The electrochemical interface between copper(111) and aqueous electrolytes

The electrochemical double layer between Cu(111) electrodes and aqueous electrolytes (F⁻ and SO²⁻₄ at various pH values) was studied by means of linear scan voltammetry and ac impedance measurements. It is found that electrochemisorption of oxygen species proceeds on the Cu(111) surface in the potential regions more negative than the electrodissolution potential of copper. The adsorption-desorption kinetics are analysed; the anodic and cathodic symmetry coefficients are found to be equal (α = β = 0.3), and the standard rate constant is k° = 4 × 10⁻¹⁰cm s⁻¹.     

The interface between Au(100) and 1-butyl-3-methyl-imidazolium-hexafluorophosphate

The electrochemical interface of Au(100) and 1-butyl-3-methyl-imidazolium hexafluorophosphate has been characterized by cyclic voltammetry, electrochemical impedance spectroscopy and in situ STM, especially in two distinct potential ranges. In the vicinity of the potential of zero total charge--the value of which has been determined by immersion experiments--charging of the double layer is rather slow, it appears as if the ions comprising the ionic liquid would slowly exchange each other at the surface. In the other, very negative region the ordering of the cations has been observed by STM.     

Interaction between chiral molecules and solvents: The importance of an interface

The recently introduced hypothesis of chiral interaction is shown here to require an additional constraint of an interface between two media in the close vicinity of the chiral molecule or element. As a result only the induced intramolecular current can exist under this constraint whereas the induced mechanical rotation is ruled out. The energy associated with this current is estimated to be of the order of (10⁻⁵–10⁻⁴)kT. Possible thermodynamical consequences of the chiral interaction for small systems are discussed.     

Link between allosteric signal transduction and functional dynamics in a multisubunit enzyme: S-adenosylhomocysteine hydrolase

S-adenosylhomocysteine hydrolase (SAHH), a cellular enzyme that plays a key role in methylation reactions including those required for maturation of viral mRNA, is an important drug target in the discovery of antiviral agents. While targeting the active site is a straightforward strategy of enzyme inhibition, evidence of allosteric modulation of active site in many enzymes underscores the molecular origin of signal transduction. Information of co-evolving sequences in SAHH family and the key residues for functional dynamics that can be identified using native topology of the enzyme provide glimpses into how the allosteric signaling network, dispersed over the molecular structure, coordinates intra- and intersubunit conformational dynamics. To study the link between the allosteric communication and functional dynamics of SAHHs, we performed Brownian dynamics simulations by building a coarse-grained model based on the holo and ligand-bound structures. The simulations of ligand-induced transition revealed that the signal of intrasubunit closure dynamics is transmitted to form intersubunit contacts, which in turn invoke a precise alignment of active site, followed by the dimer-dimer rotation that compacts the whole tetrameric structure. Further analyses of SAHH dynamics associated with ligand binding provided evidence of both induced fit and population shift mechanisms and also showed that the transition-state ensemble is akin to the ligand-bound state. Besides the formation of enzyme-ligand contacts at the active site, the allosteric couplings from the residues distal to the active site are vital to the enzymatic function.     

Isomers of C(20): an energy profile

Semiempirical calucaltions, at the PM 3 level, are used to geometrically optimize and determine the absolute energies (heats of formation) of a variety of C(20) isomers. Based on the geometrically optimized Cartesian coordinates of the ring and the bowl isomers, and the subsequent saddle-point calculation, a two-dimensional energy profile between these two isomers is generated. Performing geometry optimization on the Cartesian coordinates that correspond to energy minima within the ring-bowl profile, we have been able to identify several more isomers of C(20) that are predicted to be energitically stable. With these additional stable structures, we have identified pairs of isomers that lie adjacent to one another on the potential energy surface, as is evidenced by the form of their respective energy profiles. These adjacent pairs of isomers establish a step-wise transformation between the ring and the bowl. This process, which extends out over the three-dimensional surface, is predicted to require less energy than that of the direct, two-dimensional transformation predicted in the ring-bowl profile.     

