Computational solvation dynamics of oxyquinolines

Solvation dynamics is one key aspect of any chemical reaction. The spectral response of a solvatochromic solute on laser excitation, the so-called time-dependent Stokes shift, provides crucial information on the solute photoreactivity and the solvent reorganization. In here, we propose a novel approach for computational solvation dynamics, combining quantum-mechanical calculations, (non-)equilibrium polarizable MD simulations and sophisticated free energy calculations. Chemical alteration of the side chains of oxyquinolines in this projects change their volume (and consequently the rotational behavior) and the strength and orientation of their dipole moments. We will analyzed the corresponding solvation effects and directly juxtapose our computational results for these oxyquinolines in various solvents to the corresponding experimental data of our cooperation partner. In contrast to the standard solvatochromatic solute coumarin C153, oxyquinolines tend to decrease their dipole moment during excitation and consequently serve as a test for the general findings on solvation dynamics gained so far. Thecomputationaldecompositionintotranslational/rotational, cationic/anionicor permanent/induced dipole contributions of the Stokes shift as well as the frequency-dependent dielectric spectra will equally provide novel insight in the role of the respective solvent and identify its key contributions for solvation dynamics. For less fluid solvents long-time simulations of dozens of nanoseconds are inevitable and thus unfeasible by quantum-mechanical approaches. The major advantage of our polarizable MD simulations is the modeling of electronic effects of the solvent while these simulation periods are still manageable.

Quite generally, chemists focus on the reacting compounds and products in their investigations and pay less attention to the solvent. The solvent is often only characterized in terms of the solubility of the participating compounds. However, solvation dynamics is a key aspect in many chemical reactions as the solvent may stabilize transition states and consequently, the dynamic response and the interactions of the solvent to the transitioning molecules may decide on the outcome of a particular reaction at certain reaction conditions. In the current project, we investigated the fundamental techniques to compute the Stokes shift. We improved force fields to come closer to experimental results and applied our methods to the investigation of solvation dynamics near a tert-butyl group, trehalose and at the surface of proteins.