bluuebunny/crossref_metadata_2025 huggingface dataset:
Ionization levels of doped sulfur and selenium chalcopyrites
Authors: Tablero C.
Published: October 2009

The substitution of Ga or Cu by another element in the chalcopyrites Ga2Cu2X4 (with X=S or Se) could have important implications either for photovoltaic or spintronic applications. We present total energy spin-polarized density-functional calculations of the substituted chalcopyrite MxA2−xB2X4, with A and B equal to Ga or Cu, X=S and Se, and M=C, Si, Ge, Sn, V, Ir, Fe, Co, Ni, Rh, and Hg. The ionization levels, interesting for both spintronic and optoelectronic applications, are calculated and discussed. The donor and acceptor levels induced by substitutional M atoms are used to make predictions on the improvement in the optoelectronic performance. From these results, some doped chalcopyrites have an amphoteric behavior. In order to analyze the role of the deep gap levels in both the radiative and nonradiative processes, the dynamic acceptor and donor energies are obtained as a function of the inward and outward M-X displacements.
Single spin transport spectroscopy: Current blockade and spin decay
Authors: Kiesslich Gerold , Schaller Gernot , Emary Clive , Brandes Tobias
Published: October 2009

We present a theory of a single-electron transistor exchange-coupled to a localized spin. We show how to gain detailed quantitative knowledge about the attached spin such as spin size, exchange coupling strength, Landé g-factor, and spin decay time T1 by utilizing a robust blockade phenomenon of dc magnetotransport with accompanying noise enhancement. Our studies are of particular relevance to spin-resolved scanning single-electron transistor microscopy, electronic transport through nanomagnets, and the effect of hyperfine interaction on transport electrons by surrounding nuclear spins.
Impact of dye interlayer on the performance of organic photovoltaic devices
Authors: Lai S. L. , Lo M. F. , Chan M. Y. , Lee C. S. , Lee S. T.
Published: October 2009

The influences of buffer interlayer at the donor/acceptor interface on the open circuit voltage (VOC) of typical copper phthalocyanine (CuPc)/C60 organic photovoltaic devices are studied. Six fluorescent dyes with progressively increasing ionization potentials (IP) were used to investigate the factors influencing the VOC. The short-circuit current and fill factor of CuPc/C60 device incorporating dye interlayer are lower than those of standard bilayer device. On the other hand, the VOC increases linearly with the IP of dye material and falls off when the IP is equal to or greater than 5.6 eV, in which the energy offset between the highest occupied molecular orbitals at the interlayer/C60 heterojunction is smaller than the C60 exciton binding energy. The findings underscore the importance of energy offsets in photovoltaic responses.
Energy- and charge-transfer processes in flexible organic donor-acceptor dyads
Authors: Hofmann Christiane C. , Bauer Peter , Haque Saif A. , Thelakkat Mukundan , Köhler Jürgen
Published: October 2009

Organic donor-bridge-acceptor dyads consisting of a triphenyldiamine donor that was linked to a perylenebisimide acceptor by a flexible nonconjugated bridge have been investigated by complementary spectroscopic techniques as a function of the length and the polarity of the linker. Time-resolved fluorescence spectroscopy revealed a quenching of the donor emission accompanied by a corresponding rise in the acceptor fluorescence, which indicates an efficient energy transfer between the donor and acceptor moieties. A second fluorescence quenching process that affects the acceptor emission is ascribed to a ground-state electron transfer from the donor to the acceptor. The lifetimes of the radicals that were determined by transient-absorption spectroscopy covered the range from 10 to 100 ms.
Electric field-dependent charge transport in organic semiconductors
Authors: Li Ling , Van Winckel Steven , Genoe Jan , Heremans Paul
Published: October 2009

An analytical description is elaborated for the variable range hopping conduction mechanism in the presence of temperature and electric fields for quasi-three-dimensional organic semiconductor systems. In the proposed description, it is assumed that the localized states are randomly distributed in energy and space coordinates. The expression for the hopping conductivity is obtained for the Gaussian density of states. The model is applied to the analysis of both temperature and electric field-dependent hopping transport in organic semiconductors. It is shown that the Poole–Frenkel behavior is only valid in medium electric field regime. Moreover, we conclude that the electric field determines whether the temperature dependence of conductivity in organic semiconductors obeys the Arrhenius law.
Evidence of type-I direct recombination in InP/GaP quantum dots via magnetoluminescence
Authors: Dewitz C. v. , Hatami F. , Millot M. , Broto J. M. , Léotin J. , Masselink W. T.
Published: October 2009

Photoluminescence from InP quantum dots within a GaP matrix is investigated as a function of magnetic field. By fitting the magnetoluminescence data to the energy states of the electrons and holes confined in quantum dots in a perpendicular magnetic field, the electron-hole reduced effective mass is determined to be 0.094m0. The diamagnetic shift in the luminescence peak gives an average exciton radius of about 5 nm, smaller than the dot radius. These results indicate that the recombining electrons are primarily composed of states originating from the InP Γ valley although the conduction band discontinuity between the InP and the GaP barrier is expected to be very small or even negative.
The ejection of triatomic molecular hydrogen ions H3+ produced by the interaction of benzene molecules with ultrafast laser pulses
Authors: Kaziannis S. , Liontos I. , Karras G. , Corsi C. , Bellini M. , Kosmidis C.
Published: October 2009

