Multiply charged gas-phase NaAOT reverse micelles: Formation, encapsulation of glycine, and collision-induced dissociation

Abstract

We report a study of the generation and characterization of multiply charged, sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) aggregates in the gas phase, using electrospray ionization (ESI) guided-ion beam tandem mass spectrometry. It was found that distributions of gas-phase NaAOT aggregate size and charge are related to the surfactant states in electrosprayed solutions. ESI mass spectra of NaAOT/water/hexane reverse micellar solutions show the compositions of [(NaAOT) n Na z ] z+ , with the aggregation number ( n) and charge ( z) increase with increasing water content in solution. In contrast, gas-phase aggregates from NaAOT monomers in solution have smaller aggregation numbers and lower charges. Gas-phase NaAOT aggregates of n ≥ 13 could encapsulate one glycine molecule, and those of n ≥ 16 could encapsulate two glycine molecules. Up to five glycine molecules could be accommodated in single aggregates of n ≥ 24. Regardless of glycine encapsulation, no water molecules were detected within gas-phase NaAOT aggregates. Collision-induced dissociation (CID) was studied for mass-selected NaAOT aggregates with Xe, including measurements of product ion masses and CID cross-sections for both empty and glycine-encapsulating aggregates over the center-of-mass collision energy range of 0.1–8.0 eV. CID results provide a detailed probe of aggregate structures as well as the interactions between surfactants and the encapsulated glycine molecules. Specifically, the dependence of glycine encapsulation on aggregate size is observed in CID product ion mass spectra, too. The present study demonstrates that NaAOT surfactants are able to form reverse micelles in the gas phase, and gas-phase reverse micelles can act as nanometer-sized vehicles for transport of non-volatile biomolecules.