Structure and dynamics of solvent landscapes in charge-transfer reactions (original) (raw)
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Energy fluctuations of a solute molecule embedded in a polar solvent are investigated to depict the energy landscape for solvation dynamics. The system is modeled by a charged molecule surrounded by two layers of solvent dipolar molecules with simple rotational dynamics. Individual solvent molecules are treated as simple dipoles that can point toward or away from the central charge ͑Ising spins͒. Single-spin-flip Monte Carlo kinetics simulations are carried out in a two-dimensional lattice for different central charges, radii of outer shell, and temperatures. By analyzing the density of states as a function of energy and temperatures, we have determined the existence of multiple freezing transitions. Each of them can be associated with the freezing of a different layer of the solvent.
Solvent Reorganization Entropy of Electron Transfer in Polar Solvents
The Journal of Physical Chemistry A, 2006
We report the results of molecular dynamics simulations of the solvent reorganization energy of intramolecular electron transfer in a charge-transfer molecule dissolved in water and acetonitrile at varying temperatures. The simulations confirm the prediction of microscopic solvation theories of a positive reorganization entropy in polar solvents. The results of simulations are analyzed in terms of the splitting of the reorganization entropy into the contributions from the solute-solvent interaction and from the alteration of the solvent structure induced by the solute. These two contributions mutually cancel each other, resulting in the reorganization entropy amounting to only a fraction of each component.
Effects of Solvent and Solute Polarizability on the Reorganization Energy of Electron Transfer
The Journal of Physical Chemistry A, 2004
We report Monte Carlo simulations of the effect of solute and solvent polarizability on the solvent reorganization energy of intramolecular electron transfer. In the first set of simulations, the polarizability of the solvent is varied at constant permanent dipole of the solvent molecules (high-frequency dielectric constants is in the range 1-2.5). The reorganization energy is calculated on the solvent configurations around a nonpolar solute (charge separation transition) and around a dipolar solute (charge recombination transition). In both cases, the variation of the solvent reorganization energy does not exceed 30%, a change much smaller than predicted by dielectric continuum models. In the second set of simulations, the solute polarizability in the chargeseparated state was varied while keeping the initial state for charge separation at zero dipole moment and polarizability. The gap between the charge-separation and charge recombination reorganization energies widens substantially with increasing difference in the polarizability of the initial and final charge-transfer states. Both the effect of solute and solvent polarizability can be accurately described by analytical theories of solvent reorganization. Reorganization Energy of Electron Transfer J. Phys. Chem. A C Reorganization Energy of Electron Transfer J. Phys. Chem. A E
The Journal of Chemical Physics, 2003
The hybrid molecular-continuum model for polar solvation considered in this paper combines the dielectric continuum approximation for treating fast electronic ͑inertialess͒ polarization effects and a molecular dynamics ͑MD͒ simulation for the slow ͑inertial͒ polarization component, including orientational and translational solvent modes. The inertial polarization is generated by average charge distributions of solvent particles, composed of permanent and induced ͑electronic͒ components. MD simulations are performed in a manner consistent with the choice of solvent and solute charges such that all electrostatic interactions are scaled by the factor 1/ ϱ , where ϱ is the optical dielectric permittivity. This approach yields an ensemble of equilibrium solvent configurations adjusted to the electric field created by a charged or strongly polar solute. The electrostatic solvent response field is found as the solution of the Poisson equation including both solute and explicit solvent charges, with accurate account of electrostatic boundary conditions at the surfaces separating spatial regions with different dielectric permittivities. Both equilibrium and nonequilibrium solvation effects can be studied by means of this model, and their inertial and inertialess contributions are naturally separated. The methodology for computation of charge transfer reorganization energies is developed and applied to a model two-site dipolar system in the SPC water solvent. Three types of charge transfer reactions are considered. The standard linear-response approach yields high accuracy for each particular reaction, but proves to be significantly in error when reorganization energies of different reactions were compared. This result has a purely molecular origin and is absent within a conventional continuum solvent model.
