Charge-transfer Indices Research Articles

Valence-Topological Charge-Transfer Indices, Dipole, Isoelectric Point and Fractal: From Homo/Heterocycles to Proteins

In the present study, valence-topological charge-transfer indices are used to calculate the isoelectric point (pI). Dipoles are computed by vector semisums µvec and valence vector semisums µvec V comparing with isoelectronic series (cyclopentadiene, benzene, styrene). The valence vector semisum results intermediate between the vector semisum and experimental dipole. The steric effect stays constant and the dominating effect is electronic. The indices are applied to the dipole of homologous series of percutaneous enhancers and amino-acid pI. No collapse of Gi-Ji indices is observed in the fit of pI decreasing co-llinearity risk. A heteroatom in the π-electron systems is beneficial. Acidic and basic groups improve models of pI. Fragment fitting allows computing lysozyme pI. Globular protein models (lysozyme, myoglobin, albumin) cover a wide pI range. The purpose of the work is to extend the study to anionic lipid bilayers. The role of electrostatics is examined in cationic protein adsorption to zwitterionic phosphatidylcholine (PC) and anionic mixed PC/phosphatidylglycerol (PG) small unilamellar vesicles, via spectrofluorimetry and liquid chromatography. Parameters (partition, co-operativity) are calculated. At pI, binding reaches maximum. In Gouy-Chapman formalism, the activity coefficient rises with square charge; deviations from this model indicate asymmetric location of anionic phospholipid in the inner leaflet, in mixed vesicles for lysozyme- and myoglobin-PC/PG systems in agreement with experiments and molecular dynamics. Vesicles bind myoglobin anti-co-operatively while they attach lysozyme/albumin, co-operatively. A model is proposed for lysozyme/albumin composing two protein sub-layers with different structures and properties. The Hill coefficient reflects the subunit co-operativity of bi and tridomain proteins. Keywords: Charge distribution, dipole moment, fractal hybrid orbital analysis, isoelectric point, valence-isoelectronic series, valencetopological charge-transfer index.

An efficient computational scheme for electronic excitation spectra of molecules in solution using the symmetry-adapted cluster-configuration interaction method: the accuracy of excitation energies and intuitive charge-transfer indices.

Solvent effects on electronic excitation spectra are considerable in many situations; therefore, we propose an efficient and reliable computational scheme that is based on the symmetry-adapted cluster-configuration interaction (SAC-CI) method and the polarizable continuum model (PCM) for describing electronic excitations in solution. The new scheme combines the recently proposed first-order PCM SAC-CI method with the PTE (perturbation theory at the energy level) PCM SAC scheme. This is essentially equivalent to the usual SAC and SAC-CI computations with using the PCM Hartree-Fock orbital and integrals, except for the additional correction terms that represent solute-solvent interactions. The test calculations demonstrate that the present method is a very good approximation of the more costly iterative PCM SAC-CI method for excitation energies of closed-shell molecules in their equilibrium geometry. This method provides very accurate values of electric dipole moments but is insufficient for describing the charge-transfer (CT) indices in polar solvent. The present method accurately reproduces the absorption spectra and their solvatochromism of push-pull type 2,2'-bithiophene molecules. Significant solvent and substituent effects on these molecules are intuitively visualized using the CT indices. The present method is the simplest and theoretically consistent extension of SAC-CI method for including PCM environment, and therefore, it is useful for theoretical and computational spectroscopy.

Prediction of the linear and nonlinear optical properties of tetrahydronaphthalone derivatives via long-range corrected hybrid functionals

The linear and nonlinear optical (NLO) properties of methoxybenzylidene (1) and thiophen-2-ylmethylidene (2) tetrahydronaphthalone derivatives are studied using long-range corrected density functional theory (LC-DFT). The calculated hyperpolarisabilities indicate that both compounds have measurable NLO properties (approximately one to two times the hyperpolarisability of p-nitroaniline). Charge-transfer indices and time-dependent DFT calculations suggest that the NLO properties are a result of a charge-transfer excitation, which is typical in conjugated donor–acceptor structures. The ultraviolet–visible spectra of 1 and 2 are also predicted using gap-fitting schemes, and these data are used to assess how accurately the hyperpolarisabilities of 1 and 2 could be estimated by the solvatochromic method.

Chemically intuitive indices for charge-transfer excitation based on SAC-CI and TD-DFT calculations

A recently proposed charge-transfer (CT) index and some related quantities aimed to the description of CT excitations in push-pull donor-acceptor model systems were computed in vacuum and in ethanol by the direct symmetry-adapted cluster-configuration interaction (SAC-CI) method including solvent effects by means of the nonequilibrium state-specific approach. The effects of both solvation and electron correlations on these quantities were found to be significant. The present results are also in line with previous time-dependent (TD) density functional theory calculations and they demonstrate that SAC-CI provides a description of the excitation character close to that of TD-PBE0. Indeed, CT indices evaluated by the SAC-CI and TD-PBE0 would be useful in the field of materials chemistry, for the design and development of novel molecular materials.

