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PubMed2006

Natural bond orbital analyses of persulfoxide stabilization by remote functional groups. The conformationally induced electrostatic stabilization sulfide photooxygenation mechanism.

The conformationally induced electrostatic stabilization (CIES) sulfide photooxygenation mechanism was computationally examined using an ab initio model and extended to the study of new donor atoms. The MP2/6-31G(d) geometries and a natural population analysis of natural lone-pair orbitals on the donor atoms support the mechanism and reveal that oxygen and nitrogen donor groups are more stabilizing than sulfur.

Journal Article↗
PubMed2006

Self-assembled hemicapsules with inherent functionalities: modeling of a supramolecular electrostatic self-assembly.

An approach to functional self-assembled hemicapsules is described consisting of the use of multivalent (valency > or = 4) and divalent components, the functional groups of which have a relatively weak binding affinity. Electrostatic self-assembly of tetrakis(pyridiniummethyl)cavitand hemispheres (H) and doubly charged anions (A) in polar media gives rise to an equilibrium mixture that consists, as detected with ESI-MS, of hemicapsule H2A3, capsule H2A4, and other ion-pair associates. Fitting 1H NMR data with a model that includes (hemi)capsules and ion-pair associates gave an effective molarity (EM) for the intramolecular assembly of the host (H) with sulfate (A) of 0.19 +/- 0.02 M and binding constants of the functional [2 + 3] hemicapsules H2A3 and the [2 + 4] capsules H(2)A(4) in methanol of 3.25 x 10(12) M(-4) and 3.45 x 10(15) M(-5), respectively. A substantial amount of the functional [2 + 3] hemicapsules H2A3 with respect to [2 + 4] capsules H2A4 is present in solution, with ratios of H2A3 to H2A4 of 5.67-0.43 in the studied concentration range (0.1-25 mM of [H]tot). The [2 + 3] hemicapsules H2A3 built with sulfate linkers incorporate guests between the closely positioned pyridinium planes.

Journal Article↗
PubMed1996

The Exceptional Conformational Bias of 1,5-Diene-3,4-diols Is Controlled by Electrostatic Interactions: A Theoretical Elucidation of the Origin in Rotational Isomer Stability.

A conformational study of 1,5-hexadiene-3,4-diol (1) and 3-buten-1-ol (2) with the ab initio molecular orbital methods reveals that electrostatic interactions, rather than steric or tosional effects, control the conformational preferences of these compounds. It is found that the 1,2-dioxygen function prefers the anti arrangement due to the lone pair electron repulsion. A pseudo 1,3-diaxial oxygen/pi bond repulsion and a 1,3-diaxial attraction between an oxygen lone pair and a vinyl H are found to be responsible for the profound preference for the CO-eclipsed form in diol 1. Quantitatively, the repulsion between two gauche oxygen atoms is calculated to be approximately 1.5 kcal/mol at the MP2/6-31G//HF/6-31G level of theory. The repulsion between an oxygen atom and a pi bond at the pseudo 1,3-diaxial position is approximately 1 kcal/mol, and the attractive interaction between an oxygen atom and a vinyl H at the pseudo 1,3-diaxial position is approximately 0.5 kcal/mol, respectively, at the MP2/6-31G//MP2/6-31G level.

Journal Article↗
PubMed1997

Electrostatic Effects on the Population of Atropisomers of Charged and Dipolar Derivatives of 1,8-Di(2'-pyridyl)naphthalene.

Dicationic 1',1'-dimethyl (6a), dipolar 1',1'-dioxide (6b), and 6',6'-dipyridone (7) derivatives of 1,8-di(2'-pyridyl)naphthalene were prepared. Proton NMR failed to reveal the presence of individual anti-syn atropisomers over a wide range of temperatures. By contrast, the 1',1"-dimethyl derivative of 1-(2'-pyridyl)-8-(3"-pyridyl)naphthalene (8), a positional isomer of 6a, existed in DMSO-d(6) as a 2:1 mixture of anti to syn atropisomers at ambient temperatures. Electrostatic repulsion is believed to cause the anti atropisomer to be favored, especially in 6a where the charged regions are closer together than in 8, owing to the splayed-out arrangement of the rings. AM1 and PM3 computations confirm that the anti is highly favored over the synisomer for 6a, 6b, and 7.