The Journal of Physical Chemistry, 1990
Dielectric relaxation in apomyoglobin and in 1-0ctanol are studied by using time-resolved Stokes shift measurements of the chromophore anilino-2-aminonaphthalene-6-dimethylsulfonamide. We find that the protein dielectric function can be represented by a Debye form with longitudinal time scale 7L = 9.2 ns. The octanol measurements are consistent with the dielectric continuum model. The implications of these results on predicting the rates of adiabatic electron-transfer processes are discussed. Dielectric Relaxation and Electron-Transfer Rates ceptor electronic coupling and nuclear reorganization in proteins. The intense interest in biological electron-transfer processes has led to growing insight1-' into the factors that control donor-ac-By contrast, the dynamicsof this nuclear reorganization (sblvation) remain poorly understood. It has been shown6-18 that when (I) (a) Marcus, R.; Sutin, N.
Solvent Reorganization in Long-Range Electron Transfer: Density Matrix Approach
The Journal of Physical Chemistry A, 1998
The dynamics of charge transfer from a photoexcited donor to an acceptor coupled through a bridge is investigated by using a correlation-function approach in Liouville space that takes into account solvent dynamics with an arbitrary distribution of time scales. The time-and frequency-resolved fluorescence spectrum from the acceptor is used to probe the scaling of the ET rate with bridge size. The crossover between the coherent tunneling (transfer) and the incoherent sequential (transport) regimes and its implications on the nature of ET processes in DNA are discussed.
Charge Transfer and Polarization in Solvated Proteins from Ab Initio Molecular Dynamics
The Journal of Physical Chemistry Letters, 2011
b S Supporting Information I t has been long recognized that water plays an important role in protein structure and dynamics. Water on the protein surface, often referred to as biological water, 1 is an essential element of protein interactions 2 and enzyme function. 3 Some water molecules reside in the same location near the protein surface for a long time 1 compared with the typical relaxation time under bulk conditions. These water molecules form strong hydrogen bonds 4 and can be directly observed in accurate model-free crystallographic experiments. 5 Classical force fields have made tremendous progress in describing interactions at proteinÀwater interfaces and can accurately predict such important energetic properties as solvation free energies of amino acids. 6,7 However, most of these theoretical models use a simplified "charged ball-and-spring" representation that is incapable of describing quantum mechanical phenomena like charge transfer (CT) and electronic polarization. Recently, it was demonstrated that CT effects account for approximately one-third of the binding energy in a neutral water dimer, 8 and for stronger H-bonds, one can anticipate this contribution to be even larger. Although CT and polarization effects are typically parametrized in classical force fields implicitly as a part of the electrostatic and Lennard-Jones two-body interactions, it remains an open question as to how accurately such approximations can describe biological water. Another recent study has stressed the importance of CT interactions in proteins and suggested this missing term should be explicitly included in future classical force field parametrizations. Although the effect of explicit solvent on protein structure and function has been studied for more than two decades, 4 solvated proteins have almost exclusively been treated using nonpolarizable classical force fields. Only a few attempts have been made to study proteinÀwater systems at higher levels of theory, such as semiempirical 10À12,35 or fragment molecular orbital 13 approaches. However, even these efforts have still relied on molecular dynamics (MD) simulations with classical force fields to provide atomic coordinates for higher level calculations. More rigorous treatment of solvated proteins by means of HartreeÀFock (HF) or density functional theory (DFT) methods is clearly needed. Ideally, one would use ab initio rather than classical MD trajectories in such calculations because classical and ab initio dynamics could potentially sample configurational space quite differently. In fact, DFT MD has been applied to study model systems such as solvated glycine dipeptide, 14 and substantial CT was observed in these simulations. However, to the best of our knowledge, ab initio (HF or DFT) MD has never been used to treat entire proteins. The major obstacle to the use of ab initio methods in this context is their high computational cost. Recent single-point energy calculations of solvated rubredoxin represent an illustrative example. 15 Calculation of the energy for the resulting 2825 atoms required over 1 h on 8196 processor cores. Dynamical simulations requiring hundreds or thousands of such calculations would appear to be completely out of reach. Fortunately, graphical processing units (GPUs) (essentially consumer videogame graphics cards) have emerged as a powerful alternative to traditional processors. We have redesigned algorithms for electronic structure theory and ab initio MD to leverage the strengths of GPUs, with promising results. 16À18 In this Letter, we ABSTRACT: Charge transfer at the Bovine pancreatic trypsin inhibitor (BPTI) proteinÀwater interface was analyzed by means of ab initio BornÀOppenheimer molecular dynamics simulation of the entire protein running on graphical processing units (GPUs). The efficiency of the GPU-based quantum chemistry algorithms implemented in our TeraChem program enables us to perform these calculations on a desktop computer. Mulliken and Voronoi deformation density (VDD) population analysis reveals that between 2.0 and 3.5 electrons are transferred from surrounding water molecules to the protein over the course of the 8.8 ps simulation. Solving for the electronic structure of BPTI in the absence of surrounding water molecules (i.e., in the gas phase) leads to large intraprotein charge transfer, where approximately one electron in total is transferred from neutral to polar residues. Solvation relieves this polarization stress, leading to a neutralization of the excess positive charge of the neutral residues.