The ejection process of triatomic molecular hydrogen ions produced by the interaction of benzene with ultrafast laser pulses of moderate strong intensity (∼1014 W/cm2) is studied by means of TOF mass spectrometry. The H3+ formation can only take place through the rupture of two C–H bonds and the migration of hydrogen atoms within the molecular structure. The H3+ fragments are released with high kinetic energy (typically 2–8 eV) and at laser intensities ≥1014 W/cm2, well above that required for the double ionization of benzene, suggesting that its formation is taking place within multiply charged parent ions. The relative ejection efficiency of H3+ molecular hydrogen ions with respect to the atomic ones is found to be strongly decreasing as a function of the laser intensity and pulse duration (67–25 fs). It is concluded that the H3+ formation is only feasible within parent molecular precursors of relatively low charged states and before significant elongation of their structure takes place, while the higher multiply charged molecular ions preferentially dissociate into H+ ions. The ejection of H2+ ions is also discussed in comparison to the production of H3+ and H+ ions. Finally, by recording the mass spectra of two deuterium label isotopes of benzene (1,2-C6H4D2, 1,4-C6H4D2) it is verified that the ejection efficiency of some molecular fragments, such as D2H+, DH+, is dependent on the specific position of hydrogen atoms in the molecular skeleton prior dissociation.
Sound propagation through a rarefied gas confined between source and receptor at arbitrary Knudsen number and sound frequency
Authors: Kalempa Denize , Sharipov Felix
Published: October 2009

A sound propagation through a rarefied gas is investigated on the basis of the linearized kinetic equation taking into account the influence of receptor. A plate oscillating in the normal direction to its own plane is considered as a sound source, while a stationary parallel plate is considered as being the receptor of sound. The main parameters determining the solution of the problem are the oscillation speed parameter, which is defined as the ratio of intermolecular collision frequency to the sound frequency, and the rarefaction parameter defined as the ratio of the distance between source and receptor to the molecular mean free path. The kinetic equation is solved via a discrete velocity method with a numerical error of 0.1%. The numerical calculations are carried out for wide ranges of the oscillation and rarefaction parameters. The concept of integral phase parameter is introduced to obtain the sound speed correctly in all regimes of the gas rarefaction and sound frequency. Analytical solutions are obtained in the limits of small and large parameters of frequency and rarefaction.
Mechanisms for chemical transformations of (R,R)-tartaric acid on Cu(110): A first principles study
Authors: Zhang Ji , Lu Tao , Jiang Chen , Zou Jianwei , Cao Fengqi , Chen Yadong
Published: October 2009

Periodic density functional theory calculations are used to systematically investigate, for the first time, the mechanisms for chemical transformations of (R,R)-tartaric acid on a model Cu(110) surface. The overall potential energy surface for the chemical transformations is revealed. The calculations show that the adsorption of the intact biacid molecules of (R,R)-tartaric acid on Cu(110) surface is not strong, but upon adsorption on Cu(110), the biacid molecules will chemically transform immediately, rather than desorb from the surface. It is found that the chemical transformations of (R,R)-tartaric acid on Cu(110) is a thermodynamically favorable process, to produce the monotartrate species, bitartrate species, and H atoms. Kinetically, the initial reaction step is only one O–H bond scission in either one of the COOH group of a biacid molecule of (R,R)-tartaric acid leading to the formation of a monotartrate species and a H atom, which is an almost spontaneous process. The rate-controlling step is the O–H bond scission in the COOH group of a monotartrate species producing a bitartrate species and a H atom. The concerted reaction for simultaneously breaking the two O–H bonds in both COOH groups of a biacid molecule cannot proceed.
Thermal rectification at water/functionalized silica interfaces
Authors: Hu Ming , Goicochea Javier V. , Michel Bruno , Poulikakos Dimos
Published: October 2009

Using nonequilibrium molecular dynamics simulations, we study the thermal diode effect in a system composed of silica, self-assembled monolayers (SAMs) at the silica surface and water surrounding this system, by imposing a series of positive and negative heat currents. We have found that in the limit of large heat currents, the thermal conductance at the SAMs-water interface is about 1000 MW/m2 K at room temperature for heat flowing from the SAMs to the water and 650 MW/m2 K for heat flowing from the water to the SAMs, respectively, resulting in a thermal rectification of up to 54%. Analysis of the radial distribution function of oxygen-oxygen atoms in water indicates that the origin of the thermal rectification resides in the strong temperature dependence of the hydrogen bonds in water.
Organization of silicon nanocrystals by localized electrochemical etching
Authors: Ayari-Kanoun Asma , Drouin Dominique , Beauvais Jacques , Lysenko Vladimir , Nychyporuk Tetyana , Souifi Abdelkader
Published: October 2009

An approach to form a monolayer of organized silicon nanocrystals on a monocrystalline Si wafer is reported. Ordered arrays of nanoholes in a silicon nitride layer were obtained by combining electron beam lithography and plasma etching. Then, a short electrochemical etching current pulse led to formation of a single Si nanocrystal per each nanohole. As a result, high quality silicon nanocrystal arrays were formed with well controlled and reproducible morphologies. In future, this approach can be used to fabricate single electron devices.
Diminish the screen effect in field emission via patterned and selective edge growth of ZnO nanorod arrays
Authors: Liu Nishuang , Fang Guojia , Zeng Wei , Long Hao , Yuan Longyan , Zhao Xingzhong
Published: October 2009

