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-<i>cis</i>-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-<i>cis</i>-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 Absorption Spectra of the <i>para</i>-Coumaric Acid Chromophore in Its Different Protonation Forms

The effects of the solvent and protonation state on the electronic absorption spectrum of the <i>para</i>-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-<i>cis</i>, s-<i>trans</i>, <i>syn</i>, and <i>anti</i> 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

QM/MM Study of Substituent and Solvent Effects on the Excited State Dynamics of the Photoactive Yellow Protein Chromophore

Substituent and solvent effects on the excited state dynamics of the Photoactive Yellow Protein chromophore are studied using the average solvent electrostatic potential from molecular dynamics (ASEP/MD) method. Four molecular models were considered: the ester and thioester derivatives of the <i>p</i>-coumaric acid anion and their methylated derivatives. We found that the solvent produces dramatic modifications on the free energy profile of the S1 state: 1) Two twisted structures that are minima in the gas phase could not be located in aqueous solution. 2) Conical intersections (CIs) associated with the rotation of the single bond adjacent to the phenyl group are found for the four derivatives in water solution but only for thio derivatives in the gas phase. 3) The relative stability of minima and CIs is reverted with respect to the gas phase values, affecting the prevalent de-excitation paths. As a consequence of these changes, three competitive de-excitation channels are open in aqueous solution: the fluorescence emission from a planar minimum on S1, the <i>trans–cis</i> photoisomerization through a CI that involves the rotation of the vinyl double bond, and the nonradiative, nonreactive, de-excitation through the CI associated with the rotation of the single bond adjacent to the phenyl group. In the gas phase, the minima are the structures with the lower energy, while in solution these are the conical intersections. In solution, the de-excitation prevalent path seems to be the photoisomerization for oxo compounds, while thio compounds return to the initial <i>trans</i> ground state without emission

How Methylation Modifies the Photophysics of the Native All-<i>trans</i>-Retinal Protonated Schiff Base: A CASPT2/MD Study in Gas Phase and in Methanol

A comparison between the free-energy surfaces of the all-<i>trans</i>-retinal protonated Schiff base (RPSB) and its 10-methylated derivative in gas phase and methanol solution is performed at CASSCF//CASSCF and CASPT2//CASSCF levels. Solvent effects were included using the average solvent electrostatic potential from molecular dynamics method. This is a QM/MM (quantum mechanics/molecular mechanics) method that makes use of the mean field approximation. It is found that the methyl group bonded to C10 produces noticeable changes in the solution free-energy profile of the S₁ excited state, mainly in the relative stability of the minimum energy conical intersections (MECIs) with respect to the Franck–Condon (FC) point. The conical intersections yielding the 9-<i>cis</i> and 11-<i>cis</i> isomers are stabilized while that yielding the 13-<i>cis</i> isomer is destabilized; in fact, it becomes inaccessible by excitation to S₁. Furthermore, the planar S₁ minimum is not present in the methylated compound. The solvent notably stabilizes the S₂ excited state at the FC geometry. Therefore, if the S₂ state has an effect on the photoisomerization dynamics, it must be because it permits the RPSB population to branch around the FC point. All these changes combine to speed up the photoisomerization in the 10-methylated compound with respect to the native compound

Theoretical Study of Solvent Effects on the Ground and Low-Lying Excited Free Energy Surfaces of a Push–Pull Substituted Azobenzene

The ground and low-lying excited free energy surfaces of 4-amino-4′-cyano azobenzene, a molecule that has been proposed as building block for chiroptical switches, are studied in gas phase and a variety of solvents (benzene, chloroform, acetone, and water). Solvent effects on the absorption and emission spectra and on the <i>cis–trans</i> thermal and photo isomerizations are analyzed using two levels of calculation: TD-DFT and CASPT2/CASSCF. The solvent effects are introduced using a polarizable continuum model and a QM/MM method, which permits one to highlight the role played by specific interactions. We found that, in gas phase and in agreement with the results found for other azobenzenes, the thermal <i>cis–trans</i> isomerization follows a rotation-assisted inversion mechanism where the inversion angle must reach values close to 180° but where the rotation angle can take almost any value. On the contrary, in polar solvents the mechanism is controlled by the rotation of the CNNC angle. The change in the mechanism is mainly related to a better solvation of the nitrogen atoms of the azo group in the rotational transition state. The photoisomerization follows a rotational pathway both in gas phase and in polar and nonpolar solvents. The solvent introduces only small modifications in the nπ* free energy surface (<i>S</i>₁), but it has a larger effect on the ππ* surface (<i>S</i>₂) that, in polar solvents, gets closer to <i>S</i>₁. In fact, the <i>S</i>₂ band of the absorption spectrum is red-shifted 0.27 eV for the <i>trans</i> isomer and 0.17 eV for the <i>cis</i>. In the emission spectrum the trend is similar: only <i>S</i>₂ is appreciably affected by the solvent, but in this case a blue shift is found

Using Molecular Dynamics and Quantum Mechanics Calculations To Model Fluorescence Observables

We provide a critical examination of two different methods for generating a donoracceptor electronic coupling trajectory from a molecular dynamics (MD) trajectory and three methods for sampling that coupling trajectory, allowing the modeling of experimental observables directly from the MD simulation. In the first coupling method we perform a single quantum-mechanical (QM) calculation to characterize the excited state behavior, specifically the transition dipole moment, of the fluorescent probe, which is then mapped onto the configuration space sampled by MD. We then utilize these transition dipoles within the ideal dipole approximation (IDA) to determine the electronic coupling between the probes that mediates the transfer of energy. In the second method we perform a QM calculation on each snapshot and use the complete transition densities to calculate the electronic coupling without need for the IDA. The resulting coupling trajectories are then sampled using three methods ranging from an independent sampling of each trajectory point (the independent snapshot method) to a Markov chain treatment that accounts for the dynamics of the coupling in determining effective rates. The results show that the IDA significantly overestimates the energy transfer rate (by a factor of 2.6) during the portions of the trajectory in which the probes are close to each other. Comparison of the sampling methods shows that the Markov chain approach yields more realistic observables at both high and low FRET efficiencies. Differences between the three sampling methods are discussed in terms of the different mechanisms for averaging over structural dynamics in the system. Convergence of the Markov chain method is carefully examined. Together, the methods for estimating coupling and for sampling the coupling provide a mechanism for directly connecting the structural dynamics modeled by MD with fluorescence observables determined through FRET experiments