Showing papers in "The Journal of Physical Chemistry in 2013"

Reversal of the Hofmeister Series: Specific Ion Effects on Peptides B

Abstract: Ion-specific effects on salting-in and salting-out of proteins, protein denaturation, as well as enzymatic activity are typically rationalized in terms of the Hofmeister series. Here, we demonstrate by means of NMR spectroscopy and molecular dynamics simulations that the traditional explanation of the Hofmeister ordering of ions in terms of their bulk hydration properties is inadequate. Using triglycine as a model system, we show that the Hofmeister series for anions changes from a direct to a reversed series upon uncapping the N-terminus. Weakly hydrated anions, such as iodide and thiocyanate, interact with the peptide bond, while strongly hydrated anions like sulfate are repelled from it. In contrast, reversed order in interactions of anions is observed at the positively charged, uncapped N-terminus, and by analogy, this should also be the case at side chains of positively charged amino acids. These results demonstrate that the specific chemical and physical properties of peptides and proteins play a fundamental role in ion-specific effects. The present study thus provides a molecular rationalization of Hofmeister ordering for the anions. It also provides a route for tuning these interactions by titration or mutation of basic amino acid residues on the protein surface.

Electronic Structure Modeling of Electrochemical Reactions at Electrode/Electrolyte Interfaces in Lithium Ion Batteries

Abstract: We review recent ab initio molecular dynamics studies of electrode/electrolyte interfaces in lithium ion batteries. Our goals are to introduce experimentalists to simulation techniques applicable to models which are arguably most faithful to experimental conditions so far, and to emphasize to theorists that the inherently interdisciplinary nature of this subject requires bridging the gap between solid and liquid state perspectives. We consider liquid ethylene carbonate (EC) decomposition on lithium intercalated graphite, lithium metal, oxide-coated graphite, and spinel manganese oxide surfaces. These calculations are put in the context of more widely studied water–solid interfaces. Our main themes include kinetically controlled two-electron-induced reactions, the breaking of a previously much neglected chemical bond in EC, and electron tunneling. Future work on modeling batteries at atomic length scales requires capabilities beyond state-of-the-art, which emphasizes that applied battery research can and s...

Physicochemical Studies on Polyurethane/Siloxane Cross-Linked Films for Hydrophobic Surfaces by the Sol–Gel Process B

Abstract: A series of castor oil based polyurethane/siloxane cross-linked films were prepared using castor oil, isophorone diisocyanate, and 3-aminopropyl trimethoxysilane by the sol–gel process. Fourier transform infrared (FT-IR) spectra reveal the cross-linking interaction between polyurethane and siloxane moieties, thereby shifting the peak position of characteristic NH and CO groups to higher wavenumber. ²⁹Si (silica) solid state nuclear magnetic resonance spectra were used to prove the formation of siloxane network linkage in the polyurethane system, thereby analyzing the Si environment present in the polyurethane/siloxane cross-linked films. The activation energy values at two stages (Tₘₐₓ₁ and Tₘₐₓ₂) for the degradation of polyurethane films were increased with increasing silane ratio. The calculated activation energy values for the higher silane ratio (1.5) are 136 and 170 kJ/mol at Tₘₐₓ₁ and Tₘₐₓ₂, respectively. From contact angle measurements, we observed that increasing siloxane cross-linking increased the hydrophobicity of the films. The optical transmittance obtained from ultraviolet–visible spectra indicated that the film samples are transparent in the region 300–800 nm. The moisture sorption/desorption isotherm curve shows a characteristic behavior of type III isotherm corresponds to hydrophobic materials. Dynamic mechanical studies show that the increase in storage modulus reveals siloxane cross-linking gives rigidity to the films. Atomic force microscopic images show that the introduction of siloxane changes the surface roughness of the polyurethane films. It is found that the siloxane cross-linking can be used to obtain hydrophobic surface films having good thermal stability and optical transmittance.

Influence of Site-Dependent Pigment–Protein Interactions on Excitation Energy Transfer in Photosynthetic Light Harvesting B

Abstract: A site-dependent spectral density system–bath model of the Fenna–Matthews–Olsen (FMO) pigment–protein complex is developed using results from ground-state molecular mechanics simulations together with a partial charge difference model for how the long-range contributions to the chromophore excitation energies fluctuate with environmental configuration. A discussion of how best to consistently process the chromophore excitation energy fluctuation correlation functions calculated in these classical simulations to obtain reliable site-dependent spectral densities is presented. The calculations reveal that chromophores that are close to the protein–water interface can experience strongly dissipative environmental interactions characterized by reorganization energies that can be as much as 2–3 times those of chromophores that are buried deep in the hydrophobic protein scaffolding. Using a linearized density matrix quantum propagation method, we demonstrate that the inhomogeneous system–bath model obtained from our site-dependent spectral density calculations gives results consistent with experimental dissipation and dephasing rates. Moreover, we show that this model can simultaneously enhance the energy-transfer rate and extend the decoherence time. Finally, we explore the influence of initially exciting different chromophores and mutating local environments on energy transfer through the network. These studies suggest that different pathways, selected by varying initial photoexcitation, can exhibit significantly different relaxation times depending on whether the energy-transfer path involves chromophores at the protein–solvent interface or if all chromophores in the pathway are buried in the protein.