The authors report on the field emission from controlled selective grown zinc oxide (ZnO) nanorod arrays by hydrothermal reaction. With the combined effect from a ZnO seed layer and a passivation layer for nanorod growth, ZnO nanorods could only grow on the edge of a 4 μm diameter circle. The ZnO nanorods hollow arrays present excellent electron emission characteristics duo to its typical morphology which can significantly diminish the screen effect. By calculating the electrostatic field distribution, it was found that the electrostatic field of the ZnO nanorods hollow arrays is significantly higher than that of the solid arrays.
Electric force microscopy imaging of charge accumulation and barrier lowering at Al/pentacene junction
Authors: Kim Chaeho , Jeon D.
Published: October 2009

We investigated the current-voltage and interface properties of the Al/pentacene/Au sandwiched sample. When the bias voltage was applied between Al and Au, a rectifying current-voltage curve was obtained as expected from the energy diagram. When measured in air, however, the forward current increased slowly to saturation even though the bias voltage was fixed. Cross-sectional electrostatic force microscopy of the interface suggested that a barrier lowering due to hole accumulation at the Al/pentacene junction was responsible.
Detection of the static and kinetic pinning of domain walls in ferromagnetic nanowires
Authors: Ahn Sung-Min , Moon Kyoung-Woong , Kim Dong-Hyun , Choe Sug-Bong
Published: October 2009

Two distinct pinning mechanisms named as kinetic and static pinning of magnetic domain wall (DW) are experimentally resolved. Both the pinning situations are realized at an artificial notch on U-shaped Permalloy nanowires, depending on the initial DW states, moving or pinned. The kinetic depinning field—a critical field for a moving DW to be trapped at a notch—is revealed to be distinguishably smaller than the static depinning field—a critical field to depin a trapped DW at the notch. Based on one-dimensional collective model, the discrepancy is explained by the tilting angle of the moving DW.
Structural instability of single wall carbon nanotube edges from first principles
Authors: Zubaer Hossain M.
Published: October 2009

Ab initio simulations are carried out to examine the origin of instability of edges in carbon nanotubes. It is found that the edge atoms with double dangling bonds (DDBs) in zigzag nanotubes move radially inward and arrange themselves in a structurally stable pentagonal configuration. This leads to a spontaneous partial closure of the edge ends. Conversely, edges containing single dangling bonds deform outward by a small amount. These structural changes occur due to the inherent structural instabilities, thus, are not initiated by thermodynamics. Besides, the inherent structural instability reduces the chemical activity of the atoms at the edge with DDBs.
Synthesis and Characterization of Soluble Conjugated Poly(p-phenylenevinylene) Derivatives Constituted of Alternating Pyrazole and 1,3,4-Oxadiazole Moieties
Authors: Chang En-Ming , Huang Shin-Lin , Lee Cheng-Tien , Lin Hui-Chang , Chen Chun-Yen , Huang Yu-Ying , Lin Shao-Kai , Wong Fung Fuh
Published: October 2009

New soluble poly(p-phenylenevinylene) derivatives with 1,3,4-oxadiazole and pyrazole rings along the main chain were synthesized by Heck coupling. The new conjugated polymers are soluble in common organic solvents as a result of the fully conjugated backbone with dodecyloxy side groups. The polymers show relatively high glass-transition temperatures (up to 160°C) and good satisfactory thermal stability. Solutions of the polymers emit blue-greenish light with photoluminescence (PL) emission maxima around 490–500 nm. The PL spectrum of the polymer’s thin films, with a maximum at 515 nm, shows a red-shift (~20 nm), with respect to the solution spectrum. Cyclic voltammetry reveals that both conjugated polymers have reversible oxidation and irreversible reduction, making them n-type electroluminescent materials. The electron affinity of the new polymers was estimated as 2.73–2.74 eV. The weight-average molecular weights (M
w) of the new soluble polymers were in the range of 4790–4950.
Synthesis and Applications of Covalent Protein-DNA Conjugates
Authors: Schaeffer Patrick M. , Dixon Nicholas E.
Published: October 2009

Synthetic protein-DNA conjugates are valuable tools with applications in fields including nanobiotechnology, bioanalytical chemistry, and molecular diagnostics, and various synthetic methods for their production have been developed during the past three decades. The present article reviews current methodologies for the synthesis of covalent protein-DNA conjugates with particular focus on the regiospecificity and stoichiometry of these reactions.
Synthesis of siRNA Polyplexes Adopting a Combination of RAFT Polymerization and Thiol-ene Chemistry
Authors: Valade David , Boyer Cyrille , Davis Thomas P. , Bulmus Volga
Published: October 2009

Block copolymers of allyl methacrylate and N-(2-hydroxypropyl)methacrylamide (HPMA) with different block lengths have been synthesized by reversible addition–fragmentation chain transfer polymerization. Allyl groups were modified with cysteamine, via a thiol-ene photoreaction, with a high efficiency (~100%) as evidenced by NMR spectroscopy, yielding cationic copolymers of HPMA. Polyelectrolyte complexes of small interfering RNAs (siRNA) and the cationic block copolymers were then formed at an N/P ratio between 1 and 4 depending on the block length of the copolymers. Increasing the length of the hydrophilic block was found to decrease the efficiency of siRNA complexation. The hydrodynamic diameter of the polyplexes in 130 mM buffer solution was less than 100 nm.
The Bioconjugation of Redox Proteins to Novel Electrode Materials
Authors: Goldstein Daniel C. , Thordarson Pall , Peterson Joshua R.
Published: October 2009

