Interaction of Cyanine-D112 with Binary Lipid Mixtures: Molecular Dynamics Simulation and Differential Scanning Calorimetry Study

This comprehensive molecular dynamics (MD) simulation and experimental study investigates the lipid bilayer interactions of dye D112 for potential photodynamic therapy (PDT) applications. PDT involves formation of a reactive oxidant species in the presence of a light sensitive molecule and light, interrupting cellular functions. D112 was developed as a photographic emulsifier, and we hypothesized that its combined cationic and lipophilic nature can render a superior photosensitizing property-crucial in various light therapies. The focus of this study is to elucidate the binding and insertion mechanisms of D112 with mixed lipid bilayers of anionic dipalmitoyl-phosphatidylserine (DPPS) and zwitterionic dipalmitoyl-phosphatidylcholine (DPPC) lipids to resemble cancer cell membranes. Our studies confirm initial electrostatic binding between the positively charged moieties of D112 and negatively charged lipid headgroups. Additionally, MD simulations combined with differential scanning calorimetry (DSC) studies confirm that D112-lipid interactions are governed by enthalpy-driven nonclassical hydrophobic effects in the membrane interior. It was further noted that despite the electrostatic preference of D112 toward the anionic lipids, D112 molecules colocalized on DPPC-rich domains after insertion. Atomistic level MD studies point toward two possible insertion mechanisms for D112: harpoon and flip. Further insights from the simulation showcase the interactions of low and high aggregates of D112 with the bilayer as the concentration of D112 increases in solution. The size of aggregates modulates the orientation and degree of insertion, providing important information for future studies on membrane permeation mechanisms.