Solute-solvent Interaction Energy Research Articles

ANI/EFP: Modeling Long-Range Interactions in ANI Neural Network with Effective Fragment Potentials.

Deep learning Neural Networks (NN) have been developed in the field of molecular modeling for the purpose of circumventing the high computational cost of quantum-mechanical calculations while rivaling their accuracies. Although these networks have found great success, they generally lack the ability to accurately describe long-range interactions, which makes them unusable for extended molecular systems. Herein, we provide a method for partially retraining the deep learning general-use neural network ANI, in which the long-range interactions are represented via atomic electrostatic potentials. The electrostatic potentials, generated with polarizable effective fragment potentials (EFP), are used as an additional input feature for the network. This new ANI/EFP network can predict solute-solvent interaction energies on a trained data set with a kcal/mol accuracy. It also shows promise in predicting the interaction energies of a solute in solvent environments that have not been included in a training data set. The proposed protocol can be taken as an example and further developed, leading to highly accurate and transferable neural network potentials capable of handling long-range interactions and extended molecular systems.

Solvation behavior of chitosan monomers in aqueous [Hmim]CL solutions. Experimental and DFT studies

This paper reports densities and viscosities for the mixtures of D-glucosamine hydrochloride (GlcN·HCl) and N-acetyl-D-glucosamine (GlcNAc) with aqueous solutions of 1-hexyl-3-methylimidazolium chloride, [Hmim]Cl, between 293.15 and 318.15 K in 5 K increments and atmospheric pressure. Leveraging this data, we calculated volumetric properties such as apparent molar volume, Vϕ, limiting partial molar volume, Vϕ0, and standard molar volume of transfer, ΔVϕ0. We handled viscosity data to compute the viscosity B-coefficient and several activation parameters of viscous flow. Some of these are relevant in discussing interactions between monosaccharides (solute) and [Hmim]Cl (co-solvent) in aqueous media. The solute–solvent interactions were discussed based on ionic/ hydrophilic/ hydrophobic interactions using the co-sphere overlap model. ΔVϕ0 > 0 indicates that ionic/hydrophilic interactions predominate in the studied systems and are stronger in GlcN·HCl solutions than in GlcNAc at low [Hmim]Cl molalities. At higher [Hmim]Cl concentrations, decreasing values of ΔVϕ0 suggest the dominance of hydrophobic interactions over hydrophilic/ionic ones. We discuss (using Hepler’s constant and viscosity B-coefficient) the ability of monosaccharides to act as structure maker/breaker in aqueous [Hmim]Cl solutions. This indicates that GlcNAc is a better structure promoter than GlcN⋅HCl in aqueous [Hmim]Cl solutions. Finally, density functional theory (DFT) was used to model the molecular structure and compute the solute–solvent interaction energies using the GAMESS 2016 software.

Solvent accessible surface area-assessed molecular basis of osmolyte-induced protein stability.

In solvent-modulated protein folding, under certain physiological conditions, an equilibrium exists between the unfolded and folded states of the protein without any need to break or make a covalent bond. In this process, interactions between various protein groups (peptides) and solvent molecules are known to play a major role in determining the directionality of the chemical reaction. However, an understanding of the mechanism of action of the co(solvent) by a generic theoretical underpinning is lacking. In this study, a generic solvation model is developed based on statistical mechanics and the thermodynamic transfer free energy model by considering the microenvironment polarity of the interacting co(solvent)-protein system. According to this model, polarity and the fractional solvent-accessible surface areas contribute to the interaction energies. The present model includes various orientations of participating interactant solvent surfaces of suitable areas. As model systems, besides the backbone we consider naturally occurring amino acid residues solvated in ten different osmolytes, small organic compounds known to modulate protein stability. The present model is able to predict the correct trend of the osmolyte-peptide interactions ranging from stabilizing to destabilizing not only for the backbone but also for side chains. Our model predicts Asn, Gln, Asp, Glu, Arg and Pro to be highly stable in most of the protecting osmolytes while Ala, Val, Ile, Leu, Thr, Met, Lys, Phe, Trp and Tyr are predicted to be moderately stable, and Ser, Cys and Histidine are predicted to be least stable. However, in denaturing solvents, both backbone and side chain models show similar stabilities in urea and guanidine. One of the important aspects of this model is that it is essentially parameter-free and consistent with the electrostatics of the interaction partners that make this model suitable for estimating any solute-solvent interaction energies.