Interplay of pH and Binding of Multivalent Metal Ions: Charge Inversion and Reentrant Condensation in Protein Solutions B

Abstract: Tuning of protein surface charge is a fundamental mechanism in biological systems. Protein charge is regulated in a physiological context by pH and interaction with counterions. We report on charge inversion and the related reentrant condensation in solutions of globular proteins with different multivalent metal cations. In particular, we focus on the changes in phase behavior and charge regulation due to pH effects caused by hydrolysis of metal ions. For several proteins and metal salts, charge inversion as measured by electrophoretic light scattering is found to be a universal phenomenon, the extent of which is dependent on the specific protein-salt combination. Reentrant phase diagrams show a much narrower phase-separated regime for acidic salts such as AlCl₃ and FeCl₃ compared to neutral salts such as YCl₃ or LaCl₃. The differences between acidic and neutral salts can be explained by the interplay of pH effects and binding of the multivalent counterions. The experimental findings are reproduced with good agreement by an analytical model for protein charging taking into account ion condensation, metal ion hydrolysis, and interaction with charged amino acid side chains on the protein surface. Finally, the relationship of charge inversion and reentrant condensation is discussed, suggesting that pH variation in combination with multivalent cations provides control over both attractive and repulsive interactions between proteins.

Binding and Aggregation Mechanism of Amyloid β-Peptides onto the GM1 Ganglioside-Containing Lipid Membrane B

Abstract: Accumulation and fibril formation of amyloid β (Aβ) peptides onto a ganglioside-rich lipid membrane is a cause of neuro-disturbance diseases. To find out a measure for suppressing the nucleation of a seed for amyloid fibrils, the mechanism of the initial binding of Aβ to the membrane should be clarified. Molecular dynamics simulations were carried out to investigate the adhesion process of Aβ peptides onto a GM1-ganglioside-containing membrane. Multiple computational trials were executed to analyze the probability of occurrence of Aβ binding by using calculation models containing a mixed lipid membrane, water layer, and one, two, or three Aβs. The simulations demonstrated that Aβ peptides approached the membrane after fluctuation in the water layer and occasionally made steady contact with the membrane. Once the steady contact had been established, Aβ was unlikely to be detached from the membrane and developed into a more stably bound form. In the stably bound form, neuraminic acids on the GM1 cluster strongly held the side chain of Lys28 of Aβ, which caused deformation of the C-terminal region of the Aβ. Since the C-terminal region of the Aβ peptide contains many hydrophobic residues, its deformation on the membrane enhances the hydrophobic interaction with other Aβ peptides. The contact region of two Aβs evolved into a parallel β-sheet form, and the third Aβ was observed to be bound to the complex of two Aβs to make a bundle of Aβ peptides. Some key structures involved in the Aβ aggregation on the GM1-containing membrane were deduced from the multiple simulations.

A Continuum Model of Solvation Energies Including Electrostatic, Dispersion, and Cavity Contributions B

Abstract: Physically accurate continuum solvent models that can calculate solvation energies are crucial to explain and predict the behavior of solute particles in water. Here, we present such a model applied to small spherical ions and neutral atoms. It improves upon a basic Born electrostatic model by including a standard cavity energy and adding a dispersion component, consistent with the Born electrostatic energy and using the same cavity size parameter. We show that the well-known, puzzling differences between the solvation energies of ions of the same size is attributable to the neglected dispersion contribution. This depends on dynamic polarizability as well as size. Generally, a large cancellation exists between the cavity and dispersion contributions. This explains the surprising success of the Born model. The model accurately reproduces the solvation energies of the alkali halide ions, as well as the silver(I) and copper(I) ions with an error of 12 kJ mol–¹ (±3%). The solvation energy of the noble gases is also reproduced with an error of 2.6 kJ mol–¹ (±30%). No arbitrary fitting parameters are needed to achieve this. This model significantly improves our understanding of ionic solvation and forms a solid basis for the investigation of other ion-specific effects using a continuum solvent model.

Highly Ordered Nanorod Assemblies Extending over Device Scale Areas and in Controlled Multilayers by Electrophoretic Deposition B

Abstract: Here we describe the formation of vertically aligned nanorod assemblies over several multilayers using CdS and CdSe nanorods by electrophoretic deposition. The presence of both charge and dipole on the rods allows both field driven deposition and orientational order to form close packed arrays where each rod is vertically aligned. Comparing assembly formation in electrophoresis to spontaneous assembly in solution gives important insights into nanorod organization by these different mechanisms. We show the influence of ligand environment on net charge (zeta potential) and its influence on assembly formation in CdSe nanorods that have long chain alkyl ligands (low charge) or pyridine ligands (high charge). The experimental observations show that highly charged rods deposit too quickly to allow close-packing to occur with perpendicular alignment only occurring with a lower net charge. This is supported by simulation predicting a lower energy configuration with a preference for perpendicular alignment as the charge state decreases. The resolute order that is retained over device scale areas and over several multilayers combined with inherent scalability of electrophoretic deposition makes this approach highly attractive for large scale nanorod integration in electronic, photonic, or photovoltaic devices.

