Structure Properties Of Solutions Research Articles

Absolute Electronic Energetics and Quantitative Work Functions of Liquids from Photoelectron Spectroscopy.

ConspectusLiquid-jet photoelectron spectroscopy (LJ-PES) enabled a breakthrough in the experimental study of the electronic structure of liquid water, aqueous solutions, and volatile liquids more generally. The novelty of this technique, dating back over 25 years, lies in stabilizing a continuous, micron-diameter LJ in a vacuum environment to enable PES studies. A key quantity in PES is the most probable energy associated with vertical promotion of an electron into vacuum: the vertical ionization energy, VIE, for neutrals and cations, or vertical detachment energy, VDE, for anions. These quantities can be used to identify species, their chemical states and bonding environments, and their structural properties in solution. The ability to accurately measure VIEs and VDEs is correspondingly crucial. An associated principal challenge is the determination of these quantities with respect to well-defined energy references. Only with recently developed methods are such measurements routinely and generally viable for liquids. Practically, these methods involve the application of condensed-matter concepts to the acquisition of photoelectron (PE) spectra from liquid samples, rather than solely relying on molecular-physics treatments that have been commonly implemented since the first LJ-PES experiments. This includes explicit consideration of the traversal of electrons to and through the liquid's surface, prior to free-electron detection. Our approach to measuring VIEs and VDEs with respect to the liquid vacuum level specifically involves detecting the lowest-energy electrons emitted from the sample, which have barely enough energy to surmount the surface potential and accumulate in the low-energy tail of the liquid-phase spectrum. By applying a sufficient bias potential to the liquid sample, this low-energy spectral tail can generally be exposed, with its sharp, low-energy cutoff revealing the genuine kinetic-energy-zero in a measured spectrum, independent of any perturbing intrinsic or extrinsic potentials in the experiment. Together with a precisely known ionizing photon energy, this feature enables the straightforward determination of VIEs or VDEs, with respect to the liquid-phase vacuum level, from any PE feature of interest. Furthermore, by additionally determining solution-phase VIEs and VDEs with respect to the common equilibrated energy level in condensed matter, the Fermi level─the generally implemented reference energy in solid-state PES─solution work functions, eΦ, and liquid-vacuum surface dipole effects can be quantified. With LJs, the Fermi level can only be properly accessed by controlling unwanted surface charging and all other extrinsic potentials, which lead to energy shifts of all PE features and preclude access to accurate electronic energetics. More specifically, conditions must be engineered to minimize all undesirable potentials, while maintaining the equilibrated, intrinsic (contact) potential difference between the sample and apparatus. The establishment of these liquid-phase, accurate energy-referencing protocols importantly enables VIE and VDE determinations from near-arbitrary solutions and the quantitative distinction between bulk electronic structure and interfacial effects. We will review and exemplify these protocols for liquid water and several exemplary aqueous solutions here, with a focus on the lowest-ionization- or lowest-detachment-energy PE peaks, which importantly relate to the oxidative stabilities of aqueous-phase species.

Approximate model and algorithms for precast supply chain scheduling problem with time-dependent transportation times

This paper focuses on the precast supply chain scheduling problem with time-dependent transportation time to minimise the total weighted tardiness (PSCSP_TDT |TWT). In the problem, an order sequence and several job sequences are to be determined simultaneously. At first, through in-depth analysis of problem structure and real data from a precast manufacturer, we approximate the problem into a three-stage order scheduling problem by combining the seven production stages into one differentiation stage, and then explore some useful properties of the schedules for the approximate problem. Subsequently, to solve the small instances for the PSCSP_TDT |TWT, we propose an approximate model-based hybrid dynamic programming and heuristic (AMHDPH) and obtain a lower bound as a by-product of the algorithm. For dealing with medium-or large instances, with considering the complexity of the problem, we propose four approximate model-based hybrid iterated greedy (AMHIG) algorithms by integration of constructive heuristics, structural properties of solutions, an iterated greedy, and a correction heuristic. Comprehensive computational results show that the AMHDPH generates tight lower bounds for small instances and solves the most of small instances to optimality within 60 seconds. Whereas the best AMHIG generates feasible solutions with an average optimality gap below 5 percent for around 70 percent instances.

Molecular Dynamics Simulations of Furfural and 5-Hydroxymethylfurfural at Ambient and Hydrothermal Conditions.

