Evolution of Bonding during the Insertion of Anionic Ibuprofen into Model Cell Membranes

Descriptors of chemical bonding derived from five different analysis tools based on quantum mechanics (natural charges, electron density differences, atoms in molecules (AIM), natural bond orbitals (NBO), and non-covalent interactions (NCI) index) consistently afford a picture of a wall of weak, non-covalent intermolecular interactions separating anionic Ibuprofen from the environment. This wall, arising from the cumulative effect of a multitude of individual weak charge transfer interactions to the interstitial region between fragments, stabilizes the drug at all equilibrium positions in the free energy profile for its insertion into model cell membranes. The formal charge in anionic Ibuprofen strengthens all intermolecular interactions, having a particularly strong effect in the network of water to water hydrogen bonds in the solvent. Electron redistribution during the insertion process leads to a sensible reduction of electron delocalization in both the -CO₂^- group and the aromatic ring of Ibuprofen. Here, we conclusively show that, despite their purely classical origin, randomly chosen configurations from molecular dynamics simulations provide deep insight into the purely quantum nature of bonding interactions.