Micelles of the chiral biocompatible surfactant (1R,2S)-dodecyl-(2-hydroxy-1-methyl-2-phenylethyl)dimethylammonium bromide (DMEB): Molecular dynamics and fragmentation patterns in the gas phase

Rationale The study of self-assembly process of surfactant molecules in gas phase is of actually interest for several theoretical and technological reasons related to their possible exploitation as drug carriers, protein shields and cleaning agents in gas phase. Methods Stability and fragmentation patterns of singly and multiply charged (either positively or negatively) aggregates of the surfactant (1R,2S)-dodecyl(2-hydroxy-1-methyl-2-phenylethyl) dimethyl ammonium bromide (DMEB) in gas phase have been studied by ion mobility mass spectrometry and tandem mass spectrometry. Molecular dynamics (MD) simulations of positively and negatively singly and multiply charged DMEB aggregates have been performed to get structural and energetics information. Finally, in order to get some clues on the DMEB growth mechanism, quantum mechanics calculations were carried out. Results It has been evidenced that positively and negatively singly charged aggregates at low collision energy decompose preferentially by loss of only one DMEB molecule. Increasing the collision energy, the loss of neutrals becomes increasingly abundant. Multiply charged DMEB aggregates are unstable and decompose forming singly charged monomers or dimers. MD simulations show reverse micelle like structures with polar heads somewhat segregated into the aggregate interior. Finally a good correlation between experimental and calculated collisional cross section was found. Conclusions The fragmentation pathways of DMEB charged species evidenced for singly charged aggregates features similar to that of other detergents aggregates, but multiply charged aggregates showed a system specific behavior. QM calculations on the optimized structures (21+, 31+, 11- and 21-) indicate that the most determinant interactions are due to an OH---Br hydrogen bonding that are also involved in the link between monomeric DMEB units. The MD models gave CCS values in good agreement with experimental ones, evidenced a less strict reverse micelle like structure and a quite spread bromine anion distribution.