In this work, we present results from molecular dynamics simulations of aqueous solutions of furfural and 5-hydroxymethylfurfural, which are important intermediates in the hydrothermal carbonization processes of biomass conversion. The computations were performed both at ambient and hydrothermal conditions using a two-level factorial design varying concentration, temperature, and pressure. A number of equilibrium and dynamic properties have been computed including enthalpies and free energies of vaporization, free energies of solvation, diffusion coefficients, and rotational/reorientational correlation times. Structural properties of solutions were analyzed using radial and spatial distribution functions. It was shown that the formation of hydrogen bonds among 5-hydroxymethylfurfural molecules is preferred compared to hydrogen bonding between 5-hydroxymethylfurfural and water. In addition, our results suggest that the oxygen atoms in the furan rings of furfural and 5-hydroxymethylfurfural do not participate in hydrogen bonding to the same extent as the oxygen atoms in the hydroxyl and carbonyl groups. It is also observed that furfural molecules aggregate under certain conditions, and we show how this is affected by changes in temperature, pressure, and concentration in agreement with experimental solubility data. The analysis of the computational results provides useful insight into the structure and dynamics of the considered molecules at conditions of hydrothermal carbonization, as well as at ambient conditions.

The Structural Properties in Solution of the Intrinsically Mixed Folded Protein Ataxin-3

It has increasingly become clear over the last two decades that proteins can contain both globular domains and intrinsically unfolded regions that can both contribute to function. Although equally interesting, the disordered regions are difficult to study, because they usually do not crystallize unless bound to partners and are not easily amenable to cryo-electron microscopy studies. NMR spectroscopy remains the best technique to capture the structural features of intrinsically mixed folded proteins and describe their dynamics. These studies rely on the successful assignment of the spectrum, a task not easy per se given the limited spread of the resonances of the disordered residues. Here, we describe the structural properties of ataxin-3, the protein responsible for the neurodegenerative Machado-Joseph disease. Ataxin-3 is a 42-kDa protein containing a globular N-terminal Josephin domain and a C-terminal tail that comprises 13 polyglutamine repeats within a low complexity region. We developed a strategy that allowed us to achieve 87% assignment of the NMR spectrum using a mixed protocol based on high-dimensionality, high-resolution experiments and different labeling schemes. Thanks to the almost complete spectral assignment, we proved that the C-terminal tail is flexible, with extended helical regions, and interacts only marginally with the rest of the protein. We could also, for the first time to our knowledge, observe the structural propensity of the polyglutamine repeats within the context of the full-length protein and show that its structure is stabilized by the preceding region.

All-atom molecular dynamics analysis of kinetic and structural properties of ionic micellar solutions

All-atom molecular dynamics simulation results regarding aqueous sodium dodecyl sulfate (SDS) solutions have been presented. Both salt-free solutions with different SDS concentrations and those containing calcium chloride additives have been studied. The simulation has shown that surface-active SDS ions form stable premicellar aggregates. The obtained molecular dynamics trajectories have been used to describe both the kinetic and structural properties of solutions containing SDS molecular aggregates and the properties of individual aggregates. Aggregation kinetics has been investigated, and the characteristic sizes of the aggregates have been calculated by different methods. It has been found that the size of a premicellar aggregate with aggregation number n = 16 in a salt-free solution virtually does not depend on surfactant concentration. Radial distribution functions (RDFs) of hydrogen and oxygen atoms of water molecules relative to the center of mass of an aggregate have no local maxima near the aggregate surface; i.e., the surface is incompletely wetted with water. Corresponding RDFs of carbon atoms have one, two or three maxima depending on the surfactant concentration and the serial number of a carbon atom in the hydrocarbon radical of the surface- active ion. The study of the potentials of mean force for the interaction of sodium and calcium ions with an aggregate having aggregation number n = 32 shows that only calcium ions can be strongly bound to such an aggregate.

Effect of solvent composition on the structural and dynamical properties of sodium chloride solutions in water-methanol mixtures

Molecular dynamics (MD) simulations have been performed on sodium chloride solutions in water-methanol mixtures under ambient conditions with methanol mole fractions (xm) ranging from 0.09 to 0.87. MD simulations are performed with the flexible SPC water model and the flexible six-site model for methanol. The ions are modeled as charged Lennard–Jones spheres. The structural properties of the solutions are discussed on the basis of radial distribution functions. A chain cluster of methanol is predominant in the mixture where xm exceeds 0.7. The number of methanol molecules are higher than water molecules in the first coordination shell of sodium and chloride ions beyond xm=0.5 and 0.48 respectively. With increasing xm, the diffusion coefficients for water (DH2O) and for methanol (DCH3OH) initially decrease. From xm=0.29 DCH3OH increases while DH2O rises only from about 0.7. The two diffusion coefficients are equal at xm=0.8. The deduced first and second reorientational correlation times for the OH and the dipole moment vectors for water speed up with increasing xm. Through the τ1/τ2 ratio, the reorientational motion of water molecules can be entirely ascribed to a jump. OH methanol vector reorientational occurs by a mechanism similar to that of water.