Unveiling the conformational landscape of tizanidine and its effects on selective nucleation: Enabling controlled manipulation of polymorphs and solvates

The intricate relationship between conformational diversity and polymorphic nucleation is evident, while the underlying mechanisms remain unclear. In this work, tizanidine (TZND) was selected as the model compound to investigate its selective nucleation behavior and the association between conformations and polymorphs. Crystallization experiments were conducted using TZND in seven solvents, resulting in two polymorphs and four solvates, five of which are reported for the first time. Quantum chemical calculations were employed to explore the conformational landscape of TZND. Furthermore, 2D nuclear Overhauser effect spectroscopy and molecular dynamics were utilized to analyze the predominant conformation and its evolution in different solvents with varying concentrations. The results revealed that the dominant conformation of TZND could nucleate directly in methanol and acetone, while a conformational transition occurred during nucleation in acetonitrile and dichloromethane. Additionally, proton transfer in DMSO, chloroform, and DMF resulted in the emergence of new conformational forms. Moreover, crystallographic analysis, spectroscopic studies and quantum chemical calculations were adopted to investigate the molecular mechanisms governing selective nucleation and the formation of polymorphs or solvates. The results indicated that the free energy and solute–solvent interaction energy at specific sites influence the dominant conformation and its evolution in the solution. Finally, the nucleation mechanisms of different polymorphs and solvates were proposed and discussed, illustrating an efficient and environmentally friendly regulation method through solvent switching.

Variational implicit solvation with Legendre-transformed Poisson–Boltzmann electrostatics

The variational implicit-solvent model (VISM) is an efficient approach to biomolecular interactions, where electrostatic interactions are crucial. The total VISM free energy of a dielectric boundary (i.e. solute–solvent interface) consists of the interfacial energy, solute–solvent interaction energy and dielectric electrostatic energy. The last part is the maximum value of the classical and concave Poisson–Boltzmann (PB) energy functional of electrostatic potentials, with the maximizer being the equilibrium electrostatic potential governed by the PB equation. For the consistency of energy minimization and computational stability, here we propose alternatively to minimize the convex Legendre-transformed Poisson–Boltzmann (LTPB) electrostatic energy functional of all dielectric displacements constrained by Gauss’ Law in the solute region. Both integrable and discrete solute charge densities are treated, and the duality of the LTPB and PB functionals is established. A penalty method is designed for the constrained minimization of the LTPB functional. In application to biomolecular interactions, we minimize the total VISM free energy iteratively, while in each step of such iteration, minimize the LTPB energy. Convergence of such a min–min algorithm is shown. Our numerical results on the solvation of a single ion indicate that the LTPB performs better than the PB formulation, providing possibilities for efficient biomolecular simulations.

Solvation dynamics on the diffusion timescale elucidated using energy-represented dynamics theory.

