Averaged Solvent Electrostatic Potential Research Articles

Photophysical and photochemical properties of 3-hydroxyflavone in ethanol solution: Implicit vs explicit solvent models

Photophysical and photochemical properties of 3-hydroxyflavone in gas-phase and solution were studied using implicit (Polarizable Continuum Model, PCM) and explicit solvent (Averaged Solvent Electrostatic Potential from Molecular Dynamics calculations, ASEP/MD) models. The conformational equilibrium in the excited state between the normal (N*) and tautomeric (T*) forms and their absorption and fluorescence spectra was studied with Time-Dependent Density Functional Theory (TD-DFT) and three different functionals. The calculated transitions are in good agreement with the spectroscopic data. It was found that solvent effects on the absorption and fluorescence spectral bands are negligible. Still, they can modify appreciably the relative stability of the N* minimum with respect to the Franck-Condon position, which affects the kinetics of the reaction. This fact, together with the increase in the barrier height in protic solvents, permits us to explain the reduction of the emission signal from T* and the increase of the N* signal in ethanol solution.

Solvent Effects on the Absorption Spectra of the para-Coumaric Acid Chromophore in Its Different Protonation Forms.

The effects of the solvent and protonation state on the electronic absorption spectrum of the para-coumaric acid (pCA), a model of the photoactive yellow protein (PYP), have been studied using the ASEP/MD (averaged solvent electrostatic potential from molecular dynamics) method. Even though, in the protein, the chromophore is assumed to be in its phenolate monoanionic form, when it is found in water solution pH control can favor neutral, monoanionic, and dianionic species. As the pCA has two hydrogens susceptible of deprotonation, both carboxylate and phenolate monoanions are possible. Their relative stabilities are strongly dependent on the medium. In gas phase, the most stable isomer is the phenolate while in aqueous solution it is the carboxylate, although the population of the phenolate form is not negligible. The s-cis, s-trans, syn, and anti conformers have also been included in the study. Electronic excited states of the chromophore have been characterized by SA-CAS(14,12)-PT2/cc-pVDZ level of theory. The bright state corresponds, in all the cases, to a π → π* transition involving a charge displacement in the system. The magnitude and direction of this displacement depends on the protonation state and on the environment (gas phase or solution). In the same way, the calculated solvatochromic shift of the absorption maximum depends on the studied form, being a red shift for the neutral, carboxylate monoanion, and dianionic chromophores and a blue shift for the phenolate monoanion. Finally, the contribution that the solvent electronic polarizability has on the solvent shift was analyzed. It represents a very important part of the total solvent shift in the neutral form, but its contribution is completly negligible in the mono- and dianionic forms.

Simultaneous Solvent and Counterion Effects on the Absorption Properties of a Model of the Rhodopsin Chromophore.

The ASEP/MD (averaged solvent electrostatic potential from molecular dynamics) method was employed in studying the environment effects (solvent and counterion) on the absorption spectrum of a model of the 11-cis-retinal protonated Schiff base. Experimental studies of the absorption spectra of the rhodopsin chromophore show anomalously large solvent shifts in apolar solvents. In order to clarify their origin, we study the role of the counterion and of the solute-solvent interactions. We compare the absorption spectra in the gas phase, cyclohexane, dichloromethane, and methanol. The counterion effect was described from both a classical and quantum point of view. In the latter case, the contribution of the chromophore-counterion charge transfer to the solvent shift could be analyzed. To the best of our knowledge, this is the first time that counterion and solvent effects on the absorption properties of the 11-cis-retinal chromophore have been simultaneously examined. We conclude that the counterion-solute ionic pair in the gas phase is not a good model to represent the solvent shift in nonpolar solvents, as it does not account for the effect that the thermal agitation of the solvent has on the geometry of the ionic pair. In contrast to nonpolar solvents, the experimental solvent shift values in methanol can be exclusively explained by the polarity of the medium. In dichloromethane, the presence of the counterion does not modify the solvent shift of the first absorption band, but it affects the position of the second excited state. In the three solvents considered, the first two excited states become almost degenerate.

Solvent Effects on the Radiative and Nonradiative Decay of a Model of the Rhodopsin Chromophore.

