title: Mass spectrometry study of multiply negatively charged, gas-phase NaAOT micelles: how does charge state affect micellar structure and encapsulation?
abstract: We report the formation and characterization of multiply negatively charged sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) aggregates in the gas phase, by electrospray ionization of methanol/water solution of NaAOT followed by detection using a guided-ion-beam tandem mass spectrometer. Singly and doubly charged aggregates dominate the mass spectra with the compositions of [Na(n-z)AOT(n)](z-) (n = 1-18 and z = 1-2). Solvation by water was detected only for small aggregates [Na(n-1)AOT(n)H(2)O](-) of n = 3-9. Incorporation of glycine and tryptophan into [Na(n-z)AOT(n)](z-) aggregates was achieved, aimed at identifying effects of guest molecule hydrophobicity on micellar solubilization. Only one glycine molecule could be incorporated into each [Na(n-z)AOT(n)](z-) of n ≥ 7, and at most two glycine molecules could be hosted in that of n ≥ 13. In contrast to glycine, up to four tryptophan molecules could be accommodated within single aggregates of n ≥ 6. However, deprotonation of tryptophan significantly decrease its affinity towards aggregates. Collision-induced dissociation (CID) was carried out for mass-selected aggregate ions, including measurements of product ion mass spectra for both empty and amino acid-containing aggregates. CID results provide a probe for aggregate structures, surfactant-solute interactions, and incorporation sites of amino acids. The present data was compared with mass spectrometry results of positively charged [Na(n+z)AOT(n)](z+) aggregates. Contrary to their positive analogues, which form reverse micelles, negatively charged aggregates may adopt a direct micelle-like structure with AOT polar heads exposed and amino acids being adsorbed near the micellar outer surface.
authors.affiliation: Department of Chemistry and Biochemistry, Queens College and the Graduate Center of the City University of New York, 65-30 Kissena Blvd, Flushing, NY 11367, USA.
authors.forename: Yigang
authors.initials: Y
authors.lastname: Fang
authors.forename: Fangwei
authors.initials: F
authors.lastname: Liu
authors.forename: Jianbo
authors.initials: J
authors.lastname: Liu
mesh_terms: D000838:Anions / Q000737:chemistry; D004143:Dioctyl Sulfosuccinic Acid / Q000737:chemistry*; D005998:Glycine / Q000737:chemistry*; D008823:Micelles; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D053719:Tandem Mass Spectrometry / Q000379:methods
chemical_list: D000838:Anions; D008823:Micelles; D004143:Dioctyl Sulfosuccinic Acid; D005998:Glycine