Photoexcitation of a solute alters the solute-solvent interaction, resulting in the nonequilibrium relaxation of the solvation structure, often called a dynamic Stokes shift or solvation dynamics. Thanks to the local nature of the solute-solvent interaction, the characteristics of the local solvent environment dissolving the solute can be captured by the observation of this process. Recently, we derived the energy-represented Smoluchowski-Vlasov (ERSV) equation, a diffusion equation for molecular liquids, which can be used to analyze the solvation dynamics on the diffusion timescale. This equation expresses the time development for the solvent distribution on the solute-solvent pair interaction energy (energy coordinate). Since the energy coordinate can effectively treat the solvent flexibility in addition to the position and orientation, the ERSV equation can be utilized in various solvent systems. Here, we apply the ERSV equation to the solvation dynamics of 6-propionyl-2-dimethylamino naphthalene (Prodan) in water and different alcohol solvents (methanol, ethanol, and 1-propanol) for clarifying the differences of the relaxation processes among these solvents. Prodan is a solvent-sensitive fluorescent probe and is thus widely utilized for investigating heterogeneous environments. On the long timescale, the ERSV equation satisfactorily reproduces the relaxation time correlation functions obtained from the molecular dynamics (MD) simulations for these solvents. We reveal that the relaxation time coefficient on the diffusion timescale linearly correlates with the inverse of the translational diffusion coefficients for the alcohol solvents because of the Prodan-solvent energy distributions among the alcohols. In the case of water, the time coefficient deviates from the linear relationship for the alcohols due to the difference in the extent of importance of the collective motion between the water and alcohol solvents.

Molecular Dynamics of Hydration Shells of Adsorbates in Entropy-Driven Adsorption (Hydrophobic Bonding) to Activated Carbon Surfaces

Hydrophobic bonding is a phenomenon wherein the adsorption of solutes from aqueous solutions is driven largely by the desire of solvent molecules to interact with each other, thus squeezing out solute molecules onto the adsorbent surface. A novel computational analysis of hydration shell water dynamics was used to study the driving force for the hydrophobic bonding of five small drug molecules to activated carbon. It was demonstrated that the solvation of these drug molecules produced hydration shells of lower density and molecular mobility than bulk water, up to 10.5–14 Å distance. Excellent correlations were found between the simulated water-water hydrogen bonding lifetimes in the hydration shell and the experimental capacity constants of hydrophobic bonding (KHB) obtained from the Two-Mechanism Langmuir-Like Equation. KHB also correlated well with the solute-solvent vdW interaction energies in a manner that could allow future predictions of KHB values from simple simulations. Such correlations were not found with the capacity constant of the well-known enthalpy-driven adsorption. The driving force for hydrophobic bonding has entropic origins due to the elimination of water structuring in the hydration shells. However, unlike a typical entropy-driven process, hydrophobic bonding to activated carbon was also associated with a large exothermic enthalpy change when studied with isoperibol calorimetry.

Molecular Crowders Can Induce Collapse in Hydrophilic Polymers via Soft Attractive Interactions.

A comprehensive understanding of protein folding and biomolecular self-assembly in the intracellular environment requires obtaining a microscopic view of the crowding effects. The classical view of crowding explains biomolecular collapse in such an environment in terms of the entropic solvent excluded volume effects subjected to hard-core repulsions exerted by the inert crowders, neglecting their soft chemical interactions. In this study, the effects of nonspecific, soft interactions of molecular crowders in regulating the conformational equilibrium of hydrophilic (charged) polymers are examined. Using advanced molecular dynamics simulations, collapse free energies of an uncharged, a negatively charged, and a charge-neutral 32-mer generic polymer are computed. The strength of the polymer-crowder dispersion energy is modulated to examine its effect on polymer collapse. The results show that the crowders preferentially adsorb and drive the collapse of all three polymers. The uncharged polymer collapse is opposed by the change in solute-solvent interaction energy but is overcompensated by the favorable change in the solute-solvent entropy as observed in hydrophobic collapse. However, the negatively charged polymer collapses with a favorable change in solute-solvent interaction energy due to reduction in the dehydration energy penalty as the crowders partition to the polymer interface and shield the charged beads. The collapse of a charge-neutral polymer is opposed by the solute-solvent interaction energy but is overcompensated by the solute-solvent entropy change. However, for the strongly interacting crowders, the overall energetic penalty decreases since the crowders interact with polymer beads via cohesive bridging attractions to induce polymer collapse. These bridging attractions are found to be sensitive to the binding sites of the polymer, since they are absent in the negatively charged or uncharged polymers. These interesting differences in thermodynamic driving forces highlight the crucial role of the chemical nature of the macromolecule as well as of the crowder in determining the conformational equilibria in a crowded milieu. The results emphasize that the chemical interactions of the crowders should be explicitly considered to account for the crowding effects. The findings have implications in understanding the crowding effects on the protein free energy landscapes.

