Mass Spectrometry Study of Multiply Negatively Charged, Gas-Phase NaAOT Micelles: How Does Charge State Affect Micellar Structure and Encapsulation?

Abstract

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-(n01-18 and z01-2). Solvation by water was detected only for small aggregates [Na n-1 AOT n H 2 O] -of n03-9. 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.

Entrapment of amino acids in gas phase surfactant assemblies: The case of tryptophan confined in positively charged (1R,2S)‐dodecyl (2‐hydroxy‐1‐methyl‐2‐phenylethyl) dimethylammonium bromide aggregates

“…The solubilization of small‐sized molecules in gas phase aggregates formed by sodium bis (2‐ethylhexyl) sulfosuccinate (NaAOT) has been investigated by Fang et al In particular, studying the inclusion of some amino acids, these authors observed that their insertion in NaAOT aggregates occurs only above an aggregation number of 8. Moreover, the number of molecules which can be incorporated increases with the aggregate size; up to 5 glycine molecules can be secluded in NaAOT aggregates with aggregation number ≥ 24.…”

Charged supramolecular assemblies of surfactant molecules in gas phase

“…While for positively charged AOTNa a reverse micelle-like structure has been somewhat agreed by several authors, (Bongiorno et al, 2010;Fang, Bennett, & Liu, 2010 some disagreement emerges for negatively charged aggregates. Fang, Liu, and Liu (2013) have indeed evidenced some peculiarities of negatively charged AOTNa micelles with respect to positively charged aggregates, that more straightforwardly lead to hypothesize a direct micelle-like structure. Most evidences deal with the increased capacity of negatively charged aggregates to host glycine or tryptophan (TRP) molecules.…”