Explicit Aqueous Solvation Treatment of Epinephrine from Car-Parrinello Molecular Dynamics: Effect of Hydrogen Bonding on the Electronic Absorption Spectrum.

The electronic absorption spectrum of the neurotransmitter epinephrine (EPN) in water solution is studied, combining ab initio Car-Parrinello molecular dynamics (CPMD) with a quantum mechanical approach within the framework of the time-dependent density functional theory (TDDFT) scheme. By selecting 52 uncorrelated snapshots, the excitation modes were calculated at the LC-ωPBE/6-31+G(d) level of theory, using an optimal range-separation parameter ω, determined by means of the gap-tuning scheme in the presence of the solvent molecules. By comparing with static approaches (vacuum and implicit solvation), we show here that explicit solvation treatment dramatically enhances the photophysical properties of the EPN, especially because of the more realistic dynamic description of the molecular geometry. The agreement between the simulated and experimental spectra is demonstrated when TDDFT calculations are performed with the optimally tuned version of the DFT hybrid, not only improving the relative intensities of the absorption bands but also the λmax position. These results highlight that accounting for the nuclear motions, that is, thermal effects (of both chromophore and solvent molecules), is imperative to predict experimental absorption spectra. In this paper, we have addressed the critical importance of explicit solvation effects on the photophysics of the EPN, raking in performance when the simulation is performed based on first-principles molecular dynamics such as CPMD.