The immobilization of redox proteins on electrode surfaces has been crucial for understanding the fundamentals of electron transfer in biological systems and has led to the development of biosensors and other bioelectronic devices. Novel materials, such as carbon nanotubes, gold and other metallic nanoparticles, carbon nanofibre and mesoporous materials have been widely used in the construction of these bioelectrodes, and have been shown to greatly improve the efficiency of electron transfer between the electrode and the redox centre of the protein. The use of these materials has spawned a diversity of covalent and non-covalent techniques for protein immobilization that offer different advantages and disadvantages to the performance of the bioelectrode. This review covers the important properties of these novel electrode materials relevant to the bioconjugation of proteins, and discusses the various methods of attachment from recent examples in the literature.
Macrocyclic Thiophene-Appended Cyanido-Bridged CoIII/FeII Complexes: Precursors to Mixed-valent Poly-thiophene Hybrid Materials
Authors: Hasan Sharizal , Kilah Nathan L. , Martinez Manuel , Bernhardt Paul V.
Published: October 2009

The new cyanido-bridged mixed valent FeII/CoIII macrocyclic complexes [L2CoIII(μ-NC)FeII(CN)5]– and trans-[L2CoIII(μ-NC)FeII(CN)4(μ-CN)CoIIIL2]2+ have been prepared and characterized spectroscopically. The trinuclear complex trans-[L2Co(μ-NC)Fe(CN)4(μ-CN)CoL2](ClO4)2·11H2O has been characterized crystallographically. The di- and trinuclear complexes exhibit metal-to-metal charge transfer transitions characteristic of Class II mixed valent chromophores and their redox and spectroscopic properties have been analyzed by Hush theory. The thiophene group attached to the macrocycle L2 in these complexes may serve as a precursor to conducting polythiophene-based hybrid materials incorporating redox active transition metal ions.
Macrocyclic Ligand Design. A Large Covalently-Linked Ring System Incorporating Four Cyclam Units and its Interaction with Nickel(II), Copper(II), Zinc(II), and Cadmium(II)
Authors: Dong Ying , Lindoy Leonard F.
Published: October 2009

A cyclic, tetra-linked macrocyclic species incorporating four 1,4,8,11-tetraazacyclotetradecane (cyclam) subunits has been synthesized employing Boc amine-group protecting chemistry coupled with high dilution conditions for the cyclization step. A series of 4:1 (metal:ligand (M:L)) complexes with nickel(ii), copper(ii), zinc(ii), and cadmium(ii) has been synthesized. Visible spectophotometric (including spectrophotometric titration), EPR, NMR, and electrochemical studies are reported for individual complexes and are in general accord with similar 4:1 (M:L) coordination behaviour occurring in both the solid state and in solution. The study makes available an example of a new category of large cyclic receptors whose cavity can be tuned electronically through variation of the metal centres incorporated in the cavity walls.
Complexation of Constrained Ligands Piperazine, N-substituted Piperazines, and Thiomorpholine
Authors: Clifford Sarah E. , Tiwow Vanny , Gendron Aleasia , Maeder Marcel , Rossignoli Monica , Lawrance Geoffrey A. , Turner Peter , Blake Alexander J. , Schröder Martin
Published: October 2009

Complexation of the symmetric cyclic diamine piperazine (1,4-diazacyclohexane) has been examined in dry dimethyl formamide by spectrophotometric titrations (with Cu2+, Ni2+) to define formation constants, and by stopped-flow kinetics to define the complexation rates and reaction pathway. Initial formation of a rarely observed η1-piperazine intermediate occurs in a rapid second-order reactions. This intermediate then undergoes two competing reactions: formation of (chelated) η2-piperazine (ML) or the formation of (bridging) μ-piperazine (in M2L and M2L3, speciation depending on relative concentrations). Protonation constants and formation constants for complexation in water of N-ethylpiperazine and thiomorpholine (1-aza-4-thiocyclohexane, tm) have been determined by potentiometric titration; 1:1 complexes with first-row M2+ display a log K from ~4 to 6, with speciation that suggests chelation of the heterocycles may be involved. Complexation of thiomorpholine has been further probed by the synthesis of PdII complexes. The N-monodentate coordination mode has been confirmed in trans-[Pd(tm)2Br2] by an X-ray crystal structure. Complexation of N-(2-aminoethyl)piperazine to CuII as a bidentate ligand involving the primary and tertiary amines is also defined by an X-ray crystal structure.
EuIII Complexes of Octadentate 1-Hydroxy-2-pyridinones: Stability and Improved Photophysical Performance
Authors: Moore Evan G. , D'Aléo Anthony , Xu Jide , Raymond Kenneth N.
Published: October 2009

The luminescence properties of lanthanoid ions can be dramatically enhanced by coupling them to antenna ligands that absorb light in the UV-visible and then efficiently transfer the energy to the lanthanoid centre. The synthesis and the complexation of LnIII cations (Ln = Eu, Gd) for a ligand based on four 1-hydroxy-2-pyridinone (1,2-HOPO) chelators appended to a ligand backbone derived by linking two l-lysine units (3LI-bis-LYS) is described. This octadentate EuIII complex ([Eu(3LI-bis-LYS-1,2-HOPO)]–) has been evaluated in terms of its thermodynamic stability, UV-visible absorption and luminescence properties. For this complex, the conditional stability constant (pM) is 19.9, which is an order of magnitude higher than diethylenetriaminepentacetic acid at pH = 7.4. This EuIII complex also shows an almost two-fold increase in its luminescence quantum yield in aqueous solution (pH = 7.4) when compared with other octadentate ligands. Hence, despite a slight decrease of the molar absorption coefficient, a much higher brightness is obtained for [Eu(3LI-bis-LYS-1,2-HOPO)]–. This overall improvement was achieved by saturating the coordination sphere of the EuIII cation, yielding an increased metal-centred efficiency by excluding solvent water molecules from the metal’s inner sphere.
Reactions of Co(III)-coordinated Ornithine: Towards Inhibitors of the Polyamine Biosynthetic Pathway
Authors: Clarkson Andrea J. , Blackman Allan G. , Clark Charles R. , Sargeson Alan M.
Published: October 2009