Journal Article↗
PubMed2005

Voltammetric studies of through-space and through-bond electrostatic interactions in alkyl linked ferrocene and benzoaza-15-crown-5 receptor molecules in acetonitrile.

Six new ferrocene alkyl-benzoaza-15-crown-5 molecules with different alkyl spacer lengths were synthesized and investigated by voltammetry in acetonitrile. Their mean potentials (E(o)') were more negative than that of ferrocene. The changes were greater for the bis-substituted ligands than for the monosubstituted ones. Increasing the alkyl spacer length shifted E(o)' negatively from -CH2- to -(CH2)2- but positively from -(CH2)2- to -(CH2)4-. This unusual variation is attributed to the combined electron donating and withdrawing influences and also the steric effect from the substituents on the ferrocene moiety. Analyses of the potentials of the molecules and their fully protonated forms suggested intramolecular electrostatic signaling through not only the space but also the alkyl chain which is usually considered to be an insulator for through-bond communication. Diffusion coefficients with insignificant differences between the receptors and their fully protonated forms were derived from cyclic voltammograms, suggesting insignificant further conformational variation upon protonation.

Journal Article↗
PubMed2005

Electrostatic field and partial Fermi level pinning at the pentacene-SiO(2) interface.

Monolayer islands of pentacene deposited on silicon substrates with thermally grown oxides were studied by electric force microscopy (EFM) and scanning Kelvin probe microscopy (SKPM) in ultrahigh vacuum (UHV) after prior 10 min exposure to atmospheric ambient. On 25-nm-thick oxides, the pentacene islands are 0.5 V higher in electrostatic potential than the silicon dioxide background because of intrinsic contact potential differences. On 2-nm-thin oxides, tunneling across the oxides allows Fermi level equilibration with pentacene associated states. The surface potential difference depends on the doping of the underlying Si substrates. The Fermi level movement at the pentacene SiO(2) interface was restricted and estimated to lie between 0.3 and 0.6 eV above the pentacene valence band maximum. It is proposed that hole traps in the pentacene or at the pentacene-oxide interface are responsible for the observations.

Journal Article↗
PubMed2005

Electrostatic contributions to indole-lipid interactions.

The role of electrostatic forces in indole-lipid interactions was studied by (1)H and (2)H NMR in ether- and ester-linked phospholipid bilayers with incorporated indole. Indole-ring-current-induced (1)H NMR chemical shifts of lipid resonances in bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, 1,2-dioleoyl-sn-glycero-3-phosphocholine, 1,2-di-O-octadecenyl-sn-glycero-3-phosphocholine, and 1,2-di-O-octadecenyl-sn-glycero-3-phosphomethanol show a bimodal indole distribution, with indole residing at the upper hydrocarbon chain/glycerol region of the lipid and near the choline group, when present. (2)H NMR of indole-d(7)-incorporated lipid bilayers reveals that the former site is occupied by about two-thirds of the indole, which adopts a distinct preferred orientation with respect to the bilayer normal. The results suggest that the upper hydrocarbon chain/glycerol location is dictated by many factors, including interactions with the electric charges and dipoles, van der Waals interactions, entropic contributions, and hydrogen bonding. Indole diffusion rates are higher in lipids with ester bonds and lower in choline-containing lipids, suggesting that interactions between indole and carbonyl groups are of minor importance for lipid-indole association and that cation-pi interactions with choline drive the second indole location. Nuclear Overhauser effect spectroscopy cross-relaxation rates suggest a 30-ns lifetime for indole-lipid associations. These results may have important implications for sidedness and structural transitions in tryptophan-rich membrane proteins.

Choline↗
PubMed2005

Synthesis and photoluminescent properties of titanate layered oxides intercalated with lanthanide cations by electrostatic self-assembly methods.