Evidence of Nanometric-Sized Phosphate Clusters in Bioactive Glasses As Revealed by Solid-State 31P NMR

Abstract: Bioactive glasses are able to form strong bonds to bone. This property, crucial for medical applications, depends on the glass composition and structure. Dissolution of phosphates in melt-quenched silicate glasses raises the question of chemical homogeneity and possible formation of clusters. A detailed structural characterization of the bioactive glasses is thus highly desirable. In this work, the nature of the distribution of phosphate units in a melt-quenched bioactive glass is elucidated for the first time using 31P spin-counting solid-state NMR experiments. The structure of a dense bioactive calcium silicate glass with 2.6 mol % of phosphorus oxide is shown to exhibit nanometric-sized chemical and structural heterogeneities. Clear experimental evidence of the presence of phosphate clusters of five and six PO4 tetrahedral units embedded in the disordered polymeric silicate network is given.

A Continuum Solvent Model of the Multipolar Dispersion Solvation Energy B

Abstract: The dispersion energy is an important contribution to the total solvation energies of ions and neutral molecules. Here, we present a new continuum model calculation of these energies, based on macroscopic quantum electrodynamics. The model uses the frequency dependent multipole polarizabilities of molecules in order to accurately calculate the dispersion interaction of a solute particle with surrounding water molecules. It includes the dipole, quadrupole, and octupole moment contributions. The water is modeled via a bulk dielectric susceptibility with a spherical cavity occupied by the solute. The model invokes damping functions to account for solute–solvent wave function overlap. The assumptions made are very similar to those used in the Born model. This provides consistency and additivity of electrostatic and dispersion (quantum mechanical) interactions. The energy increases in magnitude with cation size, but decreases slightly with size for the highly polarizable anions. The higher order multipole moments are essential, making up more than 50% of the dispersion solvation energy of the fluoride ion. This method provides an accurate and simple way of calculating the notoriously problematic dispersion contribution to the solvation energy. The result establishes the importance of using accurate calculations of the dispersion energy for the modeling of solvation.

Chlorophyll Triplet Quenching and Photoprotection in the Higher Plant Monomeric Antenna Protein Lhcb5 B

Abstract: In oxygenic photosynthetic organisms, chlorophyll triplets are harmful excited states readily reacting with molecular oxygen to yield the reactive oxygen species (ROS) singlet oxygen. Carotenoids have a photoprotective role in photosynthetic membranes by preventing photoxidative damage through quenching of chlorophyll singlets and triplets. In this work we used mutation analysis to investigate the architecture of chlorophyll triplet quenching sites within Lhcb5, a monomeric antenna protein of Photosystem II. The carotenoid and chlorophyll triplet formation as well as the production of ROS molecules were studied in a family of recombinant Lhcb5 proteins either with WT sequence, mutated into individual chlorophyll binding residues or refolded in vitro to bind different xanthophyll complements. We observed a site-specific effect in the efficiency of chlorophyllcarotenoid triplet–triplet energy transfer. Thus chlorophyll (Chl) 602 and 603 appear to be particularly important for triplet–triplet energy transfer to the xanthophyll bound into site L2. Surprisingly, mutation on Chl 612, the chlorophyll with the lower energy associated and in close contact with lutein in site L1, had no effect on quenching chlorophyll triplet excited states. Finally, we present evidence for an indirect role of neoxanthin in chlorophyll triplet quenching and show that quenching of both singlet and triplet states is necessary for minimizing singlet oxygen formation.

Free Energies of Binding from Large-Scale First-Principles Quantum Mechanical Calculations: Application to Ligand Hydration Energies B

Abstract: Schemes of increasing sophistication for obtaining free energies of binding have been developed over the years, where configurational sampling is used to include the all-important entropic contributions to the free energies. However, the quality of the results will also depend on the accuracy with which the intermolecular interactions are computed at each molecular configuration. In this context, the energy change associated with the rearrangement of electrons (electronic polarization and charge transfer) upon binding is a very important effect. Classical molecular mechanics force fields do not take this effect into account explicitly, and polarizable force fields and semiempirical quantum or hybrid quantum–classical (QM/MM) calculations are increasingly employed (at higher computational cost) to compute intermolecular interactions in free-energy schemes. In this work, we investigate the use of large-scale quantum mechanical calculations from first-principles as a way of fully taking into account electronic effects in free-energy calculations. We employ a one-step free-energy perturbation (FEP) scheme from a molecular mechanical (MM) potential to a quantum mechanical (QM) potential as a correction to thermodynamic integration calculations within the MM potential. We use this approach to calculate relative free energies of hydration of small aromatic molecules. Our quantum calculations are performed on multiple configurations from classical molecular dynamics simulations. The quantum energy of each configuration is obtained from density functional theory calculations with a near-complete psinc basis set on over 600 atoms using the ONETEP program.

