Competing intramolecular vs. intermolecular hydrogen bonds in solution

Bioconformational modulation of a thymidine kinase enzyme ligand through F⋯HO intramolecular hydrogen bond

While the induced-fit of a ligand towards an enzyme is pivotally dictated by intermolecular hydrogen bonds between the small molecule and amino acid residues in the binding site, the role of intramolecular hydrogen bond as contributing interaction for a bioactive conformation is not well understood. This work reports a theoretical conformational analysis of a thymidine kinase enzyme ligand (NMF) that is prone to experience an F⋯HO intramolecular hydrogen bond, inside and outside the biological binding site. This interaction stabilizes the most favorable conformations of NMF in the gas phase and, although it can be disrupted in a biological environment due to intermolecular hydrogen bonds in some cases, these interactions are competitive in other systems. Therefore, an intramolecular hydrogen bond can affect the conformational likeliness most related to the bioactivity. Moreover, isolated conformations governed by this interaction cannot be unequivocally used to generate molecular descriptors in 3D-QSAR (Quantitative Structure-Activity Relationships), since the bioactive conformation may not be determined only by intramolecular interactions.

Photophysical properties of some novel tetraphenylimidazole derived BODIPY based fluorescent molecular rotors

In this work, we present tetraphenylimidazole-based BODIPYs (HPIB1–HPIB4) as fluorescent molecular rotors exhibiting aggregation induced emission, solid state fluorescence and appreciable sensitivity towards viscosity.

On the applicability of functional-group symmetry-adapted perturbation theory and other partitioning models for chiral recognition - the case of popular drug molecules interacting with chiral phases

The applicability of symmetry-adapted perturbation theory (SAPT) and functional-group SAPT (F-SAPT) to study chiral recognition is investigated on an example of three popular chiral drug molecules: ibuprofen, norepinephrine, and baclofen, interacting with phenethylamine or proline - two molecules that are often used as chiral phases in chromatography. The comparison of the F-SAPT with the interacting quantum atoms (IQA) approach is also provided. Accurate estimation of energetic differences of the non-covalent intermolecular complexes composed of two chiral molecules is a necessary prerequisite for the possibility of a prediction of the chiral recognition. The SAPT method with interacting molecules described on the density functional theory level provides accurate total interaction energies, while the F-SAPT approach is the most useful in determining which functional groups are responsible for strengthening or weakening of the interaction between chiral molecules. The largest difference in the interaction energies has been calculated for the baclofen-phenethylamine and norepinephrine-phenethylamine pairs, while the smallest for the ibuprofen-proline and baclofen-proline ones. In most cases, the intermolecular interaction is found to be composed of a strong directional hydrogen bond, which was stabilized by two or more weaker non-covalent interactions between groups (in accordance with the phenomological three-point rule), but in several cases more subtle factors are responsible for larger stability of one diastereoisomer, like the stabilization of the conformation involving two noninteracting functional groups attached to a chiral atom through intramolecular attraction. Additionally, the simulated IR spectra were analyzed for all pairs of diastereoisomeric complexes and the red- and blue-shifts of characteristic bond vibrations were discussed in the context of inter-group interactions.

Assessing the Conformational Equilibrium of Carboxylic Acid via Quantum Mechanical and Molecular Dynamics Studies on Acetic Acid

Accurate hydrogen placement in molecular modeling is crucial for studying the interactions and dynamics of biomolecular systems. The carboxyl functional group is a prototypical example of a functional group that requires protonation during structure preparation. To our knowledge, when in their neutral form, carboxylic acids are typically protonated in the syn conformation by default in classical molecular modeling packages, with no consideration of alternative conformations, though we are not aware of any careful examination of this topic. Here, we investigate the general belief that carboxylic acids should always be protonated in the syn conformation. We calculate and compare the relative energetic stabilities of syn and anti acetic acid using ab initio quantum mechanical calculations and atomistic molecular dynamics simulations. We focus on the carboxyl torsional potential and configurations of microhydrated acetic acid from molecular dynamics simulations, probing the effects of solvent, force field (GAFF vs GAFF2), and partial charge assignment of acetic acid. We show that while the syn conformation is the preferred state, the anti state may in some cases also be present under normal NPT conditions in solution.

