Effect of DNA Environment on Electronically Excited States of Methylene Blue Evaluated by a Three-Layered QM/QM/MM ONIOM Scheme.

Interactions between chromophores and biological environments may alter the electronic properties of the chromophores. A three-layered QM/QM/MM ONIOM scheme with electrostatic embedding is implemented to investigate the influence of an additional QM layer on excited-state calculations with respect to a standard QM/MM description. The implemented ONIOM scheme is employed to compute the electronic excitations of the photosensitizer methylene blue interacting with a solvated DNA double strand. It is shown that the additional quantum mechanical description of several nucleobases in the vertical energy calculations induces energy shifts in the excited states of methylene blue, compared to the energies of a traditional QM/MM scheme, where the solvated double strand is described fully classically. The energy shifts present an electrostatic component, caused by a charge redistribution of the environment, and an electronic-coupling component, originated by the mixing between the electronic bright state of methylene blue and a charge-transfer state between methylene blue and guanine. In addition, hydrogen bonding and stacking interactions are stronger when the environment is described quantum mechanically during the geometry optimizations than when it is fully described by molecular mechanics. These larger intermolecular interactions produce further energy shifts in the excitation energies of the photosensitizer.