The radiative and nonradiative decay of a model with five double bonds of the 11-cis-retinal protonated Schiff base was studied both in vacuum and in methanol solution using an extended version of the averaged solvent electrostatic potential from molecular dynamics data (ASEP/MD) method that allows the location of crossing points between free energy surfaces both in equilibrium and in frozen solvent conditions. The multireference quantum method CASSCF was used for the description of the states of interest, while the solvent structure was obtained from molecular dynamics simulations. Electron dynamic correlation corrections to the energy were included at CASPT2 level. Unlike in gas phase, where only two states seem to be implicated, in methanol solution, three states are necessary to describe the photoisomerization process. At the Franck-Condon point the S1 and S2 states are almost degenerate; consequently, the S1 surface has a region with an ionic character ((1)Bu-like) and another one with a covalent character ((2)Ag-like). Emission from the ionic minima is responsible for the low-frequency part of the fluorescence band, while emission from the covalent minima originates the high-frequency part. The ionic minimum is separated from the conical intersection yielding the all-trans isomer by an energy barrier that was estimated in 0.7 kcal/mol. The geometry of the optimized conical intersection was found at a torsion angle of the central double bond close to 90° both in vacuum and in methanol solution. This large torsion in addition to the accompanying charge displacements forces a strong solvent reorganization during the de-excitation process which slows down the photoisomerization kinetics in methanol with respect to the gas phase. Solvent fluctuations modulate the minima depth and the barrier height and could explain the multiexponential relaxation time observed in the experiments.

Study on the conformational equilibrium of the alanine dipeptide in water solution by using the averaged solvent electrostatic potential from molecular dynamics methodology

The ASEP/MD method has been employed for studying the solvent effect on the conformational equilibrium of the alanine dipeptide in water solution. MP2 and density functional theory (DFT) levels of theory were used and results were compared. While in gas phase cyclic structures showing intramolecular hydrogen bonds were found to be the most stable, the stability order is reversed in water solution. Intermolecular interaction with the solvent causes the predominance of extended structures as the stabilizing contacts dipeptide-water are favoured. Free-energy differences in solution were calculated and PPII, α(R), and C5 conformers were identified as the most stable at MP2 level. Experimental data from Raman and IR techniques show discrepancies about the relative abundance of α(R) y C5, our results support the Raman data. The DFT level of theory agrees with MP2 in the location and stability of PPII and α(R) forms but fails in the location of C5. MP2 results suggest the possibility of finding traces of C7eq conformer in water solution, in agreement with recent experiments.

Theoretical study of the role of solvent Stark effect in electron transitions

The possible influence of the solvent Stark effect (SSE) on the solvatochromic shift in electron transitions has been analyzed by using the ASEP/MD (averaged solvent electrostatic potential from molecular dynamics) method. With this purpose, four molecules, two polar (acrolein and formaldehyde) and two non-polar (p-difluorobenzene and trans-difluoroethene) have been studied in solvents of diverse polarity. Independently of the nature of the system we found that the contribution of SSE on the average value of the solvent shift or on the multipole moment values is negligible. In the case of centro-symmetric molecules, our results permit to discard the SSE as cause of the solvent shift found, which must be assigned to the electrostatic interaction of the solute quadrupole and higher multipoles with the solvent. As the SSE values provide also a measure of the errors introduced by the mean field approximation (MFA), these results indicate that MFA permits a very accurate determination of the solvent shift at the same time that it reduces drastically the computational cost. Finally, a new procedure suited to the ASEP/MD method has been presented that permits to estimate the inhomogeneous broadening of spectral bands, complementing the information provided by mean field theories. This procedure does not need additional quantum calculations and its computational cost is minimal.

Theoretical Study of the Competition between Intramolecular Hydrogen Bonds and Solvation in the Cys-Asn-Ser Tripeptide

A study of the competition between intra- and intermolecular hydrogen bonds and its influence on the stability of the Cys-Asn-Ser tripeptide in aqueous solution was performed by using the averaged solvent electrostatic potential from molecular dynamics method (ASEP/MD). The model combines a DFT-B3LYP/6-311+G(d) quantum treatment in the description of the solute molecule with NVT molecular dynamics simulations in the description of the solvent. In gas phase, the most stable structure adopts a C5 conformation. Somewhat higher in energy are found the PP(II) and C7eq structures. In solution, the stability order of the different conformers is reversed: the PP(II) structure becomes the most stable, and the C5 structure is strongly destabilized. The conformational equilibrium is shifted toward conformations in which the intramolecular hydrogen bonds (IHB) have been substituted with intermolecular hydrogen bonds with the water molecules. The solvent stabilizes extended structures without IHBs that are not stable in vacuum. The effect of the protonation state on the conformational equilibrium was also analyzed.