Balancing Noncovalent Interactions in the Self-Assembly of Nonplanar Aromatic Carboxylic Acid MOF Linkers at the Solution/Solid Interface: HOPG vs Au(111)

This study explores directed noncovalent bonding in the self-assembly of nonplanar aromatic carboxylic acids on gold and graphite surfaces. It is the first step in developing a new design strategy to create two-dimensional surface metal-organic frameworks (SURFMOFs). The acid molecules used are tetraphenylethene-based and are typically employed in the synthesis of three-dimensional (3D) MOF crystalline solids. They include tetraphenylethene tetracarboxylic acid, tetraphenylethene bisphenyl carboxylic acid, and tetraphenylethene tetrakis-phenyl carboxylic acid. The two-dimensional structures formed from these molecules on highly ordered pyrolytic graphite (HOPG) and Au(111) are studied by scanning tunneling microscopy in a solution environment. The process of monolayer formation and final surface linker structures are found to be strongly dependent on the combination of the molecule and substrate used and are discussed in terms of intermolecular and molecule-substrate interactions, bonding geometry, and symmetry of the acid molecules. In the case of linker self-assembly on HOPG, the molecule-substrate interactions play a significant role in the resulting surface structure. When the acid molecules are adsorbed on Au(111), the intermolecular interactions tend to dominate over the weaker molecule-substrate bonding. Additionally, the interplay of π-π interactions and hydrogen bonding that directs the surface self-assembly on different supports can be modified by varying the linker concentration. This is particularly applicable for the case of the acid molecules adsorbing on the Au(111) substrate. Precise control over predesigned surface structures and orientation of the nonplanar aromatic carboxylic linkers open up an exciting prospect for manipulating the direction of SURFMOF growth in two dimensions and potentially in 3D.

Derivatives of 3-Aminopyrazine-2-carboxamides: Synthesis, Antimicrobial Evaluation, and in Vitro Cytotoxicity

We report the design, synthesis, and in vitro antimicrobial activity of a series of N-substituted 3-aminopyrazine-2-carboxamides with free amino groups in position 3 on the pyrazine ring. Based on various substituents on the carboxamidic moiety, the series is subdivided into benzyl, alkyl, and phenyl derivatives. The three-dimensional structures of the title compounds were predicted using energy minimization and low mode molecular dynamics under AMBER10:EHT forcefield. Compounds were evaluated for antimycobacterial, antibacterial, and antifungal activities in vitro. The most active compound against Mycobacterium tuberculosis H37Rv (Mtb) was 3-amino-N-(2,4-dimethoxyphenyl)pyrazine-2-carboxamide (17, MIC = 12.5 µg/mL, 46 µM). Antimycobacterial activity against Mtb and M. kansasii along with antibacterial activity increased among the alkyl derivatives with increasing the length of carbon side chain. Antibacterial activity was observed for phenyl and alkyl derivatives, but not for benzyl derivatives. Antifungal activity was observed in all structural subtypes, mainly against Trichophyton interdigitale and Candida albicans. The four most active compounds (compounds 10, 16, 17, 20) were evaluated for their in vitro cytotoxicity in HepG2 cancer cell line; only compound 20 was found to exert some level of cytotoxicity. Compounds belonging to the current series were compared to previously published, structurally related compounds in terms of antimicrobial activity to draw structure activity relationships conclusions.

Intramolecular hydrogen bonding: An opportunity for improved design in medicinal chemistry

Recent literature shows that intramolecular hydrogen bond (IMHB) formation can positively impact upon the triad of permeability, solubility, and potency of drugs and candidates. IMHB modulation can be applied to compounds in any chemical space as a means for discovering drug candidates with both acceptable potency and absorption, distribution, metabolism, and excretion-Tox profiles. Integrating IMHB formation in design of drugs is, therefore, an exciting and timely challenge for modern medicinal chemistry. In this review, we first provide some background about IMHBs from the medicinal chemist's point of view and highlight some IMHB-associated misconceptions. Second, we propose a classification of IMHBs for drug discovery purposes, review the most common in silico tactics to include IMHBs in lead optimization and list some experimental physicochemical descriptors, which quantify the propensity of compounds to form IMHBs. By focusing on the compounds size and the number of IMHBs that can potentially be formed, we also outline the major difficulties encountered when designing compounds based on the inclusion of IMHBs. Finally, we discuss recent case studies illustrating the application of IMHB to optimize cell permeability and physicochemical properties of small molecules, cyclic peptides and macrocycles.