Interdiffusion at the interface between poly(vinylpyrrolidone) and epoxy

Electron microprobe analysis (EMP) was used to study interdiffusion in bilayer films of thermoplastic poly(vinylpyrrolidone) (PVP) and a thermoset epoxy. The bilayer films were prepared by casting a stoichiometric mixture of the uncured diglycidyl ether of bisphenol A epoxy (DGEBA) and 4,4′-diaminodiphenylsulfone (DDS) on the PVP film and then curing the system in a two-step process under a nitrogen atmosphere. For the EMP studies, the sulfur signal was used as a probe for DDS, while the nitrogen signal served as a probe for both DDS and PVP. The addition of brominated DGEBA to the conventional DGEBA in a 1: 1 weight ratio allowed the bromine signal to be used as a probe for the epoxy phase. It was found that the interfacial thickness was much larger for the film prepared from low molecular weight PVP than that from high molecular weight PVP. Interdiffusion was suppressed when the initial cure temperature in the two-step cure cycle was 130°C compared to 170°C, in which the first stage of the cure reaction dominated the interdiffusion process. More importantly, it was demonstrated that the diffusion front of the curing agent was located closer to the thermoplastic polymer phase as compared to that of the thermoset polymer in the interface region. This tendency was more significant in the system with the larger interfacial thickness. These results have important consequences on interphase structures and properties. They suggest that crosslinking of the epoxy in the interphase may be suppressed because of an insufficient amount of curing agent and that the not-fully-reacted curing agent in the PVP phase may act to plasticize this phase. © 1997 John Wiley & Sons, Inc.     

The structures of integrins and integrin-ligand complexes: implications for drug design and signal transduction

Integrins are pivotal proteins in cell-cell adhesion, signaling and apoptosis. These properties render them attractive targets for drugs, especially those involved in cancer treatment. Recently, the structures of the extracellular domains of one of the integrin subtypes was solved with X-ray crystallography in the free form as well as bound to a ligand. These structures in combination with NMR spectroscopic data, electron microscopy images, and molecular modeling provide deeper insight into the mechanism of integrin-mediated signal transduction. The structures make structure-based rational drug design possible and are certainly hallmarks in integrin research.     

Signal transduction in a coupled hormone system: selective explicit internal signal stochastic resonance and its control

Cooperative interactions of signal transduction and environmental noise are investigated with a coupled hormone system, in which selective explicit internal signal stochastic resonance (EISSR) is observed. More specifically, the large peak of a period-2 oscillation (i.e., a strong signal) is greatly amplified by the environmental noise while the small peak (i.e., a weak signal) does not exhibit cooperative interactions with noise. The EISSR phenomenon could be controlled by adjusting the frequency or amplitude of an external signal and a critical amplitude for external signal is found. Significantly, the maximal signal-to-noise ratio increases almost linearly with the increment of control parameter, despite that the magnitude of the large peak is decreased. In addition, the noise does not alter the fundamental frequencies of the strong signal and the weak signal, which implicates that the system can keep its intrinsic oscillatory state and resist the effect of environmental fluctuations.     

Electronic energy transfer between NF(b) and IF(X)

Described are the assumptions used to formulate a long-range interaction model that appears adequate to describe the reaction processes between excited nitrogen fluoride, NF(b), and iodine monofluoride, IF. With such a model, it may be possible to survey a large number of candidate receptor systems to determine which ones merit additional experimental investigation.     

Isomers of C(20): an energy profile II

Semi-empirical calculations, at the PM3 level provided within the Winmopac v2.0 software package, are used to geometrically optimize and determine the absolute energies (heats of formation) of a variety of C(20) isomers that are predicted to exist in and around the bowl and cage isomers. Using the optimized Cartesian coordinates for the bowl and the cage isomers, a saddle-point calculation was performed. The output file generated, containing energy, distance, and geometry information, is then organized into a graphical format. The resulting graph, which plots the energy of the 20-atom system as a function of the distance from the geometric midpoint, is a two-dimensional energy profile. This profile illustrates an estimation of the contours on the potential energy surface, showing energy minima and maxima that are encountered as the bowl evolves into the cage structure, or vice-versa. To expand the surface into three dimensions, geometry optimizations were performed on the sets of Cartesian coordinates that correspond to energy minima in the bowl-cage profile. Based on these optimizations, eight additional isomers of C(20) have been identified and are predicted to be energetically stable. These additional isomers were subsequently subjected to saddle-point calculations in order to identify those isomers that lie adjacent to one another on the three-dimensional surface. Two isomers that are adjacent to each other will exhibit an energy profile that progresses smoothly from the potential well of each isomer up to the saddle point separating them. Consequently, these adjacent pairs of isomers establish a step-wise transformation between the bowl and the cage. This process, which extends out over the three-dimensional surface, is predicted to require less energy than that of the direct, two-dimensional transformation predicted in the bowl-cage profile.     