Mixed quantum mechanical (QM)/classical methods provide a computationally efficient approach to modeling both ground and excited states in the condensed phase. To accurately model short-range interactions, some amount of the environment can be included in the QM region, whereas a classical model can treat long-range interactions to maintain computational affordability. The best computational protocol for these mixed QM/classical methods can be determined by examining convergence of molecular properties. Here, we compare molecular mechanical (MM) fixed point charges to a polarizable continuum model (PCM) for computing electronic excitations in solution. We computed the excitation energy of three pairs of neutral/anionic molecules in aqueous solvent, including up to 250 water molecules in the QM region. Interestingly, the convergence is similar for MM point charges and a PCM, with convergence achieved when at least one full solvation shell is treated with QM. Although the van der Waals (VDW) definition of the PCM cavity is adequate with small amounts of QM solvent, larger QM solvent layers had gaps in the VDW PCM cavity, leading to asymptotically incorrect excitation energies. Given that the VDW cavity leads to unphysical solute−solvent interactions, we advise using a solvent-excluded surface cavity for QM/PCM calculations that include QM solvent. ■ INTRODUCTION Excited states of chromophores play a key role in charge-transfer and energy-conversion processes. These chemical processes typically occur in the condensed phase and the surrounding environment can have a significant effect on the absorption and emission spectra of chromophores. 1,2 The electrostatic potential of the environment and its polarization can stabilize the ground and excited states of the chromophore and participate in charge and proton transfer with the solute. There are various models that attempt to account for the interactions between the chromophore and its environment. One approach is to describe the environment with quantum mechanics (QM), but this becomes computationally intractable for systems larger than a few hundred atoms. A more computationally feasible approach is to treat the chromophore with QM and the surrounding environment with a classical model, such as molecular mechanical (MM) point charges (the QM/MM method) 3−6 or a polarizable continuum model (PCM). 7−10 This hybrid QM/ classical approach has proven effective in modeling enzyme catalysis 11−16 and electronic excitations of chromophores in both solution 17−22 and protein environments. 23−26 When modeling ionization, including the effects of solvent polarization is essential for obtaining correct energies; 27 for ionization processes, a point charge MM model of the solvent will fail, and a method that captures long-range polarization is essential. However, it is not clear if the same polarization requirement is essential for modeling molecular excitations. Here, we focus on the effects of the QM/MM model compared to the QM/PCM model on electronic transitions of chromophores in solution. Using standard nonpolarizable MM force fields, the QM/MM method embeds the QM nuclear charges and QM electron density in a field of static point charges representative of the solvent. The static MM point charges polarize the QM electron density, but the MM point charges remain unchanged throughout the ground-and excited-state calculation. With the QM/PCM method, the QM charge density is enclosed in a cavity surrounded by a dielectric continuum, and the response of the dielectric to the QM charge density evaluated at the cavity surface is included in the effective Hamiltonian. Thus, both classical solvent models include polarization of the QM region, but only QM/PCM includes the mutual polarization of the solvent and QM charge density. This polarization may model solute−solvent interactions more accurately than the static point charges of standard MM force fields, but the dielectric continuum does not include site-to-site interactions, such as hydrogen bonding. These interactions can be modeled with MM point charges. Perhaps the best QM/classical model includes the polarizability of the MM point charges in response to the QM density. Several polarizable MM force fields have been developed using a variety of models, such as induced dipoles, 28−30 fluctuating charges, 31 and Drude oscillators, 32 but because these methods are not