Various lanthanide cations were intercalated into the interlayer of the exfoliated H(x)Ti((2-x)/4)) square(x/4)O(4) x H(2)O (HTO) by the electrostatic self-assembly deposition (ESD) and layer-by-layer self-assembly (LBL) methods. X-ray diffraction and thermal analysis data indicated that interlayer lanthanide cations existed as an aqua ion and were coordinated with 7-10 water molecules under ambient conditions. The interlayer distances were found to be in the range 6-7 Angstrom for HTO layered oxide intercalated with a lanthanide cation. Intercalation of lanthanide cations into the interlayer by the LBL method was monitored by UV-vis spectrum and X-ray diffraction. Photoluminescence properties were also discussed in detail. Eu(3+) intercalated layered oxide exhibited intense red emission at room temperature. The presence of interlayer water molecules was found to be inevitable for the emission with high intensity. The emission intensity was significantly higher for the films conditioned at 100% RH than those at 5% RH. The icelike behavior of the confined water molecules in the interlayer around lanthanide cations was believed to be contributing highly to the emission mechanism. The mechanism was illustrated and explained by data obtained under several conditions.

Journal Article↗
PubMed2006

Monte Carlo and modified Tanford-Kirkwood results for macromolecular electrostatics calculations.

The understanding of electrostatic interactions is an essential aspect of the complex correlation between structure and function of biological macromolecules. It is also important in protein engineering and design. Theoretical studies of such interactions are predominantly done within the framework of Debye-Hückel theory. A classical example is the Tanford-Kirkwood (TK) model. Besides other limitations, this model assumes an infinitesimally small macromolecule concentration. By comparison to Monte Carlo (MC) simulations, it is shown that TK predictions for the shifts in ion binding constants upon addition of salt become less reliable even at moderately macromolecular concentrations. A simple modification based on colloidal literature is suggested to the TK scheme. The modified TK models suggested here satisfactorily predict MC and experimental shifts in the calcium binding constant as a function of protein concentration for the calbindin D(9k) mutant and calmodulin.

Macromolecular Substances↗
PubMed2006

Electrostatic stitching in gel-phase supported phospholipid bilayers.

We show that mixing zwitterionic lipids with up to 20% mole % cationic lipids produces gel-phase supported lipid bilayers that are morphologically free of defects detectable using noncontact mode atomic force microscopy (AFM). This contrasts with the observation of massive defects when anionic lipid was added, and also when no charged lipid was added. Infrared measurements of headgroup orientation in the presence of cationic lipid show that the mean headgroup orientation changes only minimally when temperature is lowered from the fluid phase to the gel phase. This is consistent with a tentative explanation, based on simple electrostatic arguments, in which cationic lipids "stitch" the bilayers together. On the functional side, this study demonstrates a simple method by which to minimize defects in gel-supported phospholipid bilayers.

Gels↗
PubMed2006

The body-centered cubic structure of methyllithium tetramer crystal: staggered methyl conformation by electrostatic stabilization via intratetramer multipolarization.

The methyllithium tetramer (CH3Li)4 structure in the bcc crystal has been theoretically optimized with the use of density functional theory calculations under the periodic boundary condition. The X-ray structure shows that the methyl-group conformation in tetramer in crystal takes the staggered form rather than the eclipsed form that is taken in the isolated tetramer, i.e., the crystal packing effect, and this has been reproduced for the first time. It is concluded that the staggered form is advantageous in crystal, as a whole, due to the larger electrostatic stabilization via the induced intratetramer multipolarization, although it should cause, simultaneously, smaller destabilization in intratetramer electronic energy.

Journal Article↗
PubMed2006

Quantum mechanics/molecular mechanics electrostatic embedding with continuous and discrete functions.