Electroconformational coupling (ECC): An electric field induced enzyme oscillation for cellular energy and signal transductions

Previous work has shown that membrane ATPases can extract free energy from applied oscillating electric fields for doing chemical work, e.g. to synthesize ATP from ADP and P_i or to transport Rb and Na ions against their respective electrochemical gradient. Data of these experiments are briefly reviewed. Electroconformational Coupling (ECC) is used to interpret these results. Computer analysis of a four state cyclic enzyme mechanism reproduces many experimental features. It is shown that a coulombic interaction between an enzyme and an alternating electric field (ac) can cause the enzyme to oscillate between different conformational states. If the frequency of the applied field matches the kinetic characteristics of the system and the amplitude matches the energy level required for inducing productive catalytic cycling, a phenomenological resonance between catalytic reaction and the periodic field is generated. A condition necessary for achieving energy coupling is the kinetic bias arising from the binding energy of the ligand. Analysis indicates that only dynamic electric fields, i.e. oscillating or fluctuating fields, can propel the cyclic reaction of the enzyme catalysis, and thus be effective for transducing energy. A stationary transmembrane electric field must be modulated, e.g. by opening and closing of an ion channel, to become oscillatory in order o produce the same effect. We propose that ECC is a fundamental process of cellular energy and signal transductions. Here, many membrane associated events are reduced to Michaelis-Menten types of enzyme catalytic reactions and they are thus amenable to the quantitative analysis of chemical kinetics.     

Molecular and vibrational structure of the extracellular bacterial signal compound N-butyryl-homoserine lactone (C4-HSL)

The molecular and vibrational structure of the title compound (C4-HSL) was studied by experimental and theoretical methods. The infrared (IR) absorption spectrum was measured in the solid state and in CCl₄ suspension. The observed absorption bands were compared with transitions obtained with B3LYP/cc-pVTZ density functional theory (DFT) calculations. Two stable molecular conformations were predicted, corresponding to an endo- and an exo-conformer with similar energies. Intermolecular amide–amide hydrogen bonding in the crystal state was approximated by a simple cluster model, leading to excellent agreement with the observed solid state IR spectrum. Due to the low solubility of C4-HSL in common solvents for IR spectroscopy, such as CS₂ and CCl₄, a liquid solution spectrum of pure, monomeric C4-HSL was not obtained. However, absorbance peaks observed in oversaturated CCl₄ solution could be assigned to distinct contributions from suspended micro-crystalline aggregates and dissolved monomeric species. The key vibrational bands of the monomeric form of C4-HSL are reported here for the first time: 3425 cm^?1 [ν(N–H)], 1784 cm^?1 [ν(CO), lactone], 1688 cm^?1 [amide I], and 1494 cm^?1 [amide II] (CCl₄).     

Forces between a rigid probe particle and a liquid interface. III. Extraction of the planar half-space interaction energy E(D)

The deformation of a liquid drop (radius R0) under the probe particle (radius a) greatly complicates the interpretation by atomic force microscopy. For rigid interfaces, F(DeltaX) can be directly related to the interaction energy E(D) per unit area between planar half-spaces of probe material and drop material across a thickness D of the liquid medium by the Derjaguin approximation, [formula in text], where D(0) is the intersurface separation distance on the line of the centers of the bodies and DeltaX0 is a constant set by the somewhat arbitrary choice of origin for the separation distance DeltaX between the stage on which the drop rests and the lowest point on the probe particle. The problem of absolute intersurface separation distance is common to all surface force measurement techniques. For rigid interfaces, DeltaX0 may be established by bringing the surfaces into close (essentially hard) contact and making measurements in the constant compliance regime. For deformable interfaces, this is not possible and a general method of extracting the absolute separation distance has yet to be devised. In this paper we discuss a general algorithm for extracting E(D+DeltaX0) from F(DeltaX) data. We apply the method to constructed data to investigate the effect of data noise and to a set of real data for a sessile tetradecane droplet in water with an anionic surfactant and a bare silica probe.     

Polyorganosiloxane-europium (III) host-guest inclusion system and its energy transfer luminescence

The Eu(III) ion, as a luminescent probe, is incorporated into a novel nanotube-contained polyorganosiloxane (POS), which is obtained by coupling of ladderlike polyvinylsilsesquioxane (Vi-T) with tetramethyldisiloxane (H-MM) via hydrosilylation, to form POS-Eu(III) composite. The results from fluorescent study demonstrate that the composite is actually a host-guest clathrate which includes the Eu(III) ions in the tubelike cavity of POS and moreover, the supramolecular clathrate exhibits an obvious energy transfer process which converts the UV light absorbed by POS into the visible light generated from the Eu(III) luminescence. Molecular simulation also gives support to the formation of such a clathrate and thus results in energy transfer process. Project supponed by the Research Foundation of Molecular Science Center and Director of Institute of Chemistry, the Chinese Academy of Sciences, and Foundation of Organic Solid Laboratory, the Chinese Academy of Sciences.     

