Predicting spectral properties of polarity sensitive dyes with QM/MM simulation

Polarity sensitive, lipophilic dyes such as Laurdan report lipid packing in biomembranes, as the emission spectrum is red shifted in more polar environments. In simple membranes, the dye is more accessible to solvent in more disordered membranes, and the spectral shift is well-explained by dipolar relaxation of the solvent. However, in more complex systems other factors may contribute, especially hydrogen bonding between the environment and the chormophore. An approach has been developed in which the local environment is first sampled by classical molecular dynamics simulation of the dye in different environments, followed by prediction of the absorption spectrum by numerical quantum mechanics. Simulation results are presented for an optimized model of Laurdan and C-Laurdan for use with the CHARMM family of forcefields in a variety of membrane environments. Several different quantum methods are compared, including time-dependent density functional theory and GW-BSE.