Solvent structuring and its effect on the polymer structure and processability: the case of water-acetone poly-ε-caprolactone mixtures.

One of the most common processes to produce polymer nanoparticles is the solvent-displacement method, in which the polymer is dissolved in a "good" solvent and the solution is then mixed with an "anti-solvent". The polymer processability is therefore determined by its structural and transport properties in solutions of the pure solvents and at the intermediate compositions. In this work, we focus on poly-ε-caprolactone (PCL) which is a biocompatible polymer that finds widespread application in the pharmaceutical and biomedical fields, performing full atomistic molecular dynamics simulations of one PCL chain of different molecular weight in a solution of pure acetone (good solvent), of pure water (antisolvent), and their mixtures. Our simulations reveal that the nanostructuring of one of the solvents in the mixture leads to an unexpected identical polymer structure irrespectively of the concentration of the two solvents. In particular, although in pure solvents the behavior of the polymer is, as expected, very different, at intermediate compositions, the PCL chain shows properties very similar to those found in pure acetone as a result of the clustering of the acetone molecules in the vicinity of the polymer chain. We derive an analytical expression to predict the polymer structural properties in solution at different solvent compositions and show that the solvent clustering affects in an unpredictable way the polymer diffusion coefficient. These findings have important consequences on the optimization of the nanoparticle production process and in the implementation of continuum modeling techniques to model it.

Solution conformational features and interfacial properties of an intrinsically disordered peptide coupled to alkyl chains: a new class of peptide amphiphiles

Owing to the large panel of biological functions of peptides and their high specificity and potency, the development of peptide-based therapeutic and diagnostic tools has received increasing interest. Peptide amphiphiles (PAs) are an emerging class of molecules in which a bioactive peptide is covalently conjugated to a hydrophobic moiety. Due to the coexistence in the molecule of a hydrophilic peptide sequence and a hydrophobic group, PAs are able to self-assemble spontaneously into a variety of nanostructures, such as monolayers, bilayers, and vesicles. In this work we have synthesized a disordered peptide, henceforth called R11, and two lipophilic derivatives of R11 bearing two alkyl chains, connected or not to R11 by an ethoxylic-based linker. The structural properties in solution of these new PAs were investigated using CD and NMR. R11 lipophilic derivatives display typical features of PAs, such as the formation of micelles and unilamellar vesicles. In addition, their surface properties were studied using Langmuir monomolecular films and the results obtained support the formation of molecular aggregates upon compression of the PA films. The presence of the alkyl chains induces not only the self-assembly of these new PAs into supramolecular aggregates but also a gain of structure within the disordered peptide.

ChemInform Abstract: Properties, Structure and Reactivity of Cobaloximes

AbstractReview: 128 refs.

Structural characterisation, stability and antibody recognition of chimeric NHBA-GNA1030: An investigational vaccine component against Neisseria meningitidis

A new generation multi-component vaccine, principally directed against serogroup B Neisseria meningitidis (4CMenB), has recently been developed. One of its components, identified through reverse vaccinology, is the neisserial heparin-binding antigen (NHBA) which is included in the formulation as a novel NHBA-GNA1030 fusion protein (NHBA-FP). We describe here the biophysical characteristics of this vaccine antigen to understand better its structural properties in solution and concurrent immunogenicity prior to formulation. By deliberately stressing the protein to lose its immune responses, we were able to study the protein's structural changes at the molecular level. The unmodified NHBA-FP was found to be mainly monomeric with mass of 67kDa and secondary structure dominated by β-sheets and turns (57% average). The antigen was very stable in storage buffer. It could be forced to unfold in a low-salt buffer resulting in the exposure of one of its two tryptophan residues at 50°C. Long-term stress studies (10-15 days at 37°C) showed modification in the chromatographic and electrophoretic profiles with progressive degradation and aggregation. Since there was little change in secondary structure (as monitored by circular dichroism and tryptophan fluorescence spectroscopy), the loss of functional immunogenicity of the thermal stressed protein could be mainly attributed to the observed fragmentation and aggregation. We therefore conclude that the maintenance of the intact, non-fragmented state of the NHBA-FP is important to preserve its functional immunogenicity. This may thus be utilised as an assay for the control testing of the protein.