Luminescence and energy transfer between Ce~(3 ) and Mn~(2 ) in alkaline earth borates

The luminescence of Ce and Ce , Mn co-doped BaB8O13 and SrB4O7 prepared in air is studied. The results show that tetravalent cerium ion can be reduced to trivalent state in the hosts and gives rise to efficient luminescence. Energy transfer between Ce3 and Mn2 is possible. Mn2 ions can be efficiently sensitized by Ce 3 and exhibit green and red emissions which implied that Mn2 occupied the crystallographic sites of cations and boron sites of the anoins, respectively. The intensity ratio of red to green emission in matrix increases with the increasing of manganese concentration.     

Cooperativity and anti-cooperativity between ligand binding and the dimerization of ristocetin A: asymmetry of a homodimer complex and implications for signal transduction

Background: Recent work has indicated that dimerization is important in the mode of action of the vancomycin group of glycopeptide antibiotics. NMR studies have shown that one member of this group, ristocetin A₁ forms an asymmetric dimer with two physically different binding sites for cell wall peptides. Ligand binding by ristocetin A and dimerization are slightly anti-cooperative. In contrast, for the other glycopeptide antibiotics of the vancomycin group that have been examined so far, binding of cell wall peptides and dimerization are cooperative.Results: Here we show that the two halves of the asymmetric homodimer formed by ristocetin A have different affinities for ligand binding. One of these sites is preferentially filled before the other, and binding to this site is cooperative with dimerization. Ligand binding to the other, less favored half of the dimer, is anti-cooperative with dimerization.Conclusions: In dinner complexes, anti-cooperativity of dimerization upon ligand binding can be a result of asymmetry, in which two binding sites have different affinities for ligands. Such a system, in which one binding site is filled preferentially, may be a mechanism by which the cooperativity between ligand binding and dimerization is fine tuned and may thus have relevance to the control of signal transduction in biological systems.     

Capillary electrophoresis-atmospheric pressure chemical ionization-mass spectrometry using an orthogonal interface: Set-up and system parameters

The feasibility of atmospheric pressure chemical ionization (APCI) as an alternative ionization technique for capillary electrophoresis-mass spectrometry (CE-MS) was investigated using a grounded sheath-flow CE-MS sprayer and an orthogonal APCI source. Infusion experiments indicated that highest analyte signals were achieved when the sprayer tip was in close vicinity of the vaporizer entrance. The APCI-MS set-up enabled detection of basic, neutral, and acidic compounds, whereas apolar and ionic compounds could not be detected. In the positive ion mode, analytes could be detected in the entire transfer voltage range (0–5 kV), whereas highest signal intensities were observed when the corona discharge current was between 1000 and 2000 nA. In the negative ion mode, the transfer voltage typically was 500 V and the optimum corona discharge current was 6000 nA. Analyte signals were raised with increasing nebulizing gas pressure, but the pressure was limited to 25 psi to avoid siphoning and current drops. Signal intensities appeared to be optimal and constant over a wide range of sheath liquid flow rate (5–25 μL/min) and vaporizer temperature (200–350 °C). APCI-MS signals were unaffected by the composition of the background electrolyte (BGE), even when it contained sodium phosphate and sodium dodecyl sulfate (SDS). Consequently, BGE composition, sheath-liquid flow rate, and vaporizer temperature can be optimized with respect to the CE separation without affecting the APCI-MS response. The analysis of a mixture of basic compounds and a steroid using volatile and nonvolatile BGEs further demonstrates the feasibility of CE-APCI-MS. Detection limits (S/N = 3) were 1. 6–10 μM injected concentrations.     

Transient signals with an Antimony(V) ion-selective electrode: The relative signal return rate as a selectivity parameter

The dynamic response of a Sb(5+) ion-selective electrode based on 1,2,4,6-tetraphenylpyridinium hexachloroantimonate(V) to different ions, ClO(-)(4), HgCl3(-)(3), AuCl(-)(4),TlCl(-)(4), SbCl(-)(6) and tetraphenylborate, in a flow-injection analysis system using a flow-through cell, is studied. The transient signals obtained are different for every ion. A parameter called relative return rate, defined as the ratio of the return rate to peak height, was found to be characteristic for each ion. It was constant up to a concentration range of three decades for those ions with selectivity coefficients K(pot)(sb,j) 1, thus providing a new source of analytical selectivity. Flow injection variables were studied in order to ascertain their influence on this new parameter.     

The relationship between physical observables and the potential energy surface in the He-HF system

This paper examines the effect of infinitesimal functional variations in a rigid rotor He-HF potential surface on several different types of observables: inelastic cross sections, rate constants, and rotational energy level populations. The dynamics and kinetic observables studied were found to be sensitive to a large number of Legendre components of the potential with the region of highest sensitivity dependent upon the energy or temperature as well as the states related by the individual observable. Sensitivity to the entire surface tends to show a large degree of structure due to competition among sensitivities to the individual potential components. Significant information loss has been observed in the transition from microscopic to macroscopic observables.