Retention of strong intramolecular hydrogen bonds in high polarity solvents in binaphthalene–benzamide derivatives: extensive NMR studies

Advanced multidimensional NMR techniques have been employed to investigate the intramolecular hydrogen bonds (HBs) in a series of N,N′-([1,1′-binaphthalene]-2,2′-diyl)bis(benzamide) derivatives, with the site-specific substitution of different functional groups. The existence of intramolecular HBs and the elimination of any molecular aggregation and possible intermolecular HBs are ascertained by various experimental NMR techniques, including solvent polarity dependent modifications of HB strengths. In the fluorine substituted derivative, direct evidence for the engagement of organic fluorine in HB is obtained by the detection of heteronuclear through-space correlation and the coupling between two NMR active nuclei where the transmission of spin polarization is mediated through HBs (^1hJ_FH). The extent of reduction in the strength of ^1hJ_FH on dilution with high polarity solvents directly provided the qualitative measure of HB strength. The HB, although becoming weakened, does not get nullified even in pure high polarity solvent, which is attributed to the structural constraints. The rate of exchange of a labile hydrogen atom with the deuterium of the solvent permitted the measurement of their half-lives, that are correlated to the relative strengths of HBs. The experimental NMR findings are further validated by XRD and DFT-based theoretical computations, such as, NCI and QTAIM.

NMR studies reveal very strong hydrogen bond unbreakable even in high polarity solvents.

Tough Reversible Adhesion Properties of a Dry Self-Cleaning Biomimetic Surface

Geckos have one of the world's most efficient reversible adhesion systems. Even walking in dusty conditions, geckos can dislodge up to 80% of contaminants and recover their adhesion capability after walking as few as four steps. Thus far, artificial dry self-cleaning materials inspired by the geckos' hierarchical fibrillar structure have been only able to remove 55% of collected large particle contaminants with 30 steps. Challenges, including low mechanical strength, low stiffness, and short fatigue time keep these materials from being used in practical applications. This study involves the novel fabrication of dry self-cleaning surfaces with a high mechanical performance and an outstanding dry self-cleaning property. Imposing a load-drag-pull process similar to a gecko's foot adhesion process, our biomimetic surfaces could dislodge up to 59% of microparticles (∼8 μm) with as few as five steps. Furthermore, the surface had an excellent screening ability at low temperatures regardless of the surface roughness similarity. The surfaces were also proven to be scratch resistant. The biomimetic surfaces exhibit enhanced dry self-cleaning and mechanical properties and could be promising in applications such as reusable adhesives, biochips, aerospace satellite waste collection, and screening equipment.

The intramolecular hydrogen bond as a unit of molecular electronics: Molecular switching controlled by overcrowded intramolecular three-centered hydrogen bond

The equilibrium geometry of the 2,5-bis-[2-(pyridin-2-yl)-vinyl]-1[Formula: see text]-pyrrole calculated at the MP2/6-311[Formula: see text]G([Formula: see text],[Formula: see text]) level of theory evidences the breaking of one of the components in the three-centered intramolecular hydrogen bond due to the steric strain. For this reason, the three-centered intramolecular hydrogen bonding turns out to be asymmetric interaction involving the major and minor components. However, the reversible switching between these components under an external impact is also possible. Two different stable states with unequal geometric and electronic structure are observed in the derivatives of the 2,5-bis-[2-(pyridin-2-yl)-vinyl]-1[Formula: see text]-pyrrole. These molecules represent novel molecular switches operating due to the pendulum-like transition between the nonequivalent two-centered components of the overcrowded three-centered intramolecular hydrogen bond. Implantation of hydrogen bond as a unit of the molecular scale device enhances potential of molecular electronics and could serve as a step towards the construction of artificial biological ensembles.

