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 processes of surfactant molecules in the gas phase is of great interest for several theoretical and technological reasons related to their possible exploitation as drug carriers, protein shields and cleaning agents in the gas phase. METHODS: The 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 the 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 obtain structural and energetics information. Finally, in order to ascertain 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 sections (CCS) was found. CONCLUSIONS: The fragmentation pathways of DMEB charged species evidenced for singly charged aggregates exhibit features similar to that of other detergent aggregates, but multiply charged aggregates showed a system-specific behavior. QMcalculations on the optimized structures (21+, 31+, 11- and 21-) indicate that the most determinant interactions are due to an OH---Br hydrogen bonding that is also involved in the link between monomericDMEB units. TheMDmodels gave CCS values in good agreement with experimental ones, evidenced by a less strict reverse micelle-like structure and a reasonably spread bromine anion distribution.