Treatment of the complex Δ,Λ-[Co(en)2(S-ornithineH-κ2N,O)](CF3SO3)3 with SOCl2 in DMF gives a mixture of the expected imino complex [Co(en)2(2-iminoornithineH-κ2N,O)]3+ and the amidine complex [Co(en)2(5-{[(1E)-(dimethylamino)methylene]amino}-2-iminopentanoate-κ2N,O)]3+, which is proposed to result from the reaction of the Vilsmeier reagent [ClCH=N(CH3)2]+ with the terminal amine of coordinated ornithine. Treatment of the imino complex with nitromethane in alkaline solution gives the complex [Co(en)2(2-nitromethylornithineH-κ2N,O)]3+, which contains the potential suicide inhibitor 2-nitromethylornithine. X-ray crystal structures of Δ-[Co(en)2(S-ornithineH-κ2N,O)]Br3 and Δ,Λ-[Co(en)2(5-{[(1E)-(dimethylamino)methylene]amino}-2-iminopentanoate-κ2N,O)](ClO4)3 are reported.
Mono- and Dinuclear Copper(II) and Iron(II) Complexes of a Tetradentate Bispidine-diacetate Ligand
Authors: Comba Peter , Daumann Lena , Lefebvre Julie , Linti Gerald , Martin Bodo , Straub Johannes , Zessin Thomas
Published: October 2009

The synthesis of a new tetradentate bispidine ligand (LH2 = 2,2′-(1,5-dimethyl-3,7-diazabicyclo[3.3.1]nonan-3,7-diyl)diacetic acid), containing two tertiary amine and two carboxylic groups, is reported along with the preparation and characterization of the corresponding Cu(ii) and Fe(iii) complexes. The mononuclear [LCu(OH2)]·4H2O (1) complex contains a five-coordinate Cu(ii) centre, which adopt a square pyramidal geometry with the four donor atoms of the ligand (N2O2) occupying the equatorial plane and a water molecule occupying the axial position. An axial electron paramagnetic resonance (EPR) signature is observed for 1 (gx = 2.054, gy = 2.050, gz = 2.234; Ax = 18 × 10–4 cm–1, Ay = 20 × 10–4 cm–1, Az = 188 × 10–4 cm–1) in frozen methanolic solution (0.1 mM). Dimerization of 1 in concentrated solution (10 mM) was observed by EPR spectroscopy (g∥ = 2.24, g⊥ = 2.07, A∥ = 195 × 10–4 cm–1, and A⊥ = 12 × 10–4 cm–1 for each Cu centre). The structure of the dimeric species [LCu(OH2)]2 (1b) was determined by a combination of molecular mechanics with the simulation of the EPR spectrum (MM-EPR). The dimer has each Cu(ii) centre coordinated by the two amines and one carboxylate of one ligand (L), while the other carboxylate bridges to the second Cu(ii) centre; each coordination sphere is completed by an axial water ligand, with the Cu···Cu distance 5.5 Å (relative orientation from EPR simulation: α = 60°, β = 0°, γ = 25°). The aqueous reaction between the tetradentate ligand (L) and Fe(ii) leads to the formation of an oxo-bridged diiron(iii) complex, [LFe-(μ-O)-FeL] (2), with a Fe–O–Fe angle of 180° (dFe···Fe = 3.516 Å), as revealed by X-ray crystallography. The Mössbauer spectrum of 2 consists of one quadrupole doublet with an isomer shift (δ) of 0.37 mm s–1 and a quadrupole splitting (ΔEQ) of 0.73 mm s–1, which is consistent with S = 5/2 Fe(iii) centres. Variable-temperature magnetic susceptibility measurements show the presence of intramolecular antiferromagnetic interactions between the two Fe(iii) centres, with an exchange coupling constant J of –91(3) cm–1 (H = –2JS1·S2).
Pseudoisotopic Molecule Mass Spectrometry: A Useful Tool for Studying Assembly and Exchange in Multinuclear Complexes
Authors: Spyratou Alexandra , Clifford Sarah , Melich Xavier , Deville Claire , Tissot Matthieu , Bonvin Gregoire , Perrottet Philippe , Williams Alan
Published: October 2009

Herein, we introduce the concept of pseudoisotope to describe particles which have similar chemistry but different masses. Examples include ligands with different substituents or metal ions with identical charges and similar coordination properties. Mixtures of pseudoisotopes may be used to establish rapidly the nuclearity of polynuclear species by electrospray ionization-mass spectrometry. Pseudoisotope exchange allows the study of the dynamics of polynuclear complexes, and shows these reactions may be surprisingly slow. The evolution of the mass spectra indicates the degree of fragmentation occurring during the exchange.
Copper-Complexed Pirouetting [2]pseudorotaxanes with Sulfur-Containing End-Groups Attached to the Thread: Synthesis, Electrochemical Studies, and Deposition on Gold Electrodes
Authors: Collin Jean-Paul , Mobian Pierre , Sauvage Jean-Pierre , Sour Angélique , Yan Yi-Ming , Willner Itamar
Published: October 2009