Estimating the energy of intramolecular hydrogen bonds from 1H NMR and QTAIM calculations

The values of the downfield chemical shift of the bridge hydrogen atom were estimated for a series of compounds containing an intramolecular hydrogen bond O-HO, O-HN, O-HHal, N-HO, N-HN, C-HO, C-HN and C-HHal. Based on these values, the empirical estimation of the hydrogen bond energy was obtained by using known relationships. For the compounds containing an intramolecular hydrogen bond, the DFT B3LYP/6-311++G(d,p) method was used both for geometry optimization and for QTAIM calculations of the topological parameters (electron density ρ^BCP and the density of potential energy V in the critical point of the hydrogen bond). The calculated geometric and topological parameters of hydrogen bonds were also used to evaluate the energy of the hydrogen bond based on the equations from the literature. Comparison of calibrating energies from the ¹H NMR data with the energies predicted by calculations showed that the most reliable are the linear dependence on the topological ρ^BCP and V parameters. However, the correct prediction of the hydrogen bond energy is determined by proper fitting of the linear regression coefficients. To obtain them, new linear relationships were found between the calculated ρ^BCP and V parameters and the hydrogen bond energies obtained from empirical ¹H NMR data. These relationships allow the comparison of the energies of different types of hydrogen bonds for various molecules and biological ensembles.

New Force Field Model for Propylene Glycol: Insight to Local Structure and Dynamics

In this work we developed a new force field model (FFM) for propylene glycol (PG) based on the OPLS all-atom potential. The OPLS potential was refined using quantum chemical calculations, taking into account the densities and self-diffusion coefficients. The validation of this new FFM was carried out based on a wide range of physicochemical properties, such as density, enthalpy of vaporization, self-diffusion coefficients, isothermal compressibility, surface tension, and shear viscosity. The molecular dynamics (MD) simulations were performed over a large range of temperatures (293.15-373.15 K). The comparison with other force field models, such as OPLS, CHARMM27, and GAFF, revealed a large improvement of the results, allowing a better agreement with experimental data. Specific structural properties (radial distribution functions, hydrogen bonding and spatial distribution functions) were then analyzed in order to support the adequacy of the proposed FFM. Pure propylene glycol forms a continuous phase, displaying no microstructures. It is shown that the developed FFM gives rise to suitable results not only for pure propylene glycol but also for mixtures by testing its behavior for a 50 mol % aqueous propylene glycol solution. Furthermore, it is demonstrated that the addition of water to the PG phase produces a homogeneous solution and that the hydration interactions prevail over the propylene glycol self-association interactions.

Weak hydrogen and halogen bonding in 4-[(2,2-difluoroethoxy)methyl]pyridinium iodide and 4-[(3-chloro-2,2,3,3-tetrafluoropropoxy)methyl]pyridinium iodide

To enable a comparison between a C-H...X hydrogen bond and a halogen bond, the structures of two fluorous-substituted pyridinium iodide salts have been determined. 4-[(2,2-Difluoroethoxy)methyl]pyridinium iodide, C₈H₁₀F₂NO⁺·I^-, (1), has a -CH₂OCH₂CF₂H substituent at the para position of the pyridinium ring and 4-[(3-chloro-2,2,3,3-tetrafluoropropoxy)methyl]pyridinium iodide, C₉H₉ClF₄NO⁺·I^-, (2), has a -CH₂OCH₂CF₂CF₂Cl substituent at the para position of the pyridinium ring. In salt (1), the iodide anion is involved in one N-H...I and three C-H...I hydrogen bonds, which, together with C-H...F hydrogen bonds, link the cations and anions into a three-dimensional network. For salt (2), the iodide anion is involved in one N-H...I hydrogen bond, two C-H...I hydrogen bonds and one C-Cl...I halogen bond; additional C-H...F and C-F...F interactions link the cations and anions into a three-dimensional arrangement.

