Interplay of noncovalent interactions in antiseptic quaternary ammonium surfactant Miramistin

The molecular and crystal structure of the widely used antiseptic benzyldimethyl{3-[(1-oxotetradecyl)amino]propyl}ammonium chloride monohydrate (Miramistin, MR), C₂₆H₄₇N₂O⁺·Cl⁻·H₂O, was determined by a single-crystal X-ray diffraction study and analyzed in the framework of the QTAIM (quantum theory of atoms in molecules) approach using both periodic and molecular DFT (density functional theory) calculations. The various noncovalent intermolecular interactions of different strengths were found to be realized in the hydrophilic parts of the crystal packing (i.e. O—H...Cl, N—H...Cl, C—H...Cl, C—H...O and C—H...π). The hydrophobic parts are built up exclusively by van der Waals H...H contacts. Quantification of the interaction energies using calculated electron-density distribution revealed that the total energy of the contacts within the hydrophilic and hydrophobic regions are comparable in value. The organic MR cation adopts the bent conformation with the head group tilted back to the long-chain alkyl tail in both the crystalline and the isolated state due to stabilization of this geometry by several intramolecular C—H...π, C—H...N and H...H interactions. This conformation preference is hypothesized to play an important role in the interaction of MR with biomembranes.