A quantum mechanics/molecular mechanics (QM/MM) implementation that uses the Gaussian electrostatic model (GEM) as the MM force field is presented. GEM relies on the reproduction of electronic density by using auxiliary basis sets to calculate each component of the intermolecular interaction. This hybrid method has been used, along with a conventional QM/MM (point charges) method, to determine the polarization on the QM subsystem by the MM environment in QM/MM calculations on 10 individual H(2)O dimers and a Mg(2+)-H(2)O dimer. We observe that GEM gives the correct polarization response in cases when the MM fragment has a small charge, while the point charges produce significant over-polarization of the QM subsystem and in several cases present an opposite sign for the polarization contribution. In the case when a large charge is located in the MM subsystem, for example, the Mg(2+) ion, the opposite is observed at small distances. However, this is overcome by the use of a damped Hermite charge, which provides the correct polarization response.

Magnesium↗
PubMed2006

Balance of hydrophobic and electrostatic forces in the pH response of weak polyelectrolyte capsules.

A detailed study of the role of solution pH and ionic strength on the swelling behavior of capsules composed of the weak polyelectrolytes poly(4-vinylpyridine) (P4VP) and poly(methacrylic acid) (PMA) with different numbers of layers was carried out. The polyelectrolyte layers were assembled onto silicon oxide particles and multilayer formation was followed by zeta-potential measurements. Hollow capsules were investigated by scanning electron microscopy and atomic force microscopy. The pH-dependent behavior of P4VP/PMA capsules was probed in aqueous media using confocal laser scanning microscopy. All systems exhibited a pronounced swelling at the edges of stability, at pHs of 2 and 8.1. The swelling degree increased when more polymer material was adsorbed. The swollen state can be attributed to uncompensated positive and negative charges within the multilayers, and it is stabilized by counteracting hydrophobic interactions. The swelling was related to the electrostatic interactions by infrared spectroscopy and zeta-potential measurements. The stability of the capsules as well as the swelling degree at a given pH could be tuned, when the ionic strength of the medium was altered.

Biophysics↗
PubMed2007

Liquid-structure forces and electrostatic modulation of biomolecular interactions in solution.

Molecular interactions in solution are controlled by the bulk medium and by the forces originating in the structured region of the solvent close to the solutes. In this paper, a model of electrostatic and liquid-structure forces for dynamics simulations of biomolecules is presented. The model introduces information on the microscopic nature of the liquid in the vicinity of polar and charged groups and the associated non-pairwise character of the forces, thus improving upon conventional continuum representations. The solvent is treated as a polar and polarizable medium, with dielectric properties described by an inhomogeneous version of the Onsager theory. This treatment leads to an effective position-dependent dielectric permittivity that incorporates saturation effects of the electric field and the spatial variation of the liquid density. The non-pairwise additivity of the liquid-structure forces is represented by centers of force located at specific points in the liquid phase. These out-of-the-solute centers are positioned at the peaks of liquid density and exert local, external forces on the atoms of the solute. The density is calculated from a barometric law, using a Lennard-Jones-type solute-liquid effective interaction potential. The conceptual aspects of the model and its exact numerical solutions are discussed for single alkali and halide ions and for ion-pair interactions. The practical aspects of the model and the simplifications introduced for efficient computation of forces in molecular solutes are discussed in the context of polar and charged amino acid dimers. The model reproduces the contact and solvent-separated minima and the desolvation barriers of intermolecular potentials of mean force of amino acid dimers, as observed in atomistic dynamics simulations. Possible refinements based on an improved treatment of molecular correlations are discussed.

Biophysics↗
PubMed2006

Molecular electrostatic potential devices on graphite and silicon surfaces.

We demonstrate that molecular gates using molecular electrostatic potentials (MEP) can be used on hydrogen-passivated silicon substrates without any disturbance of their behavior in vacuum; however, the use of graphite as a substrate strongly affects such behavior. As expected, the substrate may become one more design variable. The ability to have several substrate alternatives is very important for the practical implementation of this new scenario based on molecular potentials. In general, the effect of the substrate can be predetermined by calculating the MEP of the surface as this indicates how strongly its intrinsic potential is.