Solubility determination and correlation, solvent effect and thermodynamic properties of 1,5-gluconolactone in twelve pure solvents at 288.15–328.15 K

At atmospheric pressure, the mole fraction solubility of 1,5-gluconolactone (GDL) in twelve pure solvents (methanol, ethanol, ethylene glycol (EG), n-propanol, i-propanol, n-butanol, i-butanol, t-butanol, acetone, acetic acid, acetonitrile, and dimethyl sulfoxide (DMSO)) was determined by the gravimetric method at 288.15–328.15 K and was positively associated with temperature. In alcohol solvents, the tendency towards molarity solubility at 318.15 K was as follows: EG > methanol > ethanol > t-butanol > i-propanol > n-propanol > n-butanol > i-butanol. In non-alcoholic solvents, the solubility sequence of GDL was DMSO > acetic acid > acetone > acetonitrile. Hirshfeld surface (HS) analysis and solute–solvent interaction energy were used to investigate the intermolecular interactions. The solvent effect was analyzed by the KAT-LSER model, which indicated that solute–solvent interaction had a great effect on solubility. In addition, the experimental values of solubility were correlated by three models (the modified Apelblat equation, van′t Hoff equation, and λh equation), and the corresponding relative deviation (RD), average relative deviation (ARD), and root-mean-square deviation (RMSD) were calculated. The applicability of three selected thermodynamic models was assessed by the Akaike Information Criterion (AIC). It can be concluded that the modified Apelblat equation was the best correlation model. Finally, the apparent thermodynamic properties of Gibbs energy, enthalpy, and entropy were computed using the van′t Hoff equation. All positive values demonstrated that the dissolution of GDL was a non-spontaneous, endothermic, and entropy-driven process.

Effect of the QM Size, Basis Set, and Polarization on QM/MM Interaction Energy Decomposition Analysis.

Herein, an Energy Decomposition Analysis (EDA) scheme extended to the framework of QM/MM calculations in the context of electrostatic embeddings (QM/MM-EDA) including atomic charges and dipoles is applied to assess the effect of the QM region size on the convergence of the different interaction energy components, namely, electrostatic, Pauli, and polarization, for cationic, anionic, and neutral systems interacting with a strong polar environment (water). Significant improvements are found when the bulk solvent environment is described by a MM potential in the EDA scheme as compared to pure QM calculations that neglect bulk solvation. The predominant electrostatic interaction requires sizable QM regions. The results reported here show that it is necessary to include a surprisingly large number of water molecules in the QM region to obtain converged values for this energy term, contrary to most cluster models often employed in the literature. Both the improvement of the QM wave function by means of a larger basis set and the introduction of polarization into the MM region through a polarizable force field do not translate to a faster convergence with the QM region size, but they lead to better results for the different interaction energy components. The results obtained in this work provide insight into the effect of each energy component on the convergence of the solute-solvent interaction energy with the QM region size. This information can be used to improve the MM FFs and embedding schemes employed in QM/MM calculations of solvated systems.

Analysis of solid-liquid equilibrium behavior of highly water-soluble beet herbicide metamitron in thirteen pure solvents using experiments and molecular simulations

Metamitron (MET), a highly water-soluble herbicide for sugar beets, has posed a significant threat to aquatic species due to its high solubility in water; therefore, understanding its solid–liquid phase equilibrium data in different solvents is essential for the future development of more environmentally friendly forms. In this study, the solubility data of MET in 12 pure solvents were determined gravimetrically and analyzed using theoretical methods (primarily Molecular electrostatic potential surface analysis, Hirshfeld surface analysis, Reduced Density Gradient analysis, radial distribution function, and solute–solvent interaction energy) to further investigate the solid–liquid equilibrium behavior. Although MET's high solubility in water contributes to environmental contamination, its value is low in comparison to that of other organic solvents. Four models, including the Apelblat equation, the λh equation, the van't Hoff equation, and the NRTL, were used to examine all solid–liquid equilibrium data, and an ARD% below 5 confirmed its good correlation. The MET solubility data is anticipated to provide solid guidance for the future development of more effective and environmentally acceptable green formulations.