Two copper [2]pseudorotaxanes incorporating a macrocycle with two chelating sites (2,9-diphenyl-1,10-phenanthroline and 2,2′,6′,2″-terpyridine) and a thread based on crescent-shaped 2,2′-bipyridine derivatives have been synthesized and characterized. One of the threads was a hindering 2,2′-bipyridine functionalized by two thioctic-ended arms. The second thread was an 8,8′-diphenyl-3,3′-biisoquinoline functionalized by two thioether-ended arms. The electrochemical studies of the two copper [2]pseudorotaxanes, both in solution and anchored on a gold surface, showed only fast-moving systems in solution. The reasons of the inertness of the deposited complexes on gold electrode have been explored.
Dependence of the Properties of Cobalt(III) Cage Complex as a Function of the Derivatization of Amine Substituents
Authors: Sargeson Alan M. , Lay Peter A.
Published: October 2009

Control of redox properties of cobalt macrobicyclic hexaamine (cage) complexes by substituent modification is important for their use as electron-transfer agents, and the resultant derivatives can also change the lipophilicity of the complexes for a variety of biological and other applications. Such derivatization is also important for incorporating cage complexes into a range of redoxactive conjugates. Here, the derivatization of the amine groups in the 1 and 8 positions of [Co(sar)]3+ (sar = sarcophagine = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) are reported. The synthesis and properties of methylamide (from the reactions with acetic anhydride), arylimine (from Schiff base reactions), benzylamine, phthalimido, and tosylate derivatives are described. These reactions provide synthons that have the potential to act as precursors for building a range of conjugates containing metal cage complexes, including dimers. The effects of the substituents on the ligand conformations, which affect other chemical and physical properties of the cage complexes, are discussed.
An Investigation into the Potential of SarAr for Use in 64Cu Radioimmunotherapy
Authors: Di Bartolo Nadine , Smith Suzanne V. , Hetherington Eric , Sargeson Alan
Published: October 2009

The B72.3 monoclonal antibody was radiolabelled with 123I, and with 111In and 64Cu, using DTPA and SarAr, respectively. Their biodistribution in tumour-bearing nude mice was used to calculate the dosimetry of their respective therapeutic analogue, using 131I, 90Y, 67Cu, and 64Cu. Two dosimetry models were used: one using the classical approach and a second model that takes into consideration the chemical stability of the radiolabelling methods employed and the biological clearance of each radioimmunoconjugate. Results clearly show that the 64Cu-SarAr-B72.3 could be used as a therapeutic agent and, theoretically, be at least as effective as any of the other therapeutic radionuclides currently studied, such as 131I, 90Y, and 67Cu.
Culture rather than genes provides greater scope for the evolution of large-scale human prosociality
Authors: Bell Adrian V. , Richerson Peter J. , McElreath Richard
Published: October 2009

Whether competition among large groups played an important role in human social evolution is dependent on how variation, whether cultural or genetic, is maintained between groups. Comparisons between genetic and cultural differentiation between neighboring groups show how natural selection on large groups is more plausible on cultural rather than genetic variation.
Pentamidine reverses the splicing defects associated with myotonic dystrophy
Authors: Warf M. Bryan , Nakamori Masayuki , Matthys Catherine M. , Thornton Charles A. , Berglund J. Andrew
Published: October 2009

Myotonic dystrophy (DM) is a genetic disorder caused by the expression (as RNA) of expanded CTG or CCTG repeats. The alternative splicing factor MBNL1 is sequestered to the expanded RNA repeats, resulting in missplicing of a subset of pre-mRNAs linked to symptoms found in DM patients. Current data suggest that if MBNL1 is released from sequestration, disease symptoms may be alleviated. We identified the small molecules pentamidine and neomycin B as compounds that disrupt MBNL1 binding to CUG repeats in vitro. We show in cell culture that pentamidine was able to reverse the missplicing of 2 pre-mRNAs affected in DM, whereas neomycin B had no effect. Pentamidine also significantly reduced the formation of ribonuclear foci in tissue culture cells, releasing MBNL1 from the foci in the treated cells. Furthermore, pentamidine partially rescued splicing defects of 2 pre-mRNAs in mice expressing expanded CUG repeats.
Genetic variation in dopaminergic neuromodulation influences the ability to rapidly and flexibly adapt decisions
Authors: Krugel Lea K. , Biele Guido , Mohr Peter N. C. , Li Shu-Chen , Heekeren Hauke R.
Published: October 2009

The ability to rapidly and flexibly adapt decisions to available rewards is crucial for survival in dynamic environments. Reward-based decisions are guided by reward expectations that are updated based on prediction errors, and processing of these errors involves dopaminergic neuromodulation in the striatum. To test the hypothesis that the
COMT
gene Val
158
Met polymorphism leads to interindividual differences in reward-based learning, we used the neuromodulatory role of dopamine in signaling prediction errors. We show a behavioral advantage for the phylogenetically ancestral Val/Val genotype in an instrumental reversal learning task that requires rapid and flexible adaptation of decisions to changing reward contingencies in a dynamic environment. Implementing a reinforcement learning model with a dynamic learning rate to estimate prediction error and learning rate for each trial, we discovered that a higher and more flexible learning rate underlies the advantage of the Val/Val genotype. Model-based fMRI analysis revealed that greater and more differentiated striatal fMRI responses to prediction errors reflect this advantage on the neurobiological level. Learning rate-dependent changes in effective connectivity between the striatum and prefrontal cortex were greater in the Val/Val than Met/Met genotype, suggesting that the advantage results from a downstream effect of the prefrontal cortex that is presumably mediated by differences in dopamine metabolism. These results show a critical role of dopamine in processing the weight a particular prediction error has on the expectation updating for the next decision, thereby providing important insights into neurobiological mechanisms underlying the ability to rapidly and flexibly adapt decisions to changing reward contingencies.
Immune synapse formation determines interaction forces between T cells and antigen-presenting cells measured by atomic force microscopy
Authors: Hosseini Babak H. , Louban Ilia , Djandji Dominik , Wabnitz Guido H. , Deeg Janosch , Bulbuc Nadja , Samstag Yvonne , Gunzer Matthias , Spatz Joachim P. , Hämmerling Günter J.
Published: October 2009