Journal Article↗
PubMed2006

Transferability of quantum topological atoms in terms of electrostatic interaction energy.

Understanding atomic transferability is important to guide the design of a force field. Atoms in molecules are defined and computed according to the theory of quantum chemical topology (QCT). The electron density associated with such topological atoms is conveniently described by high-rank multipole moments. Here, we assess the transferability of atoms by means of their electrostatic interaction energy, using a convergent multipole expansion. The test systems are (H2O)3 and serine...(H2O)5. The effect of a varying electron density cutoff (i.e., truncating the atoms) is discussed and the effect of polarization is quantified.

Journal Article↗
PubMed2004

Electrostatic interactions between double layers: influence of surface roughness, regulation, and chemical heterogeneities.

Electrostatic interactions between two surfaces as measured by atomic force microscopy (AFM) are usually analyzed in terms of DLVO theory. The discrepancies often observed between the experimental and theoretical behavior are usually ascribed to the occurrence of chemical regulation processes and/or to the presence of surface chemical or morphological heterogeneities (roughness). In this paper, a two-gradient mean-field lattice analysis is elaborated to quantifying double layer interactions between nonplanar surfaces. It allows for the implementation of the aforementioned sources of deviation from DLVO predictions. Two types of ion-surface interaction ensure the adjustment of charges and potentials upon double layer overlap, i.e., specific ionic adsorption at the surfaces and/or the presence of charge-determining ions for the surfaces considered. Upon double layer overlap, charges and potentials are adjusted via reequilibrium of the different ion adsorption processes. Roughness is modeled by grafting asperities on supporting planar surfaces, with their respective positions, shapes, and chemical properties being assigned at will. Local potential and charge distributions are derived by numerically solving the nonlinear Poisson-Boltzmann equation under the boundary conditions imposed by the surface profiles and regulation mechanism chosen. Finite size of the ions is taken into account. A number of characteristic situations are briefly discussed. It is shown how the surface irregularities are reflected in the Gibbs energy of interaction.

Journal Article↗
PubMed2004

Orientational behavior of thermotropic liquid crystals on surfaces presenting electrostatically bound vesicular stomatitis virus.

We report the orientational behavior of nematic phases of 4-cyano-4'-pentylbiphenyl (5CB) on cationic, anionic, and nonionic surfaces before and after contact of these surfaces with solutions containing the negatively charged vesicular stomatitis virus (VSV). The surfaces were prepared on evaporated films of gold by either adsorption of poly-L-lysine (cationic) or formation of self-assembled monolayers (SAMs) from HS(CH2)2SO3- (anionic) or HS(CH2)11(OCH2CH2)4OH (nonionic). Prior to treatment with virus, we measured the initial orientation of 5CB (delta epsilon = epsilon(parallel) - epsilon(perpendicular) > 0) to be parallel to the cationic surfaces (planar anchoring) but perpendicular (homeotropic) after equilibration for 5 days. A similar transition from planar to homeotropic orientation of 5CB was observed on the anionic surfaces. Only planar orientations of 5CB were observed on the nonionic surfaces. Because N-(4-methoxybenzylidene)-4-butylaniline (MBBA, delta epsilon = epsilon(parallel) - epsilon(perpendicular) < 0) exhibited planar alignment on all surfaces, the time-dependent alignment of 5CB on the ionic surfaces is consistent with a dipolar coupling between the 5CB and electrical double layers formed at the ionic interfaces. Treatment ofpoly-L-lysine-coated gold films (cationic) with purified solutions of VSV containing 10(8)-10(10) plaque-forming units per milliliter (pfu/mL) led to the homeotropic alignment of 5CB immediately after contact of 5CB with the surface. In contrast, treatment of anionic surfaces and nonionic surfaces with solutions of VSV containing approximately 10(10) pfu/mL did not cause immediate homeotropic alignment of 5CB. These results and others suggest that homeotropic alignment of 5CB on cationic surfaces treated with VSV of titer > or = 10(8) pfu/mL reflects the presence of virus electrostatically bound to these surfaces.

Anions↗