Analytical Solution to the Flory-Huggins Model.

A self-consistent analytical solution for binodal concentrations of the two-component Flory–Huggins phase separation model is derived. We show that this form extends the validity of the Ginzburg–Landau expansion away from the critical point to cover the whole phase space. Furthermore, this analytical solution reveals an exponential scaling law of the dilute phase binodal concentration as a function of the interaction strength and chain length. We demonstrate explicitly the power of this approach by fitting experimental protein liquid–liquid phase separation boundaries to determine the effective chain length and solute–solvent interaction energies. Moreover, we demonstrate that this strategy allows us to resolve differences in interaction energy contributions of individual amino acids. This analytical framework can serve as a new way to decode the protein sequence grammar for liquid–liquid phase separation.

Application of Polarisable Continuum Modelling to assess Minoxidil solubility in mixed solvents

ABSTRACT Experimental solubility of minoxidil was obtained in binary aqueous mixtures of ethanol, methanol and acetonitrile at 298.2 K. The effects of pH and micellisation were investigated on the solubility of the drug. Experimental solubility data was correlated by three co-solvency models, i.e. Yalkowsky, CNIBS/R-K, and the modified Wilson model. Density functional theory and hybrid functional B3LYP method was used with Polarisable Continuum Model (PCM) to correlate the experimental solubility data in various mass fractions of the binary solvent mixtures. We report the results of solute–solvent interaction energy as free energy of solvation (∆G sol ) of minoxidil in various mass fractions of ethanol + water mixtures. Total electrostatic energy, dipole moment, total Gibbs free energy and dielectric constant (ε) of minoxidil were also investigated.

Contributions of higher-order proximal distribution functions to solvent structure around proteins.

The proximal distribution function (pDF) quantifies the probability of finding a solvent molecule in the vicinity of solutes. The approach constitutes a hierarchically organized theory for constructing approximate solvation structures around solutes. Given the assumption of universality of atom cluster-specific solvation, reconstruction of the solvent distribution around arbitrary molecules provides a computationally convenient route to solvation thermodynamics. Previously, such solvent reconstructions usually considered the contribution of the nearest-neighbor distribution only. We extend the pDF reconstruction algorithm to terms including next-nearest-neighbor contribution. As a test, small molecules (alanine and butane) are examined. The analysis is then extended to include the protein myoglobin in the P6 crystal unit cell. Molecular dynamics simulations are performed, and solvent density distributions around the solute molecules are compared with the results from different pDF reconstruction models. It is shown that the next-nearest-neighbor modification significantly improves the reconstruction of the solvent number density distribution in concave regions and between solute molecules. The probability densities are then used to calculate the solute-solvent non-bonded interaction energies including van der Waals and electrostatic, which are found to be in good agreement with the simulated values.

Solubility of xenon in liquid n-alkanes and cycloalkanes by computer simulation. Towards the perfect anaesthetic

The solubility (Henry’s constant) of xenon in a series of n-alkanes (n-heptane, n-octane, n-nonane, n-dodecane and n-hexadecane) and in two cycloalkanes (cyclopentane and cyclohexane) has been obtained by Monte Carlo computer simulations as a function of temperature, at a reference pressure of 100 kPa and compared with experimental results from literature. The n-alkanes and cycloalkanes were modelled with the united atom TraPPE force field, using optimized non-bonded parameters in a transferable fashion for each solvent family. Standard enthalpies of solvation were calculated from the temperature dependence of the Henry’s constants. Solute-solvent interaction energies were also estimated for all systems by Molecular Dynamics. The agreement between the simulated Henry’s constants and experimental data from the literature is excellent in all cases. For all systems involving n-alkanes, the temperature dependence of the standard enthalpies of solvation displays a maximum at very similar reduced temperatures, 0.51 < TR < 0.55. These results confirm the behaviour previously observed experimentally for n-pentane and n-hexane. Furthermore, the simulation results provide a large coherent set of data that allows extending the analysis to a large number of longer n-alkanes, including those for which no experimental data is available.Structural details of the solutions were also studied calculating the radial distribution functions xenon/CHn for all the systems at similar thermodynamic states. The results confirm the enrichment of methyl groups within xenon’s first coordination sphere relatively to methylene groups. The effect is more pronounced the longer the n-alkane chain.