During adaptive immune responses, T lymphocytes recognize antigenic peptides presented by MHC molecules on antigen-presenting cells (APCs). This recognition results in the formation of a so-called immune synapse (IS) at the T-cell/APC interface, which is crucial for T-cell activation. The molecular composition of the IS has been extensively studied, but little is known about the biophysics and interaction forces between T cells and APCs. Here, we report the measurement of interaction forces between T cells and APCs employing atomic force microscopy (AFM). For these investigations, specific T cells were selected that recognize an antigenic peptide presented by MHC-class II molecules on APCs. Dynamic analysis of T-cell/APC interaction by AFM revealed that in the presence of antigen interaction forces increased from 1 to 2 nN at early time-points to a maximum of ≈14 nN after 30 min and decreased again after 60 min. These data correlate with the kinetics of synapse formation that also reached a maximum after 30 min, as determined by high-throughput multispectral imaging flow cytometry. Because the integrin lymphocyte function antigen-1 (LFA-1) and its counterpart intercellular adhesion molecule-1 (ICAM-1) are prominent members of a mature IS, the effect of a small molecular inhibitor for LFA-1, BIRT377, was investigated. BIRT377 almost completely abolish the interaction forces, emphasizing the importance of LFA-1/ICAM-1-interactions for firm T-cell/APC adhesion. In conclusion, using biophysical measurements, this study provides precise values for the interaction forces between T cells and APCs and demonstrates that these forces develop over time and are highest when synapse formation is maximal.
Heritability of reproductive fitness traits in a human population
Authors: Kosova Gülüm , Abney Mark , Ober Carole
Published: October 2009

The genetic basis of fitness traits has been studied widely in animals, yet the contribution of genetic variation to these traits in humans is controversial. In particular, it is difficult to disentangle genetic versus environmental effects on fertility, because of within-family correlations of sociocultural, economic, and other nongenetic factors that influence family sizes. In this study, we investigated the genetic architecture of reproductive fitness traits in a fertile human population whose communal lifestyle assures uniform and equal access to resources. Our study revealed significant heritabilities for reproductive traits in both men and women, after accounting for common household effects shared among siblings and demographic changes in reproductive practices. Furthermore, our results indicate that both autosomal and X-linked additive and dominance variances contribute to these traits. We therefore propose that reproductive traits should be amenable to genetic mapping studies, and the results we present here will facilitate the search for the novel genes influencing natural fertility in humans.
Endosomes: A legitimate platform for the signaling train
Authors: Murphy Jane E. , Padilla Benjamin E. , Hasdemir Burcu , Cottrell Graeme S. , Bunnett Nigel W.
Published: October 2009

Although long regarded as a conduit for the degradation or recycling of cell surface receptors, the endosomal system is also an essential site of signal transduction. Activated receptors accumulate in endosomes, and certain signaling components are exclusively localized to endosomes. Receptors can continue to transmit signals from endosomes that are different from those that arise from the plasma membrane, resulting in distinct physiological responses. Endosomal signaling is widespread in metazoans and plants, where it transmits signals for diverse receptor families that regulate essential processes including growth, differentiation and survival. Receptor signaling at endosomal membranes is tightly regulated by mechanisms that control agonist availability, receptor coupling to signaling machinery, and the subcellular localization of signaling components. Drugs that target mechanisms that initiate and terminate receptor signaling at the plasma membrane are widespread and effective treatments for disease. Selective disruption of receptor signaling in endosomes, which can be accomplished by targeting endosomal-specific signaling pathways or by selective delivery of drugs to the endosomal network, may provide novel therapies for disease.
Dual role for the methyltransferase G9a in the maintenance of β-globin gene transcription in adult erythroid cells
Authors: Chaturvedi Chandra-Prakash , Hosey Alison M. , Palii Carmen , Perez-Iratxeta Carolina , Nakatani Yoshihiro , Ranish Jeffrey A. , Dilworth F. Jeffrey , Brand Marjorie
Published: October 2009

Using a proteomics screen, we have identified the methyltransferase G9a as an interacting partner of the hematopoietic activator NF-E2. We show that G9a is recruited to the β-globin locus in a NF-E2-dependent manner and spreads over the entire locus. While G9a is often regarded as a corepressor, knocking down this protein in differentiating adult erythroid cells leads to repression of the adult β

maj
globin

gene and aberrant reactivation of the embryonic β
-like globin
gene

E
y

. While in adult cells G9a maintains

E
y

in a repressed state via dimethylation of histone H3 at lysines 9 and 27, it activates β

maj

transcription in a methyltransferase-independent manner. Interestingly, the demethylase UTX is recruited to the β

maj

(but not the

E
y

) promoter where it antagonizes G9a-dependent H3K27 dimethylation. Collectively, these results reveal a dual role for G9a in maintaining proper expression (both repression and activation) of the β
-globin
genes in differentiating adult erythroid cells.
Structural and biochemical insights into 2′-O-methylation at the 3′-terminal nucleotide of RNA by Hen1
Authors: Mui Chan Chio , Zhou Chun , Brunzelle Joseph S. , Huang Raven H.
Published: October 2009