Influence of Ions on Water DiffusionA Neutron Scattering Study B

Abstract: Using quasielastic neutron scattering spectroscopy, we measured the averaged translational diffusion of water in solutions of biologically relevant salts, NaCl, a kosmotrope, and KCl, a chaotrope. The analysis revealed the striking difference in the influence of these ions on water dynamics. While the averaged water diffusion slows down in the presence of the structure making (kosmotrope) Na⁺ ion, the diffusion becomes faster in the presence of the structure breaking (chaotrope) K⁺ ion. The latter means that, despite strong Coulombic interactions introduced by the K⁺ ions, their disruption of the hydrogen-bonding network is so significant that it leads to faster diffusion of the water molecules.

Calorimetric and Structural Studies of Tetrabutylammonium Bromide Ionic Clathrate Hydrates B

Abstract: In the present work, characteristic properties of tetrabutylammonium bromide (TBAB) ionic clathrate hydrates structures were studied by single-crystal X-ray structure analysis. The structures of three different tetragonal TBAB ionic clathrate hydrates that were formed in our experiments were based on the same water lattice of tetragonal structure I (TS-I) differing in the ways of including bromide anions and arranging tetrabutylammonium cations. We demonstrated that (1) Br– can be included into the water lattice, replacing two water molecules, (2) the butyl group of the cation can be inserted not only in large T and P cavities but also in small D cavities of the water lattice TS-I, and (3) one of the reasons for polytypism of ionic clathrate hydrates on the basis of TS-I is the occurrence of alternative modes of arrangements of four-compartment cavities in adjacent layers of the water framework. The compositions of three TBAB ionic clathrate hydrates TBAB·38.1H₂O, TBAB·32.5H₂O, and TBAB·26.4H₂O were determined by chemical analysis, and their enthalpies of fusion were measured by differential scanning calorimetry (DSC). From the obtained results, the enthalpies of the TBAB hydrate formation from TBAB and water were calculated thermodynamically.

Saturation Behavior in X-ray Raman Scattering Spectra of Aqueous LiCl B

Abstract: We report a study on the hydrogen-bond network of water in aqueous LiCl solutions using X-ray Raman scattering (XRS) spectroscopy. A wide concentration range of 0–17 mol/kg was covered. We find that the XRS spectral features change systematically at low concentrations and saturate at 11 mol/kg. This behavior suggests a gradual destruction in the hydrogen-bond network until the saturation concentration. The surprisingly large concentration required for the saturation supports an interpretation in which the ions affect the structure of water only within their first hydration shell. The study is complemented by density-functional-theory calculations and molecular dynamics simulations.

Correlated Ions in a Calcium Channel Model: A Poisson–Fermi Theory B

Abstract: We derive a continuum model, called the Poisson–Fermi equation, of biological calcium channels (of cardiac muscle, for example) designed to deal with crowded systems in which ionic species and side chains nearly fill space. The model is evaluated in three dimensions. It includes steric and correlation effects and is derived from classical hard-sphere lattice models of configurational entropy of finite size ions and solvent molecules. The maximum allowable close packing (saturation) condition is satisfied by all ionic species with different sizes and valences in a channel system, as shown theoretically and numerically. Unphysical overcrowding (“divergence”) predicted by the Gouy–Chapman diffuse model (produced by a Boltzmann distribution of point charges with large potentials) does not occur with the Fermi-like distribution. Using probability theory, we also provide an analytical description of the implicit dielectric model of ionic solutions that gives rise to a global and a local formula for the chemical potential. In this primitive model, ions are treated as hard spheres and solvent molecules are described as a dielectric medium. The Poisson–Fermi equation is a local formula dealing with different correlations at different places. The correlation effects are apparent in our numerical results. Our results show variations of dielectric permittivity from bath to channel pore described by a new dielectric function derived as an output from the Poisson–Fermi analysis. The results are consistent with existing theoretical and experimental results. The binding curve of Poisson–Fermi is shown to match Monte Carlo data and illustrates the anomalous mole fraction effect of calcium channels, an effective blockage of permeation of sodium ions by a tiny concentration (or number) of calcium ions.

Graphene in Ionic Liquids: Collective van der Waals Interaction and Hindrance of Self-Assembly Pathway B

Abstract: Over the past decade, there has been much controversy regarding the microscopic mechanism by which the π-electron-rich carbon nanomaterials such as graphene and carbon nanotubes can be dispersed in ionic liquids. Through a combination of a quantum mechanical calculation on the level of density functional theory, an extensive molecular dynamics study on the time scale of microseconds, and a kinetic analysis at the experimental time scale, we have demonstrated that collective van der Waals forces between ionic liquids and graphene are able to describe both the short-ranged cation−π interaction and the long-ranged dispersion interaction and this microscopic interaction drives two graphene plates trapped in their metastable state while two graphene plates easily self-assemble into graphite in water.