Solvent Effects on Internal Conversions and Intersystem Crossings: The Radiationless De-Excitation of Acrolein in Water

An extended version of the averaged solvent electrostatic potential from molecular dynamics data (ASEP/MD) method oriented to the study of the solvent effects on internal conversion and intersystem crossing processes is presented. The method allows for the location of crossing points between free energy surfaces both in equilibrium and in frozen solvent conditions. The ground and excited states of the solute molecule are described at the complete active space self-consistent field (CASSCF) level while the solvent structure is obtained from molecular dynamics simulations. As an application, we studied the nonradiative de-excitation of s-trans-acrolein 1(n --> pi*) in aqueous solution. We found that the solvent modifies the relative stability of the different crossing points but not enough as to alter the relative order of stability with respect to the in vacuo situation. The relaxation through an equilibrium path involves a strong solvent reorganization. On the contrary, the nonequilibrium path does not involve solvent motion and the de-excitation could proceed with the same speed as in vacuo.

A CASPT2//CASSCF Study of Vertical and Adiabatic Electron Transitions of Acrolein in Water Solution

The 1(n --> pi*) excited-state of acrolein in liquid water was studied theoretically by using the averaged solvent electrostatic potential from molecular dynamics method (ASEP/MD). The model combines a multireference perturbational CASPT2//CASSCF treatment in the description of the solute molecule with NVT molecular dynamics simulations in the description of the solvent. In this paper, we present two alternative methods for calculating solvent shift on adiabatic transitions and their performance is analyzed. In the first method, the free energy change during an adiabatic transition is calculated classically by using the free energy perturbation method. In the second method, it is calculated from the quantum values of the vertical absorption and emission electron transition energies. The 1(n --> pi*) excitation is accompanied by a charge flux from the oxygen to the carbon skeleton, this charge flux decreases the dipole moment of the excited-state with respect to the ground state value and, consequently, the solute-solvent interaction energy. This effect destabilizes the excited-state with respect to the ground state and produces a blue shift in the absorption and emission bands. For the emission process, there also exists an additional destabilization due to a partial desolvation of the excited state. The effect of the solvent electron polarization, the inclusion of the solute electron correlation, and the use of relaxed geometries in solution on the calculated solvent shift of the absorption and emission spectra are also analyzed.

An efficient statistically converged average configuration for solvent effects

Using statistically uncorrelated solute–solvent configurations generated by Monte Carlo simulation a simpler and efficient implementation of the averaged solvent electrostatic potential is made. An average configuration alone is used such that one single quantum mechanical calculation reproduces the converged statistical average obtained from the entire simulation. Applications are presented for solvent effects in a variety of properties of acetone and aminopurine in water. In all cases, excellent agreement is obtained using the average configuration and the average from the full statistical distribution.

Comparison of three effective Hamiltonian models of increasing complexity: Triazene in water as a test case

A critical comparison of widely used solvation models is reported. It is illustrated by a study of the triazene molecule in liquid water. We consider the following approaches: (1) a continuum model based on multicentric multipole expansions of the charge distribution, (2) the averaged solvent electrostatic potential from molecular dynamics (ASEP/MD) method, and (3) molecular dynamics simulations using a combined quantum mechanics/molecular mechanics potential (QM/MM/MD). We find that the solvation induces appreciable changes in the geometry and charge distribution of triazene. These changes are only qualitatively reproduced by the dielectric continuum model, which clearly underestimates induced dipole moments and solute-solvent interaction energy. We also show that the use of effective point charges placed on solute nuclei during the classical simulations may cause significant errors in the description of the solvent structure. The addition of charges representing nitrogen atom lone pairs is compulsory to reproduce the QM/MM/MD simulation results. Moreover, our results validate the use of the mean field approximation in the study of solvent effects. A major conclusion of this study is that the ASEP/MD method constitutes a reliable alternative to the much more computationally demanding QM/MM/MD methods.

Theoretical study of the relative stability of rotational conformers of alpha and beta-D-glucopyranose in gas phase and aqueous solution.