Thermodynamic and structural description of relative solubility of the flavonoid rutin by DFT calculations and molecular dynamics simulations

Understanding the solubility of flavonoids is of relevance in view of the broad application such as antioxidant and antitumor agents. The solubility of flavonoid rutin has been experimentally investigated in a homologous series of alcohols, acetone, n-heptane and water solvents. In this work theoretical evaluation of Gibbs free energy of solvation was performed at the Density Functional Theory (DFT) and Molecular Dynamics (MD) levels, with the ωB97x-D functional and multistate Bennett acceptance ratio (MBAR) method, respectively. The degree of solubility of a given solute among various solvents is dictated by solute-solvent interactions, which may be represented by the solvation energy. Our calculated relative energy values due to solvent effects, using quantum chemical methods (PCM and SMD solvent models) do not follow strictly the observed solubility trend. However, the PCM model including solute-solvent dispersion interaction, solute-solvent repulsion interaction and solute cavitation energy contribution to the total energy (named DIS,REP,CAV) yielded an overall agreement with experimental solubility profile, with exception of ethanol solvent. A similar satisfactory accordance with experiment was found for MBresults, with a discrepancy also found for 1-butanol, which deserve further investigation. Analysis of spatial and radial distribution functions (SDF and RDF) for pure solvents and the analysis of the contributions of vdW and electrostatic terms for the total free energy reveal that a balance between dispersive and electrostatic interactions between the rutin and the solvent is necessary to solubilize the flavonoid, which corroborates with the values of these energy contributions to the solvation-free energy calculated by the MBAR method. The use of explicit solvent molecules in DFT calculations is precluded due to the size, flexibility, and many sites for solvent interactions of rutin molecule. Our results indicate that in general DFT-PCM (DIS,REP,CAV) and MBAR levels of calculations can satisfactorily predict the solvation free energy and solubility trend for flavonoid rutin along a series of solvents, and also provide a reasonable estimate of relative solubilities of other flavonoids, information that can be relevant for further molecular modeling studies.

Bifonazole dissolved in numerous aqueous alcohol mixtures: Solvent effect, enthalpy–entropy compensation, extended Hildebrand solubility parameter approach and preferential solvation

The accessible mole fraction solubility of bifonazole in n-propanol/methanol/isopropanol/propylene glycol (PG) (1) + water (2) mixtures at 298.15 K was investigated in the light of molecular interactions between solvent and solute via extended Hildebrand solubility approach (EHSA). For aqueous methanol/PG systems within entire mole fraction range of methanol/PG, aqueous n-propanol system within mole fraction range of n-propanol from 0 to 0.8 and aqueous isopropanol system within mole fraction range of isopropanol from 0 to 0.7, the solute–solvent interaction energies are larger than the ones of geometric mean for regular solution, asserting that there exists some association between bifonazole and mixtures; whereas within the other composition regions for the aqueous isopropanol/n-propanol systems, the solutions consisting of bifonazole present regular behavior at several solubility points. The inverse Kirkwood–Buff integrals tool was utilized to investigate the local mole fractions of bifonazole in the solutions of alcohol (1) + water (2) in the light of the solubility. For the aqueous solutions of isopropanol/n-propanol with middle and isopropanol/n-propanol-rich mole fractions, preferential solvation of bifonazole was observed for isopropanol/n-propanol. Bifonazole was not solvated preferentially by PG/methanol in the above composition regions for aqueous PG/methanol mixtures. The solvent effect was quantitatively described by modeling the solubility change by means of the linear solvation energy relationships. The solubility parameter and dipolarity-polarizability of system descriptors presented the dominant contributions to variation of solubility values. The transfer and dissolution properties, e.g. entropy, Gibbs free energy change and enthalpy were calculated. In addition, the analysis of enthalpy–entropy compensation was performed, specifying that the variation of bifonazole solubility in the four aqueous alcohol solutions was controlled by two different mechanisms, enthalpy-driven and entropy-driven.