Ionic Liquid Expedites Partition of Curcumin into Solid Gel Phase but Discourages Partition into Liquid Crystalline Phase of 1,2-Dimyristoyl-sn-glycero-3-phosphocholine Liposomes B

Abstract: The hydrolysis of curcumin in alkaline and neutral buffer conditions is of interest because of the therapeutic applicability of curcumin. We show that hydrolysis of curcumin can be remarkably suppressed in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes. The fluorescence of curcumin sensitively detects the phase transition temperature of liposomes. However, at greater concentrations, curcumin affects the phase transition temperature, encouraging fusion of two membrane phases. The interaction of curcumin with DMPC is found to be strong, with a partition coefficient value of Kₚ = 2.78 × 10⁵ in the solid gel phase, which dramatically increases in the liquid crystalline phase to Kₚ = 1.15 × 10⁶. The importance of ionic liquids as green solvents has drawn interest because of their toxicological effect on human health; however, the impact of ionic liquids (ILs) on liposomes is not yet understood. The present study establishes that ILs such as 1-methyl-3-octylimidazolium chloride (moic) affect the permeability and fluidity of liposomes and thus influence parition of curcumin into DMPC liposomes, helping in the solid gel phase but diminishing in the liquid crystalline phase. The Kₚ value of curcumin does not change appreciably with moic concentration in the solid gel state but decreases with moic concentration in the liquid crystalline phase. Curcumin, a rotor sensitive to detect phase transition temperature, is applied to investigate the influence of ionic liquids such as 1-methyl-3-octylimidazolium chloride, 1-buytl-3-methyl imadazolium tetrafluoroborate, and 1-benzyl-3-methyl imidazolium tetrafluoroborate on DMPC liposome properties. 1-Methyl-3-octylimidazolium chloride lowers the phase transition temperature, but 1-buytl-3-methyl imidazolium tetrafluoroborate and 1-benzyl-3-methyl imidazolium tetrafluoroborate do not perceptibly modify the phase transition temperature; rather, they broaden the phase transition.

Investigations on the Interactions of Proteins with Polyampholyte-Coated Magnetite Nanoparticles B

Abstract: Magnetite nanoparticles have been widely used in biomedical applications, especially as contrast agents in magnetic resonance imaging. In this work, the antifouling property of polyampholyte-coating (poly(acrylic acid) (PAA)-co-3-(diethylamino)-propylamine (DEAPA)) is systematically demonstrated. Polyampholyte-coated magnetite nanoparticles (NP1) and PAA-coated magnetite nanoparticles (NP2) were synthesized to investigate their interactions with BSA and lysozyme (LYZ) by high-resolution turbidimetric titration, dynamic light scattering (DLS), and isothermal titration calorimetry (ITC) in phosphate buffer saline (PBS) buffer with pH 7.4. The abundant carboxyl groups of NP2 and polyampholyte coating of NP1 were well proven by TGA, ζ-potential, and titration methods. Turbidity change shows that NP1 have no interaction with both proteins other than NP2 having adsorption with LYZ, which was further confirmed by DLS. Besides, ITC gives the exact enthalpy change and unveils the binding stoichiometry for each interaction. All characterizations demonstrate the antifouling property of NP1 to both negatively charged protein BSA and positively charged protein LYZ. The polyampholyte-coated magnetite nanoparticles were shown to be a promising material to eliminate the strong interaction with proteins in complex medium, for example, when it is applied for MRA contrast agents with long in vivo circulation time.

Unraveling the Role of the Protein Environment for [FeFe]-Hydrogenase: A New Application of Coarse-Graining B

Abstract: Hydrogenase enzymes are natural biocatalysts that might be harnessed to reduce the cost of hydrogen gas production. [FeFe]-hydrogenases are the most effective of three such enzymes at catalyzing H⁺ reduction. In this study, we develop and apply a novel combination of all-atom molecular dynamics and coarse-grained (CG) analysis to characterize two important steps of the catalytic cycle of [FeFe]-hydrogenase. The first is the electron transport through FeS clusters to the active site. We use a Marcus formulation to compute the free energy and the reorganization energy of three electron transport steps and decompose these values into contributions from the CG protein sites and the solvent. The three-step transport process is found to be downhill with relative free energies of −11.7 for the first step, −14.8 for the second step, and −17.1 kcal/mol for the third step. The electron-transport process is also found to activate a water channel suggesting a coupled mechanism for proton and electron transport to the active site. The channel opening is orchestrated by three CG sites located in the active-site domain of the protein with one of the sites also contributing a strong attractive electrostatic potential (ESP) to help shuttle protons to the active site. Overall, our CG analysis points to a concerted mechanism of electron and proton delivery to the active site in these proteins thus providing important insight for the development of biomimetic devices.

