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Reverse micelles, with the surfactant polarheads oriented around an internal aqueous core and the chains forming the outer surface, represent polar microenvironments.  They are often generated in apolar solvents for solubilization and catalysis.  We are, however, interested in generating reverse micelles in the gas phase, with the ultimate goal of using gas-phase reverse micelles as nano-reactors to probe chemistry of single molecules confined in the micellar core.  During the first year of this grant period, we have successfully generated and characterized large sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) reverse micelles in the gas phase, and have demonstrated that gas-phase reverse micelles could host extra metal ions as well as amino acids in their interior.

The experiment was performed on a guided-ion-beam tandem mass spectrometer, developed in the PI's lab.  AOT reverse micelles were prepared in anhydrous hexane solution containing 5 x 10^-3 M NaAOT.  Water was added to the solution to achieve required w₀ (= [water]/[AOT]).  The micelle solution was sprayed into the ambient atmosphere by nanoelectrospray operating in the positive ion mode.  Charged droplets were fed into a heated desolvation capillary, underwent continuous desolvation and converted to gas-phase charged micelles.  Charged micelles were focused by a hexapole ion guide, and passed into the mass spectrometer. 

It was found that, within the instrument detection range (m/z = 1- 4000), gas-phase reverse micelles generated from the hexane solution have the composition of [(NaAOT)_nNa_z]^z+, with n in the range of 2 - 44 and z in the range of 1-5.  Each micelle hosts extra Na⁺ (in addition to AOT^- counter ions), which accounts for its overall charge.  The relative abundances of gas-phase micelles decrease with increasing the micelle size and charge.  In contrast, electrospray ionization of methanol/water (1:1) solution of NaAOT (5 x 10^-3 M) only produced singly and doubly charged gas-phase aggregates, with small n.  This implies that gas-phase aggregates retain a memory of their original states in solution.  In hexane solution AOT forms reverse micelles, and the micelles preserve their structures when transferred from solution to the gas-phase, at least to some extent.  In contrast, in methanol/water solution, no reverse micelles exist, and self-assembling of AOT must take place in the gas phase during evaporation, rather than starting from the original solution.  The preservation of reverse micellar structure in the gas phase is exemplified by the fact that heavier aggregates form in the gas phase upon increasing w₀ in the original hexane solution of micelles.  The maximum aggregation number of observed gas-phase micelles, n_max, is 22 at w₀ = 5, increasing to 35 at w₀ = 6.5, and to 44 at w₀ = 10.  The actual n_max could be larger for w₀ = 10, since the m/z of n = 44 approached the instrument detection limit.  The variation of n_max of gas-phase micelles follows a similar trend as compared to the micelle solution.  In solution, typical aggregation numbers are around 18, 32 and 72 at w = 5, 6.5 and 10, respectively.  Note that gas-phase micelles underwent collisions with air and solvent molecules, yielding small micelles by sequential losses of AOT and Na⁺.  Water molecules were absent in most gas-phase micelles due to dissociation in collisions, and loss of water does not affect the stability of gas-phase micelles.  It is worth mentioning that theoretical simulations by Bongiorno et al. (J. Mass. Spectrom. 2005, 40, 1618) and Wang et al. (Biochem. 2009, 48, 1006) also suggested that reverse micellar structure is energetically favorable in the vacuum, independent of water. 

Further support for preservation of the reverse micellar structure in the gas phase is found in the study of encapsulation of amino acids in the gas-phase micelles.  AOT reverse micelles were prepared in hexane (w₀ = 10), into which glycine or proline was incorporated ([AOT]/[amino acid] = 5:1).  The micelles were then transferred to the gas phase by electrospray.  It was found that micelles of n ³ 8 could entrap one amino acid, and those of n ³ 15 could entrap two amino acids. Up to three amino acids could be accommodated in single micelles of n ³ 23.  Assuming that the reverse micelle is spherical, its core diameter d can be estimated using d = (nxA/pi)^0.5, where A is the area of the AOT polar head (0.52 nm²).  Accordingly, d is 1.1, 1.6 and 2.0 nm, respectively, for n = 8, 15 and 23.  The orientation-averaged radii of glycine and proline are 0.6 - 0.7 nm.  If we take into account the extra Na⁺ contained in the core, the maximum number of encapsulated amino acids roughly matches the core size of the micelles.  Collision-induced dissociation of size-selected micelles, aimed at further elucidating the micellar structure, is now underway.

Our next step is to study the reaction of micelle-encapsulated amino acid with electronically excited singlet molecular oxygen (¹O₂), to illustrate that micelles could be used as gas-phase nano-sized reactors.  To successfully perform this experiment, it is essential to first understand the reaction mechanism of gas-phase bare amino acids with ¹O₂.  For this reason, the reaction of protonated tyrosine with ¹O₂ was examined in the gas phase using ion-beam scattering methods.  Tyrosine was chosen because it is known to be reactive with ¹O₂.  Only one product channel was observed from this reaction, corresponding to generation of H₂O₂ via transfer of two hydrogen atoms from protonated tyrosine.  Despite being exoergic, the reaction is in competition with physical quenching of ¹O₂ and is very inefficient.  At low collision energies, the reaction may be mediated by intermediate complexes, and shows inhibition by collision energy.  At high collision energies, a small fraction of H₂O₂ is produced via a direct, concerted mechanism where two H atoms are transferred simultaneously, but most H₂O₂ is formed by dissociation of hydroperoxide intermediates.  The results have been accepted for publication on J. Phys. Chem. A.  These findings provide a foundation for understanding what will happen in the much more complicated micelle reaction.