Solvent effect on the absorption spectra of coumarin 120 in water: A combined quantum mechanical and molecular mechanical study

Abstract

The solvent effect on the absorption spectra of coumarin 120 (C120) in water was studied utilizing the combined quantum mechanical∕molecular mechanical (QM∕MM) method. In molecular dynamics (MD) simulation, a new sampling scheme was introduced to provide enough samples for both solute and solvent molecules to obtain the average physical properties of the molecules in solution. We sampled the structure of the solute and solvent molecules separately. First, we executed a QM∕MM MD simulation, where we sampled the solute molecule in solution. Next, we chose random solute structures from this simulation and performed classical MD simulation for each chosen solute structure with its geometry fixed. This new scheme allowed us to sample the solute molecule quantum mechanically and sample many solvent structures classically. Excitation energy calculations using the selected samples were carried out by the generalized multiconfigurational perturbation theory. We succeeded in constructing the absorption spectra and realizing the red shift of the absorption spectra found in polar solvents. To understand the motion of C120 in water, we carried out principal component analysis and found that the motion of the methyl group made the largest contribution and the motion of the amino group the second largest. The solvent effect on the absorption spectrum was studied by decomposing it in two components: the effect from the distortion of the solute molecule and the field effect from the solvent molecules. The solvent effect from the solvent molecules shows large contribution to the solvent shift of the peak of the absorption spectrum, while the solvent effect from the solute molecule shows no contribution. The solvent effect from the solute molecule mainly contributes to the broadening of the absorption spectrum. In the solvent effect, the variation in C-C bond length has the largest contribution on the absorption spectrum from the solute molecule. For the solvent effect on the absorption spectrum from the solvent molecules, the solvent structure around the amino group of C120 plays the key role.

Solvent Effect on the Fluorescence Spectra of Coumarin 120 in Water: A Combined Quantum Mechanical and Molecular Mechanical Study

“…Hence, the interaction between the solute and solvent molecules increases. This was also found in the simulation of C120 in water for the ground state [33]. The bond alternation influences the π orbitals, which plays an important role in the π → π * excitation.…”

“…1) in water solution. The constructed fluorescence spectra, which consider variation in electronic structures accompanied with molecular distortion, can be compared with the solvent effect of absorption spectra we have studied previously [33].…”

Solvatochromic probe in molecular solvents: implicit versus explicit solvent model

“…Quantum-chemical computations may be performed for the whole solute-solvent system, or the solvent molecules may be replaced by point charges in QM/MM approach or by polarizable multipoles (polarizable embedding methods [7,8]). Some recent examples include works using classical MD or MC methods [7][8][9][10][11][12][13][14][15][16][17] or ab initio molecular dynamics [18][19][20][21][22][23].…”

Comparison of different computational methods for water structure optimisation

“…It can be caused by the absence of the specific hydrogen bonding term in AMBER99 force field. This result should be considered before choosing the force field for combined QM/MM techniques (Hao et al, 2003, Murugan, 2011, Sakata et al, 2011. First peak represents the radius of first hydration shell of the water molecule.…”

“…Radial distribution function for oxygen-oxygen couple represents the oxygen density variations as the function of a distance r from the origin (another oxygen atom). All results are compared with the experimental RDF taken from X-ray scattering experiment data (Sakata et al, 2011). Molecular mechanic approach with AMBER99 force field is the simplest and fastest method.…”