Topological Charge-Transfer Indices: From Small Molecules to Proteins

Valence-topological charge-transfer indices are applied to the calculation of dipole moment-pH at the isoelectric point. Dipole moments calculated by algebraic-vector semisums of charge-transfer indices are defined. The ability of indices, for the description of molecular charge distribution, is established by comparing them with the dipole moment of the valence-isoelectronic series of cyclopentadiene-benzene-styrene. Both charge-transfer indices are proposed: vector semisums μvec-μvec V. The μvec V is intermediate between μvec and μexperiment. The steric effect is almost constant along series and the dominating effect is electronic. The indices are applied to the calculation of the dipole moments of the homologous series of percutaneous enhancers and the isoelectric point of 21 amino acids. In most fits no superimposition of the corresponding Gk-Jk/Gk VJk V pairs is observed, which diminishes the risk of collinearity. The inclusion of heteroatoms in the π-electron systems is beneficial for the description of isoelectric point, because of either the role of additional p-orbitals provided by heteroatom or the role of steric factors in the π-electron conjugation. The use of (valence) chargetransfer indices gives limited results for amino-acid isoelectric points. The inclusion of the number of acidic/basic groups improves the models, especially for amino acids with more than two functional groups. The fitting line for 21 amino acids is used to estimate the lysozyme isoelectric point by replacing (1+Δn/nT) with (M+Δn)/nT. The lysozyme fragment results can estimate the isoelectric point of the whole protein within 1-13% error.

Improvement of charge-transfer indices for multifunctional amino acids: Application to lysozyme†

Valence topological Charge-Transfer (CT) indices are applied to the calculation of pH at the isoelectric point (pI). The model is generalized for molecules with heteroatoms. The ability of the indices for the description of molecular charge distribution is established by comparing them with the pI of 21 amino acids. Linear correlation models are obtained. The CT indices improve multivariable regression equations for pI. The variance decreases by 95%. No superimposition of the corresponding Gk –Jk and pairs is observed in most fits, which diminishes the risk of collinearity. The inclusion of heteroatoms in π-electron system is beneficial for the description of pI, the because of either the role of the additional p orbitals provided by heteroatom or the role of steric factors in π-electron conjugation. The use of CT/valence CT indices {Gk , Jk , } gives limited results for modelling amino acid pI. The inclusion of the number of acidic/basic groups improves the models. The effect is specially noticeable for amino acids with more than two functional groups. The fitting line for 21 amino acids is used to estimate lysozyme pI replacing (1 + Δn/n T) with (M + Δn)/n T. The results for lysozyme fragments can estimate the pI of the whole protein with 1–13% error. †Presented at the 13th International Workshop on QSARs in the Environmental Sciences (QSAR 2008), 8–12 June 2008, Syracuse, USA.

Valence Topological Charge-Transfer Indices for Reflecting Polarity: Correction for Heteromolecules

Valence topological charge-transfer (CT) indices are applied to the calculation of dipole moments μ. The μ calculated by algebraic and vector semisums of the CT indices are defined. The model is generalized for molecules with heteroatoms and corrected for sp3-heteromolecules. The ability of the indices for the description of the molecular charge distribution is established by comparing them with μ of the valence-isoelectronic series of cyclopentadiene, benzene and styrene. Two CT indices, μvec (vector semisum of vertex-pair μ) and μvecV (valence μvec) are proposed. The μvecV behaviour is intermediate between μvec and μexperiment. The correction is produced in the correct direction. The best results are obtained for the greatest group. Inclusion of the heteroatom in the π-electron system is beneficial for the description of μ, owing to either the role of additional p and/or d orbitals provided by the heteroatom or the role of steric factors in the π-electron conjugation. The steric effect is almost constant along the series and the dominating effect is electronic. Inclusion of the heteroatom enhances μ, which can improve the solubility of the molecule. For heteroatoms in the same group, the ring size and the degree of ring flattering are inversely proportional to their electronegativity.

Valence Topological Charge-Transfer Indices for Dipole Moments: Percutaneous Enhancers

Valence topological charge-transfer (CT) indices are applied to the calculation of dipole moments. The algebraic and vector semisum CT indices are defined. The combination of CT indices allows the estimation of the dipole moments. The model is generalized for molecules with heteroatoms. The ability of the indices for the description of the molecular charge distribution is established by comparing them with the dipole moments of homologous series of percutaneous enhancers (phenyl alcohols and 4-alkylanilines). Linear and quadratic correlation models are obtained. CT indices improve the multivariable quadratic regression equations for the dipole moment. The variance decreases 97% (4-alkylanilines). No superposition of the corresponding G(k)-J(k)and G(k)(V)-J(k)(V) pairs is observed in the fits, which diminishes the risk of co-linearity. The inclusion of the heteroatom in the pi-electron system is beneficial for the description of the dipole moment, owing to either the role of the additional p orbitals provided by the heteroatom or the role of steric factors in the pi-electron conjugation. Inclusion of a conjugated double bond in the alkyl chain lends to more rigid structures with dipole moment variations lower than 1%.