Long-Range Charge Separation in a Ferrocene-(zinc Porphyrin)-Naphtalenediimide Triad. Asymmetric Role of 1,2,3-Triazole Linkers

Abstract: New dyad and triad systems based on a zinc porphyrin (ZnP), a naphthalenediimide (NDI), and a ferrocene (Fc) as molecular components, linked by 1,2,3-triazole bridges, ZnP-NDI (3) and Fc-ZnP-NDI (4), have been synthesized. Their photophysical behavior has been investigated by both visible excitation of the ZnP chromophore and UV excitation of the NDI unit. Dyad 3 exhibits relatively inefficient quenching of the ZnP singlet excited state, slow charge separation, and fast charge recombination processes. Excitation of the NDI chromophore, on the other hand, leads to charge separation by both singlet and triplet quenching pathways, with the singlet charge-separated (CS) state recombining in a subnanosecond time scale and the triplet CS state decaying in ca. 90 ns. In the triad system 4, primary formation of the Fc-ZnP+-NDI- charge-separated state is followed by a secondary hole shift process from ZnP to Fc. The product of the stepwise charge separation, Fc+-ZnP-NDI-, undergoes recombination to the ground state in 1.9 μs. The charge-separated states are always formed more efficiently upon NDI excitation than upon ZnP excitation. DFT calculations on a bridge-acceptor fragment show that the bridge is expected to mediate a fast donor-to-bridge-to-acceptor electron cascade following excitation of the acceptor. More generally, triazole bridges may behave asymmetrically with respect to photoinduced electron transfer in dyads, kinetically favoring hole-transfer pathways triggered by excitation of the acceptor over electron-transfer pathways promoted by excitation of the donor.

Assessing the Accuracy of Inhomogeneous Fluid Solvation Theory in Predicting Hydration Free Energies of Simple Solutes

Abstract: Accurate prediction of hydration free energies is a key objective of any free energy method that is applied to modeling and understanding interactions in the aqueous phase. Inhomogeneous fluid solvation theory (IFST) is a statistical mechanical method for calculating solvation free energies by quantifying the effect of a solute acting as a perturbation to bulk water. IFST has found wide application in understanding hydration phenomena in biological systems, but quantitative applications have not been comprehensively assessed. In this study, we report the hydration free energies of six simple solutes calculated using IFST and independently using free energy perturbation (FEP). This facilitates a validation of IFST that is independent of the accuracy of the force field. The results demonstrate that IFST shows good agreement with FEP, with an R² coefficient of determination of 0.99 and a mean unsigned difference of 0.7 kcal/mol. However, sampling is a major issue that plagues IFST calculations and the results suggest that a histogram method may require prohibitively long simulations to achieve convergence of the entropies, for bin sizes which effectively capture the underlying probability distributions. Results also highlight the sensitivity of IFST to the reference interaction energy of a water molecule in bulk, with a difference of 0.01 kcal/mol changing the predicted hydration free energies by approximately 2.4 kcal/mol for the systems studied here. One of the major advantages of IFST over perturbation methods such as FEP is that the systems are spatially decomposed to consider the contribution of specific regions to the total solvation free energies. Visualizing these contributions can yield detailed insights into solvation thermodynamics. An insight from this work is the identification and explanation of regions with unfavorable free energy density relative to bulk water. These regions contribute unfavorably to the hydration free energy. Further work is necessary before IFST can be extended to yield accurate predictions of binding free energies, but the work presented here demonstrates its potential.

Alkali Halide Solutions under Thermal Gradients: Soret Coefficients and Heat Transfer Mechanisms B

Abstract: We report an extensive analysis of the non-equilibrium response of alkali halide aqueous solutions (Na⁺/K⁺–Cl–) to thermal gradients using state of the art non-equilibrium molecular dynamics simulations and thermal diffusion forced Rayleigh scattering experiments. The coupling between the thermal gradient and the resulting ionic salt mass flux is quantified through the Soret coefficient. We find the Soret coefficient is of the order of 10–³ K–¹ for a wide range of concentrations. These relatively simple solutions feature a very rich behavior. The Soret coefficient decreases with concentration at high temperatures (higher than T ∼ 315 K), whereas it increases at lower temperatures. In agreement with previous experiments, we find evidence for sign inversion in the Soret coefficient of NaCl and KCl solutions. We use an atomistic non-equilibrium molecular dynamics approach to compute the Soret coefficients in a wide range of conditions and to attain further microscopic insight on the heat transport mechanism and the behavior of the Soret coefficient in aqueous solutions. The models employed in this work reproduce the magnitude of the Soret coefficient, and the general dependence of this coefficient with temperature and salt concentration. We use the computer simulations as a microscopic approach to establish a correlation between the sign and magnitude of the Soret coefficients and ionic solvation and hydrogen bond structure of the solutions. Finally, we report an analysis of heat transport in ionic solution by quantifying the solution thermal conductivity as a function of concentration. The simulations accurately reproduce the decrease of the thermal conductivity with increasing salt concentration that is observed in experiments. An explanation of this behavior is provided.

