Evidence for micellar structure in the gas phase

Area of Science:

• Analytical Chemistry
• Biophysical Chemistry
• Supramolecular Chemistry

Background:

• Micelles and reverse micelles are crucial supramolecular structures with distinct surfactant arrangements.
• Understanding surfactant behavior and interactions within these assemblies is vital for various applications.
• Electrospray mass spectrometry (ES-MS) offers a powerful tool for analyzing non-covalent complexes in solution.

Purpose of the Study:

• To compare the behavior of micelles, reverse micelles, and myoglobin-encapsulating reverse micelles using ES-MS.
• To investigate the preservation of surfactant orientation and ion exchange during electrospray.
• To determine the stoichiometry of surfactant molecules associated with encapsulated myoglobin.

Main Methods:

• Utilized electrospray mass spectrometry (ES-MS) to analyze micelle and reverse micelle preparations.
• Employed tandem mass spectrometry (MS/MS) to probe ion dissociation and headgroup exchange.
• Controlled micelle formation by adjusting aqueous and organic phase compositions using cetyltrimethylammonium bromide (CTAB) as the surfactant.

Main Results:

• Regular micelles showed extensive bromide-to-acetate ion exchange (>90%), while reverse micelles exhibited minimal exchange, indicating headgroup protection.
• Tandem mass spectra revealed polydispersed assemblies of several hundred CTAB molecules in both micelle types.
• Reverse micelles encapsulating myoglobin contained approximately 270 CTAB molecules per holo myoglobin molecule.

Conclusions:

• Solution-phase orientation of surfactants is maintained during electrospray ionization.
• Surfactant interactions with encapsulated proteins are preserved throughout the ES-MS process.
• ES-MS is a suitable technique for characterizing the structure and stoichiometry of self-assembled surfactant systems and protein-surfactant complexes.