Alchemical transformations for concerted hydration free energy estimation with explicit solvation.

We present a family of alchemical perturbation potentials that enable the calculation of hydration free energies of small- to medium-sized molecules in a single concerted alchemical coupling step instead of the commonly used sequence of two distinct coupling steps for Lennard-Jones and electrostatic interactions. The perturbation potentials we employ are non-linear functions of the solute-solvent interaction energy designed to focus sampling near entropic bottlenecks along the alchemical pathway. We present a general framework to optimize the parameters of alchemical perturbation potentials of this kind. The optimization procedure is based on the λ-function formalism and the maximum-likelihood parameter estimation procedure we developed earlier to avoid the occurrence of multi-modal distributions of the coupling energy along the alchemical path. A novel soft-core function applied to the overall solute-solvent interaction energy rather than individual interatomic pair potentials critical for this result is also presented. Because it does not require modifications of core force and energy routines, the soft-core formulation can be easily deployed in molecular dynamics simulation codes. We illustrate the method by applying it to the estimation of the hydration free energy in water droplets of compounds of varying size and complexity. In each case, we show that convergence of the hydration free energy is achieved rapidly. This work paves the way for the ongoing development of more streamlined algorithms to estimate free energies of molecular binding with explicit solvation.

Solubilisation of dexamethasone: experimental data, co-solvency and Polarised Continuum Modelling

ABSTRACT Experimental solubility of dexamethasone was obtained in binary aqueous mixtures of ethanol, methanol and acetonitrile at 298.2 K. The effects of micellization and pH were also investigated on the solubility of the drug. Experimental solubility data in the binary solvent mixtures were correlated by three co-solvency models, i.e. Yalkowsky, CNIBS/R-K, and the modified Wilson model. A new principle has also been investigated with the purpose of modelling drug solubility in the binary solvent mixtures using Polarised Continuum Model (PCM) by using the hybrid functional B3LYP method and 6–311++G** basis set. By matching experimental data with PCM modelling, we investigated the outcome of drug solubility in various mass fractions of binary solvent mixtures. Here we report the solute-solvent interaction energy as free energy of solvation (∆Gsol), total electrostatic energy, dipole moment, total Gibbs-free energy and dielectric constant (ε) of dexamethasone in various mass fractions of ethanol + water mixtures.

Exact long-range Coulombic energy calculation for net charged systems neutralized by uniformly distributed background charge using fast multipole method and its application to efficient free energy calculation.

In molecular dynamics (MD) calculations of the free energies of ions and ionic molecules, we often encounter net charged molecular systems where the electrical neutrality condition is broken. This charge causes a problem in the evaluation of long-range Coulombic interactions under periodic boundary conditions. A standard remedy for this problem is to consider a hypothetical homogeneous background charge density to neutralize the total system. Here, we present a new expression for the evaluation of Coulombic interactions for such systems including background charge using the fast multipole method (FMM). Furthermore, an efficient scheme is developed to evaluate solute-solvent interaction energies using the FMM, reducing the computational burden for the far-field part. We calculate the hydration free energies of Mg2+, Na+, and Cl- ions dissolved in a neutral solvent using the new expression. The calculated free energies show good agreement with the results obtained using the well-established particle mesh Ewald method. This demonstrates the validity of the proposed expression. This work should make a contribution to highly parallelized MD calculations for large-scale charged systems (particularly, those with over million particles).