The alpha-beta anomer energy difference and the stability of 10 rotamers of counterclockwise D-glucopyranose were studied in vacuo and in aqueous solution at the B3LYP/6-31+G(d,p) level. To obtain the solute charge distribution and the solvent structure around it, we used the averaged solvent electrostatic potential from molecular dynamics method, ASEP/MD, which alternates molecular dynamics and quantum mechanics calculations in an iterative procedure. The main characteristics of the anomeric equilibrium, both in vacuo and in solution, are well reproduced. The relative stability of the different anomers is related to the availability of the free pairs of electrons in the anomeric oxygen to interact with the water molecules. The influence of solvation in the conformer equilibrium is also analyzed.

An averaged solvent electrostatic potential from molecular dynamics study of the anomeric equilibrium of D-xylose in aqueous solution

We applied the free-energy perturbation method together with the averaged solvent electrostatic potential from molecular dynamics (ASEP/MD) method to study the anomeric equilibrium of d-xylose in aqueous solution. The level of calculation, 6-311G++(2d,2p) basis set and density functional theory, permits one to explain the main characteristics of the anomeric equilibrium of d-xylopyranose: in vacuo, the anomeric effect predominates and the α form is the stabler. In water, solvation leads to the β form being the stabler. A comparison between the performances of the ASEP/MD and polarizable continuum models is also presented.

Theoretical Study of Liquid Hydrogen Fluoride. Application of the Averaged Solvent Electrostatic Potential/Molecular Dynamics Method

We applied a Quantum Mechanics/Molecular Mechanics method (QM/MM) that makes use of the mean field approximation to study the polarization of hydrogen fluoride (HF) in its liquid phase. The method is based on the calculation of the Averaged Solvent Electrostatic Potential from Molecular Dynamics data (ASEP/MD). Our model considers the HF molecule to be nonrigid, the H−F bond length can vary, and includes the effect of the electron correlation calculated at the Moller−Plesset second-order (MP2) level and the effect of the solvent polarization. The H−F bond elongates and undergoes strong polarization when it passes from the gas to the liquid phase. The ASEP/MD method provides an adequate description of this polarization, and reproduces adequately both the thermodynamics and the structure of the liquid. A comparison between the performances of two-site and three-site models for the HF molecule is also presented.

An iterative procedure to determine Lennard-Jones parameters for their use in quantum mechanics/molecular mechanics liquid state simulations

A new procedure to optimize Lennard-Jones (LJ) parameters for liquid state simulations using combined ab initio quantum mechanical and molecular mechanical potentials (QM/MM) is described. In this paper, the LJ parameters are determined in an iterative way taking advantage of the similarity between QM/MM simulations and computations using the averaged solvent electrostatic potential (ASEP) QM/MM approach. The procedure is illustrated through the study of structural and thermodynamic properties of liquid water. The optimized parameters improve significantly the results of previous QM/MM simulations at the same theoretical level.

A theoretical study of liquid alcohols using averaged solvent electrostatic potentials obtained from molecular dynamics simulations: Methanol, ethanol and propanol

We applied a quantum mechanics/molecular mechanics method that makes use of the mean field approximation to study the polarization of several alcohols in the liquid phase. The method is based on the calculation of the averaged solvent electrostatic potential from molecular dynamics data. Because of the reduced number of quantum calculations that our approximation involves, it permits the use of flexible basis sets, the consideration of the electron correlation and the solvent and solute polarization. We found that the molecules studied undergo strong polarization when they pass from the gas to the liquid phase. From this point of view, the polarization methanol displays a behavior lightly different from ethanol and propanol. The vaporization energies are very well reproduced especially when the correlation energy is included. The differences with the experimental values are less than 3% in the three systems studied. Finally, we consider the effect on the thermodynamics and the structure of the solution of the choice of the Lennard-Jones parameters.

A mean field approach that combines quantum mechanics and molecular dynamics simulation: the water molecule in liquid water

A mean field theory for the study of solvent effects that considers polarizable solvent molecules is proposed. The model, which combines quantum chemistry and simulation calculations, splits the system into three parts: 1. (a) the solute, treated quantum-mechanically; 2. (b) polarizable solvent molecules, whose positions are obtained from molecular dynamics data; 3. (c) a dielectric continuum. An averaged solvent electrostatic potential, obtained by averaging over the solvent configurations, is included in the solute Hamiltonian and electric and energy properties are obtained. The method provides results for the dipole moment and solute-solvent interaction which agree with the experimental values and with the results obtained by other workers.