Raman and ROA Spectra of (−)- and (+)-2-Br-Hexahelicene: Experimental and DFT Studies of a π-Conjugated Chiral System B

Abstract: The Raman optical activity (ROA) spectra of both enantiomers of 2-Br-hexahelicene in chloroform solution have been measured in the range 1700–300 cm–¹. Density functional theory (DFT) calculations accurately reproduce the observed features. The most intense ROA features are also the most intense Raman features, in the region 1350–1400 cm–¹, and correspond to the so-called D-modes, which play a major role in coronene and other PAHs (polycyclic aromatic hydrocarbons). Together with a detailed analysis of the normal mode structure, the polarizability tensors for the intense Raman features are investigated and related to the principal characteristics of helicene systems, namely, chirality and π-conjugation. Through electron–phonon coupling analysis, we propose a mechanism that justifies the intense ROA signals.

Renewable Feedstocks in Green Solvents: Thermodynamic Study on Phase Diagrams of d-Sorbitol and Xylitol with Dicyanamide Based Ionic Liquids B

Abstract: Experimental and theoretical studies on thermodynamic properties of three ionic liquids based on dicyanamide anion (namely, 1-butyl-3-methylimidazolium dicyanamide, 1-butyl-1-methylpyrrolidinium dicyanamide, and 1-butyl-1-methylpiperidinium dicyanamide) and their binary mixtures with sugar alcohols (d-sorbitol and xylitol) were conducted in order to assess the applicability of the salts ionic liquids for dissolution of those biomass-related materials. Density and dynamic viscosity (at ambient pressure) of pure ionic liquids are reported in the temperature range from T = 293.15 to 363.15 K. Solid–liquid equilibrium phase diagrams in binary systems {sugar alcohol + ionic liquid} were measured with dynamic method up to the fusion temperature of sugar alcohol. The impact of the chemical structure of both the ionic liquid and sugar alcohol were established and discussed. For the very first time, the experimental solubility data were reproduced and analyzed in terms of equation of state rooted in statistical mechanics. For this purpose, perturbed-chain statistical associating fluid theory (PC-SAFT) was employed. In particular, new molecular schemes for the ionic liquids, d-sorbitol, and xylitol were proposed, and then the pure chemicals were parametrized by using available density and vapor pressure data. The model allowed accurate correlation of pure fluid properties for both ionic liquids and sugar alcohols, when the association term is taken into account. The results of solid–liquid equilibria modeling were also satisfactory. However, one or two adjustable binary corrections to the adopted combining rules were required to be adjusted in order to accurately capture the phase behavior. It was shown that a consistent thermodynamic description of extremely complex systems can be achieved by using relatively simple (but physically grounded) theoretical tools and molecular schemes.

Cis Carotenoids: Colorful Molecules and Free Radical Quenchers B

Abstract: We present a density functional theory (DFT) and time-dependent density functional theory (TD-DFT) study on the stability, antioxidant properties with respect to the single electron transfer mechanism, and electronic absorption spectra of some isomers (9-cis, 13-cis, and 15-cis) of carotenoids such as astaxanthin, lycopene, and those present in virgin olive oil (lutein, β-carotene, neoxanthin, antheraxanthin, violaxanthin, neochrome, luteoxanthin, mutatoxanthin, and violaxanthin). In general, the calculated relative stability of the cis isomers appears to be in line with experimental observations. It is predicted that the above-mentioned carotenoids (cis and trans isomers) will transfer one electron to the •OH radical. However, this transference is not plausible with radicals such as •OOH, •OC₂H₅, •OOC₂H₅, •NO₂, and •OOCH₂CHCH₂. On the other hand, some carotenoids (β-carotene, lycopene, lutein, astaxanthin, violaxanthin, and antheraxanthin) will likely accept, in a medium of low polarity, one electron from the radical •O₂–. However, neoxanthin, auroxanthin, mutatoxanthin, luteoxanthin, and neochrome would not participate in such an electronic transfer mechanism. The TD-DFT studies show that neutral species of the cis and trans isomers maintain the same color. On the contrary, the ionic species undergo a “bleaching” process where the absorption wavelengths shift to longer values (>700 nm). Additionally, the formation of a complex between astaxanthin and Cu²⁺ is explored as well as the effect that the metal atom will have in the UV–vis spectrum.

Intramolecular Triplet Energy Transfer in Anthracene-Based Platinum Acetylide Oligomers B

Abstract: Platinum acetylide oligomers that contain an anthracene moiety have been synthesized and subjected to photophysical characterization. Spectroscopic measurement and DFT calculations reveal that both the singlet and triplet energy levels of the anthracene segment are lower than those of the platinum acetylide segment. Thus, the platinum acetylide segment acts as a sensitizer to populate the triplet state of the anthrancene segment via intramolecular triplet–triplet energy transfer. The objective of this work is to understand the mechanisms of energy-transfer dynamics in these systems. Fluorescence quenching and the dominant triplet absorption that arises from the anthracene segment in the transient absorption spectrum of Pt4An give clear evidence that energy transfer adopts an indirect mechanism, which begins with singlet–triplet energy transfer from the anthracene segment to the platinum acetylide segment followed by triplet–triplet energy transfer to the anthracene segment.