Valence topological charge-transfer indices for dipole moments

Valence topological charge-transfer (CT) indices are applied to the calculation ofdipole moments. The dipole moments calculated by algebraic and vector semisums of the CT indices are defined. The combination of the CT indices allows the estimation of the dipole moments. The model is generalized for molecules with heteroatoms. The ability of the indices for the description of the molecular charge distribution is established by comparing them with the dipole moment of the valence-isoelectronic series of benzene and styrene. Two CT indices, micro(vec) (vector semisum of vertex-pair dipole moments) and microVvec (valence micro(vec)) are proposed. micro(vec) and microVvec are important for the prediction of micro(experiment), because (micro(experiment) - micro(vec)) and (microVvec - micro(experiment)) always have the same sign. In general, the best results are obtained for the greatest group that can be studied. The inclusion of the heteroatom in the pi-electron system is beneficial for the description of the dipole moment, owing to either the role of additional p and/or d orbitals provided by the heteroatom or the role of steric factors in the pi-electron conjugation. The steric effect is almost constant along the studied series and the dominating effect is the electronic one. Inclusion of the heteroatom enhances the dipole moment, which, in turn, can improve the solubility of the molecule. Inclusion of the heteroatom enhances the dipole moment, which can improve the solubility of the molecule.

Valence Topological Charge-Transfer Indices for Dipole Moments

New valence topological charge-transfer indices are applied to the calculation of dipole moments. The algebraic and vector semisum charge-transfer indices are defined. The combination of the charge-transfer indices allows the estimation of the dipole moments. The model is generalized for molecules with heteroatoms. The ability of the indices for the description of the molecular charge distribution is established by comparing them with the dipole moments of a homologous series of phenyl alcohols. Linear and non-linear correlation models are obtained. The new charge-transfer indices improve the multivariable non-linear regression equations for the dipole moment. When comparing with previous results, the variance decreases 92%. No superposition of the corresponding Gk–Jk and – pairs is observed. This diminishes the risk of co-linearity. Inclusion of the oxygen atom in the π-electron system is beneficial for the description of the dipole moment, owing to either the role of the additional p orbitals provided by the heteroatom or the role of steric factors in the π-electron conjugation. Linear and non-linear correlations between the fractal dimension and various descriptors point not only to a homogeneous molecular structure but also to the ability to predict and tailor drug properties.

Valence topological charge-transfer indices for dipole moments

New topological valence charge-transfer (CT) indices are applied to the calculation of dipole moments. The dipole moments calculated by algebraic and vector semisums of the CT indices are defined. The combination of the CT indices allows the estimation of the dipole moments. The model is generalized for molecules with heteroatoms. The ability of the indices for the description of the molecular charge distribution is established by comparing them with the dipole moment of the valence-isoelectronic series of benzene and styrene. Two new CT indices, μ vec (vector semisum of vertex-pair dipoles) and μ vec V (valence μ vec) are proposed. Inclusion of the heteroatom in the π-electron system is beneficial for the description of the dipole moment, owing to either the role of additional p and/or d orbitals provided by the heteroatom or the role of steric factors in the π-electron conjugation. The steric effect is almost constant along the studied series and the dominating effect is the electronic one.

A new topological index to elucidate apolar hydrocarbons.

A new topological charge-transfer index is applied to elucidate the polar character of hydrocarbons. The dipole moments calculated by algebraic and vector semisums of charge-transfer indices are defined in this work. The combination of the charge-transfer indices allows the estimation of the molecular dipole moments. The ability of the indices for the description of the molecular charge distribution is established by comparing them with the dipole moment of a heterogeneous set of 57 polar and 53 apolar hydrocarbons. Linear and nonlinear correlation models are obtained. The new charge-transfer index, mu(vec), elucidates whether hydrocarbons are apolar, and improves the multivariable nonlinear regression equation for the dipole moment. When comparing with previous results, smaller superpositions of the corresponding pairs Gk, Jk are observed in our fits. This diminishes the risk of colinearity.

The nature of electronic transitions in absorption spectra of phenyl-substituted 1,3,4-oxadiazoles and 1,3,4-sym-triazoles

The electronic absorption spectra of 1,3,4-oxadiazole, 1,3,4-sym-triazole and their phenyl derivatives were calculated within the π-electron approximation, by the PPP-Cl procedure. The nature of the electronic transitions was analysed in terms of special quantum-mechanical excitation indices: excitation-localization indices and charge-transfer indices. An orbital analysis was performed within the framework of the structure-orbital approach. The transitions calculated for unsubstituted azoles should be treated as perturbed ones of the models-cys-azabutadiene molecule. In the case of phenyl derivatives one can classify the transitions as being either local or molecular in character. The dependence of their characteristics on the π-system structure has been studied for the latter transitions. The results of the theoretical treatment are in good agreement with experimental data.