. The association of reductase with cytochromes P450 and cytochrome c may rely on different mechanisms. The data from experiments using anti-peptide (P450c) antibodies supports the important role of P450c lysine residues 271/279 and 458/460 in the interaction with reductase. ^

Long-time quantum simulation of the primary charge separation in bacterial photosynthesis.

Accurate quantum mechanical simulations of the primary charge transfer in photosynthetic reaction centers are reported. The process is modeled by three coupled electronic states corresponding to the photoexcited chlorophyll special pair (donor), the reduced bacteriopheophytin (acceptor), and the reduced accessory chlorophyll (bridge) that interact with a dissipative medium of protein and solvent degrees of freedom. The time evolution of the excited special pair is followed over 17 ps by using a fully quantum mechanical path integral scheme. We find that a free energy of the reduced accessory chlorophyll state approximately equal to 400 cm(-1) lower than that of the excited special pair state yields state populations in agreement with experimental results on wild-type and modified reaction centers. For this energetic configuration electron transfer is a two-step process.

Hydrophobicity and surface electrostatic charge of conidia of the mycoparasite Coniothyrium minitans

The effect of increasing culture age on cell surface hydrophobicity (CSH) and cell surface electrostatic charge (measured as zeta potential) of conidia from five isolates of Coniothyrium minitans representing three different morphological types was examined. Conidial CSH of three isolates (A2 960/1, CH1 and CH2) decreased with culture age, whereas CSH of two others (B 1300/2 and IMI 134523) remained high for the whole 42 day experimental period. In contrast, cell surface electrostatic charge decreased uniformly in conidia of all five isolates for the first 34 d and then rose slightly at 42 d. The variation in cell surface electrostatic charge (spectrum width) of the sampled conidia decreased with age for all five isolates. In all five isolates cell surface electrostatic charge of conidia became increasingly negative as the pH of the buffer used to suspend conidia was increased from pH 3.0 to 9.0. No relationship between colony morphology of C. minitans and conidial CSH and cell surface electrostatic charge was found.

Solvation-Driven Charge Transfer and Localization in Metal Complexes

In any physicochemical process in liquids, the dynamical response of the solvent to the solutes out of equilibrium plays a crucial role in the rates and products: the solvent molecules react to the changes in volume and electron density of the solutes to minimize the free energy of the solution, thus modulating the activation barriers and stabilizing (or destabilizing) intermediate states. In charge transfer (CT) processes in polar solvents, the response of the solvent always assists the formation of charge separation states by stabilizing the energy of the localized charges. A deep understanding of the solvation mechanisms and time scales is therefore essential for a correct description of any photochemical process in dense phase and for designing molecular devices based on photosensitizers with CT excited states. In the last two decades, with the advent of ultrafast time-resolved spectroscopies, microscopic models describing the relevant case of polar solvation (where both the solvent and the solute molecules have a permanent electric dipole and the mutual interaction is mainly dipole−dipole) have dramatically progressed. Regardless of the details of each model, they all assume that the effect of the electrostatic fields of the solvent molecules on the internal electronic dynamics of the solute are perturbative and that the solvent−solute coupling is mainly an electrostatic interaction between the constant permanent dipoles of the solute and the solvent molecules. This well-established picture has proven to quantitatively rationalize spectroscopic effects of environmental and electric dynamics (time-resolved Stokes shifts, inhomogeneous broadening, etc.). However, recent computational and experimental studies, including ours, have shown that further improvement is required. Indeed, in the last years we investigated several molecular complexes exhibiting photoexcited CT states, and we found that the current description of the formation and stabilization of CT states in an important group of molecules such as transition metal complexes is inaccurate. In particular, we proved that the solvent molecules are not just spectators of intramolecular electron density redistribution but significantly modulate it. Our results solicit further development of quantum mechanics computational methods to treat the solute and (at least) the closest solvent molecules including the nonperturbative treatment of the effects of local electrostatics and direct solvent−solute interactions to describe the dynamical changes of the solute excited states during the solvent response.

Observation of charge-dependent azimuthal correlations and possible local strong parity violation in heavy-ion collisions

Parity (P)-odd domains, corresponding to nontrivial topological solutions of the QCD vacuum, might be created during relativistic heavy-ion collisions. These domains are predicted to lead to charge separation of quarks along the orbital momentum of the system created in noncentral collisions. To study this effect, we investigate a three-particle mixed-harmonics azimuthal correlator which is a P-even observable, but directly sensitive to the charge-separation effect. We report measurements of this observable using the STAR detector in Au + Au and Cu + Cu collisions at root s(NN) = 200 and 62 GeV. The results are presented as a function of collision centrality, particle separation in rapidity, and particle transverse momentum. A signal consistent with several of the theoretical expectations is detected in all four data sets. We compare our results to the predictions of existing event generators and discuss in detail possible contributions from other effects that are not related to P violation.

The effects of electron-hole separation on the photoconductivity of individual metal oxide nanowires

The responses of individual ZnO nanowires to UV light demonstrate that the persistent photoconductivity (PPC) state is directly related to the electron¿hole separation near the surface. Our results demonstrate that the electrical transport in these nanomaterials is influenced by the surface in two different ways. On the one hand, the effective mobility and the density of free carriers are determined by recombination mechanisms assisted by the oxidizing molecules in air. This phenomenon can also be blocked by surface passivation. On the other hand, the surface built-in potential separates the photogenerated electron¿hole pairs and accumulates holes at the surface. After illumination, the charge separation makes the electron¿hole recombination difficult and originates PPC. This effect is quickly reverted after increasing either the probing current (self-heating by Joule dissipation) or the oxygen content in air (favouring the surface recombination mechanisms). The model for PPC in individual nanowires presented here illustrates the intrinsic potential of metal oxide nanowires to develop optoelectronic devices or optochemical sensors with better and new performances.

Charge-dependent translocation of Bordetella pertussis adenylate cyclase toxin into eukaryotic cells: Implication for the in vivo delivery of CD8+ T cell epitopes into antigen-presenting cells

Bordetella pertussis secretes a calmodulin-activated adenylate cyclase toxin, CyaA, that is able to deliver its N-terminal catalytic domain (400-aa residues) into the cytosol of eukaryotic target cells, directly through the cytoplasmic membrane. We have previously shown that CyaA can be used as a vehicle to deliver T cell epitopes, inserted within the catalytic domain of the toxin, into antigen-presenting cells and can trigger specific class I-restricted CD8+ cytotoxic T cell responses in vivo. Here, we constructed a series of recombinant toxins harboring at the same insertion site various peptide sequences of 11–25 amino acids, corresponding to defined CD8+ T cell epitopes and differing in the charge of the inserted sequence. We show that inserted peptide sequences containing net negative charges (−1 or −2) decreased or completely blocked (charge of −4) the internalization of the toxin into target cells in vitro and abolished the induction of cytotoxic T cell responses in vivo. The blocking of translocation due to the inserted acidic sequences can be relieved by appropriate mutations in the flanking region of CyaA that counterbalance the inserted charges. Our data indicate that (i) the electrostatic charge of the peptides inserted within the catalytic domain of CyaA is critical for its translocation into eukaryotic cells and (ii) the delivery of T cell epitopes into the cytosol of antigen-presenting cells by recombinant CyaA toxins is essential for the in vivo stimulation of specific cytotoxic T cells. These findings will help to engineer improved recombinant CyaA vectors able to stimulate more efficiently cellular immunity.