Application of hydrophilic interaction chromatography for simultaneous separation of six impurities of mildronate substance

The possibility of separating the impurities of mildronate, an antiischemic drug, by hydrophilic interaction chromatography (HILIC) was investigated on different polar stationary phases (silica, amino, cyano and zwitterionic sulfobetaine). The investigations have shown that HILIC is a useful alternative to reversed phase and ion-pair chromatography. The impact of HILIC separation conditions (acetonitrile content, buffer pH in mobile phase) on retention and selectivity has been systematically studied. Importance of these factors was found to be dependent on the structural properties of solutes. A HILIC method using a zwitterionic sulfobetaine stationary phase was developed and validated to determine six impurities in the drug substance. The method was validated in terms of specificity, limit of quantitation, limit of detection, linearity, accuracy and precision.

Ionic correlations in the inhomogeneous atmosphere surrounding cylindrical polyions: Catalytic effects of polyions

The structural properties of linear polyelectrolyte solutions in the presence of a salt as evidenced through ionic correlations in the inhomogeneous atmosphere around a polyion and their consequence such as the catalytic potential are studied by using Monte Carlo simulation techniques. The simulations are performed on the cylindrical cell model where a uniformly charged hard cylinder mimics the linear polyion, which is caged in its own cylindrical cell containing counterions and salt. The cell (volume) average of the interionic correlations is presented as a function of the polyion and salt concentrations and ion radius. These results are utilized to study the catalytic effects of polyions as manifested through the changes in the collision frequency between ions in the double layer surrounding the polyion relative to that in the pure electrolyte solution. The reported results suggest a strong influence of the added salt/polyelectrolyte concentration ratio on the structural properties of the solution and hence on ion-ion collision frequency. The machine simulations are supplemented by nonlinear Poisson-Boltzmann results. Fair agreement between two different theoretical methods of calculating the collision frequency is obtained.

Rheological and structural properties of dilute active filament solutions

The rheology and the structure of a dilute semiflexible biofilament solution, like F-actin, interacting via molecular motors is probed by molecular dynamics simulations. Oscillatory external shear is used to measure the storage and loss moduli as a function of motor activity in a range of frequencies and for low shear rates. The overall effect of the motor activity on the rheological properties is interpreted as an increase of the temperature, with the effective temperature proportional to the density of motors. However, the effect of motors on the structural properties of the solution, such as the orientation correlation function, is opposite: the motors drastically increase the orientation correlation length whereas thermal fluctuations decrease it.

Mercedes Benz model of neutral amino-acid side chains

We present a 2-d coarse-grain model of polar and non-polar amino-acid side chains, and investigate their thermodynamic and structural properties in solution. The molecules interact via a Lennard-Jones potential and a local treatment of the hydrogen bond energy. Such a form allows very fast highly converged calculations. Estimates of the Gibbs free energy of hydration (GFEH) of the solutes show good qualitative agreement with experiments. We find moreover that contributions to the GFEH are linked to the perturbations of the solvent hydration shell structure. For large hydrophobic solutes, unfavorable GFEH results mainly from an increase of the water–water energy contribution, while for hydrophilic solutes the increase in solvent energy is compensated by the solute–solvent energy. The model appears therefore to capture the main features of non-charged amino-acids hydration.

Investigation of the enhanced solubility of fluorinated methanes in CO 2 by Monte Carlo simulation: Absolute free energy of solvation and structural properties of solution

The absolute free energy of solvation of methane (CH 4) and its fluorinated forms (CH 3F, CH 2F 2, CHF 3 and CF 4) have been computed via statistical perturbation theory (SPT) in the NPT ensemble at four thermodynamical states (whitin liquid and supercritical regions), in the context of Monte Carlo Simulations. Thermodynamical interpretation of the observed trend in the absolute free energy of solvation in different states reveals an exothermic solvation with Δ S slv < 0 (entropically unfavorable solvation) that the intermolecular interactions play an important role in the solvation process. The fluorinated methanes are confirmed to control the mutual arrangement of neighboring CO 2 molecules and the axis of CO 2 molecules near F atom of fluorinated solutes make a right angle with F(solute)–C(CO 2) axis. Moreover, the energetic distribution along with structural and orientational distributions confirm the existence of a direct interaction between CO 2 and F atom, although the extent of this interaction is not very large.