Hydrophilic and Hydrophobic Hydration of Sodium Propanoate and Sodium Butanoate in Aqueous Solution B

Abstract: Aqueous solutions of sodium propanoate (NaOPr) and n-butanoate (NaOBu) have been studied at concentrations of c ≲ 3 M by broadband dielectric relaxation spectroscopy over the frequency range 0.2 ≤ ν/GHz ≤ 89 at 25 °C. Three relaxation modes were resolved, centered at (approximately) 1, 8, and 18 GHz, for both sets of solutions. The two faster modes were assigned to the cooperative relaxation of “slow” and bulk water molecules. Detailed analysis of the spectra indicated that both OPr– and OBu– were strongly hydrated, with ∼23 and ∼33 slow water molecules per anion, respectively, at infinite dilution. These effective hydration numbers include ∼6 water molecules hydrophilically bound to the carboxylate moiety, with the remainder arising from the hydrophobic hydration of the nonpolar alkyl chains. The latter shows a characteristic rapid decrease with increasing solute concentration, which facilitated the separation of the hydrophobic and hydrophilic contributions. The lowest frequency mode was a composite with contributions from ion-cloud, ion-pair, and anion relaxations. Although this low intensity mode provided specific evidence of weak ion pairing between Na⁺(aq) and the carboxylate anions, reliable estimates of the association constant could not be made because of its composite nature.

Exploring the Molecular Mechanism of Trimethylamine-N-oxide’s Ability to Counteract the Protein Denaturing Effects of Urea B

Abstract: Protein denaturation in highly concentrated urea solution is a well-known phenomenon. The counteracting effect of a naturally occurring osmolyte, trimethylamine-N-oxide (TMAO), against urea-conferred protein denaturation is also well-established. However, what is largely unknown is the mechanism by which TMAO counteracts this denaturation. To provide a molecular level understanding of how TMAO protects proteins in highly concentrated urea solution, we report here the structural, energetic, and dynamical properties of N-methylacetamide (NMA) solutions that also contain urea and/or TMAO. The solute NMA is of interest mainly because it contains the peptide linkage in addition to hydrophobic sites and represents the typical solvent-exposed state of proteins. Molecular dynamics computer simulation technique is employed in this study. Analysis of solvation characteristics reveals dehydration of NMA and reduction in hydrogen bond number between NMA oxygen and water upon addition of TMAO. The effect of TMAO on NMA–urea interaction is found to be insignificant. Because TMAO cannot donate its hydrogen to NMA oxygen, the total number of hydrogen bonds formed by NMA oxygen with solution species decreases in the presence of TMAO. In solution, TMAO is found to interact strongly with water and urea. Solvation of TMAO makes the water hydrogen bonding network relatively stronger and reduces relaxation of urea–water hydrogen bonds. Implications of these results for counteracting mechanism of TMAO are discussed.

Ion Distributions at the Water/1,2-Dichloroethane Interface: Potential of Mean Force Approach to Analyzing X-ray Reflectivity and Interfacial Tension Measurements

Abstract: We present X-ray reflectivity and interfacial tension measurements of the electrified liquid/liquid interface between two immiscible electrolyte solutions for the purpose of understanding the dependence of interfacial ion distributions on the applied electric potential difference across the interface. The aqueous phase contains alkali-metal chlorides, including LiCl, NaCl, RbCl, or CsCl, and the organic phase is a 1,2-dichloroethane solution of bis(triphenylphosphor anylidene) ammonium tetrakis(pentafluorophenyl)borate (BTPPATPFB). Selected data for a subset of electric potential differences are analyzed to determine the potentials of mean force for Li⁺, Rb⁺, Cs⁺, BTPPA⁺, and TPFB–. These potentials of mean force are then used to analyze both X-ray reflectivity and interfacial tension data measured over a wide range of electric potential differences. Comparison of X-ray reflectivity data for strongly hydrated alkali-metal ions (Li⁺ and Na⁺), for which ion pairing to TPFB– ions across the interface is not expected, to data for weakly hydrated alkali-metal ions (Rb⁺ and Cs⁺) indicates that the Gibbs energy of adsorption due to ion pairing at the interface must be small (<1 kBT per ion pair) for both the CsCl and RbCl samples. This paper demonstrates the applicability of the Poisson–Boltzmann potential of mean force approach to the analysis of X-ray reflectivity measurements that probe the nanoscale ion distribution and the consequences of these underlying distributions for thermodynamic studies, such as interfacial tension measurements, that yield quantities related to the integrated ion distribution.