A Study about Regioisomeric Hydroquinones with Multiple Intramolecular Hydrogen Bonding

A theoretical exploration about hydrogen bonding in a series of synthetic regioisomeric antitumor tricyclic hydroquinones is presented. The stabilization energy for the intramolecular hydrogen bond (IHB) formation in four structurally different situations were evaluated: (a) IHB between the proton of a phenolic hydroxyl group and an ortho-carbonyl group (forming a six-membered ring); (b) between the oxygen atom of a phenolic hydroxyl group and the proton of an hydroxyalkyl group (seven membered ring); (c) between the proton of a phenolic hydroxyl group with the oxygen atom of the hydroxyl group of a hydroxyalkyl moiety (seven-membered ring); and (d) between the proton of a phenolic hydroxyl group and an oxygen atom directly bonded to the aromatic ring in ortho position (five-membered ring). A conformational analysis for the rotation around the hydroxyalkyl substituent is also performed. It is observed that there is a correspondence between the conformational energies and the IHB. The strongest intramolecular hydrogen bonds are those involving a phenolic proton and a carbonyl oxygen atom, forming a six-membered ring, and the weakest are those involving a phenolic proton with the oxygen atom of the chromenone, forming five-membered rings. Additionally, the synthesis and structural assignment of two pairs of regioisomeric hydroquinones, by 2D-NMR experiments, are reported. These results can be useful in the design of biologically-active molecules.

Solvent-dependent dynamics of hydrogen bonding structure 5-(methylthio)-1, 3, 4-thiadiazole-2(3H)-thione as determined by Raman spectroscopy and theoretical calculation

The vibration spectra of 5-(methylthio)-1,3,4-thiadiazole-2(3H)-thione (MTTN) in acetonitrile (CH₃CN), methanol (CH₃OH) and water (H₂O) solvents were collected and evaluated via deuterium isotopic substitution Raman spectroscopic experiments. These experiments were combined with the quantum chemical theoretical calculations using the PCM solvent model and normal mode analysis. The results confirmed that the MTTN in CH₃CN, CH₃OH and H₂O have hydrogen bonding (H-bonding) MTTN(solvent)_n clusters that produce significantly different Raman intensity patterns in different solvents. Combined with the normal Raman assignment, most resonance Raman spectra were assigned to the vibration modes of the H-bonding MTTN(CH₃CN), MTTN(CH₃OH)₃ and MTTN(H₂O)₃ clusters in CH₃CN, CH₃OH and H₂O. The theoretically-predicted frequencies and intensities in different surrounding environments enabled reliable assignments of Raman bands. The intermolecular>NH⋯O and >NH⋯N H-bonding interactions are key constituents of stable thione structures in MTTN. This underlines the significant structural differences of MTTN in CH₃CN, CH₃OH and H₂O. H-bonding perturbation of MTTN reveal important insights about the intermolecular excited state proton transfer (ESPT) reaction mechanisms in the Franck-Condon region structural dynamics of the thione→thiol tautomer in CH₃OH and H₂O.

H-Bond: Τhe Chemistry-Biology H-Bridge

H-bonding, as a non covalent stabilizing interaction of diverse nature, has a central role in the structure, function and dynamics of chemical and biological processes, pivotal to molecular recognition and eventually to drug design. Types of conventional and non conventional (H-H, dihydrogen, H- π, CH- π, anti- , proton coordination and H-S) H-bonding interactions are discussed as well as features emerging from their interplay, such as cooperativity (σ- and π-) effects and allostery. Its utility in many applications is described. Catalysis, proton and electron transfer processes in various materials or supramolecular architectures of preorganized hosts for guest binding, are front-line technology. The H-bond-related concept of proton transfer (PT) addresses energy issues or deciphering the mechanism of many natural and synthetic processes. PT is also of paramount importance in the functions of cells and is assisted by large complex proteins embedded in membranes. Both intermolecular and intramolecular PT in H-bonded systems has received attention, theoretically and experimentally, using prototype molecules. It is found in rearrangement reactions, protein functions, and enzyme reactions or across proton channels and pumps. Investigations on the competition between intra- and intermolecular H bonding are discussed. Of particular interest is the H-bond furcation, a common phenomenon in protein-ligand binding. Multiple H-bonding (H-bond furcation) is observed in supramolecular structures.