Effect of dielectric and structural properties of solutions on the polymerizability of diallylammonium-type monomers

New polymerization systems— N , N -diallyl- N -methylammonium trifluoroacetate (D1TFA) and N , N -diallylammonium trifluoroacetate (D2TFA)—exhibit phenomena that distinguish their behavior during radical polymerization (Scheme 1) from the known cases, in particular, from the quaternary N , N -diallyl- N , N -dimethylammonium chloride (D3C). These phenomena are evidence of a substantial effect of the medium on the basic polymerization characteristics of new cationogenic monomers: the initial polymerization rate V , the average length of the polymer chain P n , and the constant of chain transfer to the monomer C m [1].

Structural Basis of Cellulosome Efficiency Explored by Small Angle X-ray Scattering

Cellulose, the main structural component of plant cell walls, is the most abundant carbohydrate polymer in nature. To break down plant cell walls, anaerobic microorganisms have evolved a large extracellular enzyme complex termed cellulosome. This megadalton catalytic machinery organizes an enzymatic assembly, tenaciously bound to a scaffolding protein via specialized intermodular "cohesin-dockerin" interactions that serve to enhance synergistic activity among the different catalytic subunits. Here, we report the solution structure properties of cellulosome-like assemblies analyzed by small angle x-ray scattering and molecular dynamics. The atomic models, generated by our strategy for the free chimeric scaffoldin and for binary and ternary complexes, reveal the existence of various conformations due to intrinsic structural flexibility with no, or only coincidental, inter-cohesin interactions. These results provide primary evidence concerning the mechanisms by which these protein assemblies attain their remarkable synergy. The data suggest that the motional freedom of the scaffoldin allows precise positioning of the complexed enzymes according to the topography of the substrate, whereas short-scale motions permitted by residual flexibility of the enzyme linkers allow "fine-tuning" of individual catalytic domains.

Recent Developments in the Synthetic and Structural Chemistry of Lithium Zincates

AbstractFor Abstract see ChemInform Abstract in Full Text.

Self-consistent integral equation theory for solutions of finite extensible semiflexible polyelectrolyte chains

We investigate the structural and conformational properties of solutions containing semiflexible polyelectrolyte chains using a self-consistent integral equation theory approach. A one-component system is considered where the polyelectrolyte chains interact with each other via a Debye–Hückel potential. Nonelectrostatic interactions among the polymers are taken into account by a self-consistently determined solvation potential. The conformational properties of the polymer chain are determined from a variational calculation with a semiflexible reference chain. The finite chain extensibility is taken into account by constraints for the bond lengths and bond angles using Lagrangian multipliers. The scaling relation for the size of an isolated semiflexible chain with respect to chain length exhibits a transition from rodlike to excluded volume type for a given Debye screening length. For flexible chains in solution, the theory provides conformational properties which are in excellent agreement with computer simulation results. The bare chain stiffness has a pronounced influence on the conformational and structural properties of the solution. In the semidilute regime a pronounced liquidlike order is obtained for flexible polyelectrolyte chains which diminishes with increasing bare persistence length. This process is accompanied by a shift of the structural peaks to smaller length scales.

Iron coordination chemistry with tetra-, penta- and hexadentate bispidine-type ligands

Described is the synthesis of tetra-, penta- and hexadentate bispidine ligands with two tertiary amine and two, three or four additional donors (pyridine, phenolate or alcoholate; bispidine=3,7-diazabicyclo[3.3.1]nonanone, coordinating substituents at positions 2,4; 2,4,7; 2,3,4; 2,3,4,7) and of their hexa-coordinate iron(II) complexes. Crystal structural analyses reveal that all complexes are six-coordinate, with one or two co-ligands, and all structures with the tetradentate bispidine ligand are asymmetrical with respect to the two tertiary amine donors, with short FeN1 and long FeN2 bonds (N1: position 3, N2: position 7). This is the same structural type as found for the Jahn–Teller labile copper(II) compounds, the manganese(II) and chromium(III) complexes but different from copper(I), zinc(II) and some cobalt(II) complexes with MN1≥MN2. Additional donors at N2 modify the structures, but do not lead to a change to the other structural type; additional donors at N1 lead to structures with MN1∼MN2. Solution studies (NMR, UV–Vis, electrochemistry, magnetism) indicate that the co-ligands may be substituted by solvent, with the donors trans to N2 being more labile than those trans to N1, but the over-all structural properties in solution are similar to those in the solid state. The complexes are stable towards oxidation, all except one have high spin electronic configuration. The oxidation potentials strongly depend on the two co-ligands.