Small RNAs of ≈20–30 nt have diverse and important biological roles in eukaryotic organisms. After being generated by Dicer or Piwi proteins, all small RNAs in plants and a subset of small RNAs in animals are further modified at their 3′-terminal nucleotides via 2′-O-methylation, carried out by the
S
-adenosylmethionine-dependent methyltransferase (MTase) Hen1. Methylation at the 3′ terminus is vital for biological functions of these small RNAs. Here, we report four crystal structures of the MTase domain of a bacterial homolog of Hen1 from
Clostridium thermocellum
and
Anabaena variabilis
, which are enzymatically indistinguishable from the eukaryotic Hen1 in their ability to methylate small single-stranded RNAs. The structures reveal that, in addition to the core fold of the MTase domain shared by other RNA and DNA MTases, the MTase domain of Hen1 possesses a motif and a domain that are highly conserved and are unique to Hen1. The unique motif and domain are likely to be involved in RNA substrate recognition and catalysis. The structures allowed us to construct a docking model of an RNA substrate bound to the MTase domain of bacterial Hen1, which is likely similar to that of the eukaryotic counterpart. The model, supported by mutational studies, provides insight into RNA substrate specificity and catalytic mechanism of Hen1.
Nitric oxide permits hypoxia-induced lymphatic perfusion by controlling arterial-lymphatic conduits in zebrafish and glass catfish
Authors: Dahl Ejby Jensen Lasse , Cao Renhai , Hedlund Eva-Maria , Söll Iris , Lundberg Jon O. , Hauptmann Giselbert , Steffensen John Fleng , Cao Yihai
Published: October 2009

The blood and lymphatic vasculatures are structurally and functionally coupled in controlling tissue perfusion, extracellular interstitial fluids, and immune surveillance. Little is known, however, about the molecular mechanisms that underlie the regulation of bloodlymphatic vessel connections and lymphatic perfusion. Here we show in the adult zebrafish and glass catfish (
Kryptopterus bicirrhis
) that blood-lymphatic conduits directly connect arterial vessels to the lymphatic system. Under hypoxic conditions, arterial-lymphatic conduits (ALCs) became highly dilated and linearized by NO-induced vascular relaxation, which led to blood perfusion into the lymphatic system. NO blockage almost completely abrogated hypoxia-induced ALC relaxation and lymphatic perfusion. These findings uncover mechanisms underlying hypoxia-induced oxygen compensation by perfusion of existing lymphatics in fish. Our results might also imply that the hypoxia-induced NO pathway contributes to development of progression of pathologies, including promotion of lymphatic metastasis by modulating arterial-lymphatic conduits, in the mammalian system.
An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis
Authors: Surovell Todd A. , Holliday Vance T. , Gingerich Joseph A. M. , Ketron Caroline , Haynes C. Vance , Hilman Ilene , Wagner Daniel P. , Johnson Eileen , Claeys Philippe
Published: October 2009

Based on elevated concentrations of a set of “impact markers” at the onset of the Younger Dryas stadial from sedimentary contexts across North America, Firestone, Kennett, West, and others have argued that 12.9 ka the Earth experienced an impact by an extraterrestrial body, an event that had devastating ecological consequences for humans, plants, and animals in the New World [Firestone RB, et al. (2007)
Proc. Natl. Acad. Sci. USA
104:16016–16021]. Herein, we report the results of an independent analysis of magnetic minerals and microspherules from seven sites of similar age, including two examined by Firestone et al. We were unable to reproduce any results of the Firestone et al. study and find no support for Younger Dryas extraterrestrial impact.
Identification of a physiological E2 module for the human anaphase-promoting complex
Authors: Williamson Adam , Wickliffe Katherine E. , Mellone Barbara G. , Song Ling , Karpen Gary H. , Rape Michael
Published: October 2009

Ubiquitination by the anaphase-promoting complex (APC/C) is essential for proliferation in all eukaryotes. The human APC/C promotes the degradation of mitotic regulators by assembling K11-linked ubiquitin chains, the formation of which is initiated by its E2 UbcH10. Here, we identify the conserved Ube2S as a K11-specific chain elongating E2 for human and
Drosophila
APC/C. Ube2S depends on the cell cycle-dependent association with the APC/C activators Cdc20 and Cdh1 for its activity. While depletion of Ube2S already inhibits APC/C in cells, the loss of the complete UbcH10/Ube2S-module leads to dramatic stabilization of APC/C substrates, severe spindle defects, and a strong mitotic delay. Ube2S and UbcH10 are tightly co-regulated in the cell cycle by APC/C-dependent degradation. We conclude that UbcH10 and Ube2S constitute a physiological E2-module for APC/C, the activity of which is required for spindle assembly and cell division.
Mechanochemical model for myosin V
Authors: Craig Erin M. , Linke Heiner
Published: October 2009

A rigorous numerical test of a hypothetical mechanism of a molecular motor should model explicitly the diffusive motion of the motor's degrees of freedom as well as the transition rates between the motor's chemical states. We present such a Brownian dynamics, mechanochemcial model of the coarse-grain structure of the dimeric, linear motor myosin V. Compared with run-length data, our model provides strong support for a proposed strain-controlled gating mechanism that enhances processivity. We demonstrate that the diffusion rate of a detached motor head during motor stepping is self-consistent with known kinetic rate constants and can explain the motor's key performance features, such as speed and stall force. We present illustrative and realistic animations of motor stepping in the presence of thermal noise. The quantitative success and illustrative power of this type of model suggest that it will be useful in testing our understanding of a range of biological and synthetic motors.