Specific Noncovalent Association of Chiral Large-Ring Hexaimines: Ion Mobility Mass Spectrometry and PM7 Study

Ion mobility mass spectrometry and PM7 semiempirical calculations are effective complementary methods to study gas phase formation of noncovalent complexes from vaselike macrocycles. The specific association of large-ring chiral hexaimines, derived from enantiomerically pure trans-1,2-diaminocyclohexane and various isophthaldehydes, is driven mostly by CH-π and π-π stacking interactions. The isotrianglimine macrocycles are prone to form two types of aggregates: tail-to-tail and head-to-head (capsule) dimers. The stability of the tail-to-tail dimers is affected by the size and electronic properties of the substituents at the C-5 position of the aromatic ring. Electron-withdrawing groups stabilize the aggregate, whereas bulky or electron-donating groups destabilize the complexes.

Comment on "Structural and vibrational studies on 1-(5-Methyl- [1,3,4] thiadiazol-2-yl)-pyrolidin-2-ol" [Spectrochimica Acta Part A, 152 (2016) 252-261]. The importance of intramolecular OH⋯N hydrogen bonding in the conformational properties of thiadiazol-pyrrolidin-2-ol bearing species

The title paper [1] reports a study on the spectroscopic and physicochemical properties of 1-(5-methyl- [1,3,4]thiadiazol-2-yl)-pyrrolidin-2-ol (MTPN) based on experimental and theoretical data. The latter ones are based on the computed molecular structure for a rather unusual conformer. Here, after a careful analysis of the conformational space of MTPN, the most stable conformation was determined for the molecule isolated in a vacuum, which results to be 21.9kJ/mol more stable than the conformer reported previously. Our study also includes the closely related species 1-(5-trifluoromethyl- [1,3,4]thiadiazol-2-yl)-pyrrolidin-2-ol (FMTPN). An intramolecular OH⋯N hydrogen bond determines the conformational behavior of the [1,3,4]thiadiazol-2-yl)-pyrrolidin-2-ol group as demonstrated by Natural Bond Orbital population analysis.

Reactivity of Biliatresone, a Natural Biliary Toxin, with Glutathione, Histamine, and Amino Acids

In our previous work, we identified a natural toxin, biliatresone, from Dysphania glomulifera and D. littoralis, endemic plants associated with outbreaks of biliary atresia in Australian neonatal livestock. Biliatresone is a very rare isoflavonoid with an α-methylene ketone between two phenyls, 1,2-diaryl-2-propenone, along with methylenedioxy, dimethoxyl, and hydroxyl functional groups, that causes extrahepatic biliary toxicity in zebrafish. The toxic core of biliatresone is a methylene in the α-position relative to the ketone of 1,2-diaryl-2-propenone that serves as an electrophilic Michael acceptor. The α-methylene of biliatresone spontaneously conjugated with water and methanol (MeOH), respectively, via Michael addition in a reverse phase high-performance liquid chromatography (RP-HPLC) analysis. We here report the reactivity of biliatresone toward glutathione (GSH), several amino acids, and other thiol- or imidazole-containing biomolecules. LC-MS and HPLC analysis of the conjugation reaction showed the reactivity of biliatresone to be in the order histidine > N-acetyl-d-cysteine (D-NAC) = N-acetyl-l-cysteine (L-NAC) > histamine > glutathione ≥ cysteine ≫ glycine > glutamate > phenylalanine, while serine and adenine had no reactivity due to intramolecular hydrogen bonding in the protic solvents. The reactivity of ethyl vinyl ketone (EVK, 1-penten-3-one), an example of a highly reactive α,ß-unsaturated ketone, toward GSH gave a 6.7-fold lower reaction rate constant than that of biliatresone. The reaction rate constant of synthetic 1,2-diaryl-2-propen-1-one (DP), a core structure of the toxic molecule, was 10-fold and 1.5-fold weaker in potency compared to the reaction rate constants of biliatresone and EVK, respectively. These results demostrated that the methylenedioxy, dimethoxyl, and hydroxyl functional groups of biliatresone contribute to the stronger reactivity of the Michael acceptor α-methylene ketone toward nucleophiles compared to that of DP and EVK.

Theoretical in-Solution Conformational/Tautomeric Analyses for Chain Systems with Conjugated Double Bonds Involving Nitrogen(s)

Conformational/tautomeric transformations for X=CH–CH=Y structures (X = CH₂, O, NH and Y = NH) have been studied in the gas phase, in dichloromethane and in aqueous solutions. The paper is a continuation of a former study where s-cis/s-trans conformational equilibria were predicted for analogues. The s-trans conformation is preferred for the present molecules in the gas phase on the basis of its lowest internal free energy as calculated at the B97D/aug-cc-pvqz and CCSD(T)_CBS (coupled-cluster singles and doubles with non-iterative triples extrapolated to the complete basis set) levels. Transition state barriers are of 29–36 kJ/mol for rotations about the central C–C bonds. In solution, an s-trans form is still favored on the basis of its considerably lower internal free energy compared with the s-cis forms as calculated by IEF-PCM (integral-equation formalism of the polarizable continuum dielectric solvent model) at the theoretical levels indicated. A tetrahydrate model in the supermolecule/continuum approach helped explore the 2solute-solvent hydrogen bond pattern. The calculated transition state barrier for rotation about the C–C bond decreased to 27 kJ/mol for the tetrahydrate. Considering explicit solvent models, relative solvation free energies were calculated by means of the free energy perturbation method through Monte Carlo simulations. These calculated values differ remarkably from those by the PCM approach in aqueous solution, nonetheless the same prevalent conformation was predicted by the two methods. Aqueous solution structure-characteristics were determined by Monte Carlo. Equilibration of conformers/tautomers through water-assisted double proton-relay is discussed. This mechanism is not viable, however, in non-protic solvents where the calculated potential of mean force curve does not predict remarkable solute dimerization and subsequent favorable orientation.

Development of linseed oil-TiO2 green nanocomposites as antimicrobial coatings

This study deals with the preparation and characterization of acrylated epoxidized linseed oil (AELO) based bionanocomposites for antimicrobial coating applications. AELO was synthesized from epoxidized vegetable oils, crosslinked with an acrylic monomer, reinforced with anatase TiO₂ nanoparticles and then subjected to UV irradiation to yield the cured nanocomposite coatings. The effect of TiO₂ loading on the morphology, barrier, thermal, mechanical, tribological and antibacterial performance of the coatings has been comprehensively investigated. FT-IR spectra indicated the existence of strong TiO₂-AELO hydrogen bonding interactions. The nanoparticles were randomly dispersed within the bioresin, significantly reducing its water absorption and gas permeability whilst increasing its thermal stability. They also promoted remarkable enhancements of both static and dynamic mechanical properties such as storage and Young's moduli, hardness, impact resistance and glass transition temperature. Strong reductions in the coefficient of friction and the wear rate were attained in the nanocomposites with the highest TiO₂ loadings. The coatings were found to display antimicrobial activity even in the absence of UV light, and the bactericidal effect against Staphylococcus aureus was higher than on Escherichia coli. Furthermore, the antimicrobial activity improved with increasing nanoparticle concentration. The use of these "green" nanocomposite coatings could be a suitable and inexpensive method to prevent microbial proliferation in public places, particularly in medical centers where there is higher risk of infections.

Mathematical modeling and physical reality in noncovalent interactions

The Hellmann-Feynman theorem provides a straightforward interpretation of noncovalent bonding in terms of Coulombic interactions, which encompass polarization (and accordingly include dispersion). Exchange, Pauli repulsion, orbitals, etc., are part of the mathematics of obtaining the system's wave function and subsequently its electronic density. They do not correspond to physical forces. Charge transfer, in the context of noncovalent interactions, is equivalent to polarization. The key point is that mathematical models must not be confused with physical reality.