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During the 2008-2009 grant period, we successfully utilized electrospray ionization (ESI) to generate multiply-charged sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) reverse micelles (RMs) in the gas phase, and characterized micelle compositions using a guided-ion-beam tandem mass spectrometer.  We capitalized on that discovery during the 2009-2010 year by focusing on structure of micellar assemblies and applications of gas-phase RMs in probing chemistry of molecules confined within. 

We first report results on encapsulation of various amino acids in gas-phase RMs, aimed at determining driving forces for solubilization in gas-phase RMs.  We used glycine (G), tryptophan (W) and protonated tryptophan (WH) as examples to summarize our findings.  Integration into gas-phase RMs depends upon amino acid hydrophobicity and charge.  Hydrophilic amino acids, e.g. glycine, are encapsulated within the micellar core via electrostatic interactions (because they are solvated by Na⁺).  Hydrophobic amino acids, e.g. tryptophan, are adsorbed in the surfactant layer.  A strong correlation was found between RM size and its incorporation capability.  Accommodation of amino acids requires micellar aggregation numbers ³ 9.  RMs can each incorporate more than one hydrophilic amino acid, with the maximum volume of encapsulated molecules matching micellar core size; however, RMs can at most only incorporate single hydrophobic amino acid.  Similar results were obtained for tyrosine and protonated tyrosine. 

It is intuitive to assume that the hydrophobic effect drives solubilization of tryptophan and tyrosine in gas-phase RMs.  However, control experiments using phenol, naphthol and phloretic acid showed that none of these model hydrophobic molecules (without ⁺H₃N-C-COO^- backbone) were observed in gas-phase RMs.  Obviously, the hydrophobic effect (and hydrogen bonding) do not provide sufficient driving force needed for solubilization of hydrophobic amino acids in gas-phase RMs.  Consequently, hydrophobic amino acids must adopt zwitterionic forms, and are intercalated between surfactants through complementary effects of electrostatic interactions between zwitterionic backbones and AOT heads, and hydrophobic interactions between side chains and AOT tails.  This mechanism is consistent with the size dependence and the low incorporation capability of W in RMs (i.e. one for each micelle).  Small RMs have large curvatures, and cannot provide much interfacial area (near head regions) for incorporating multiple W.  Protonation of W could increase its incorporation, and displaces its site location from the interfacial region to the micellar core.  Consequently, the encapsulation behavior of WH resembles that of glycine.  Based on the incorporation efficiencies of W and WH, we estimated that, primarily due to extra electrostatic interactions, protonation improves incorporation of WH by a factor of 2-3. 

Further evidence of solute-surfactant interactions and incorporation sites of hosted amino acids was obtained from collision-induced dissociation (CID) of mass-selected micellar ions with Xe.  CID of G- and WH-encapsulating micellar ions yielded complex fragment ions, and exclusion of amino acids resulted in the collapse of whole micellar structures.  On the other hand, CID of W-encapsulating micellar ions corresponds to stripping W (and only W moiety) off RMs.  CID results provided direct evidence that G and WH are predominately hosted inside the micellar core, while W must adsorb to the micellar surface and acts as a co-surfactant. 

Different interactions between amino acids and RMs may be used to manipulate amino acid incorporation sites, and makes selective incorporation of different amino acids possible.  We carried out encapsulation experiments in the presence of two amino acids, aspartic acid and tryptophan.  Aspartic acid has a pI of 2.8, lower than tryptophan's pI of 5.9.  The mass spectrum of NaAOT (1.0 mM)/aspartic acid (0.1 mM) showed abundant micellar ions occupied by protonated aspartic acid.  Surprisingly, all ion peaks associated with encapsulation of protonated aspartic acid vanished after adding 0.1 mM tryptophan to the ESI solution.  Instead, only protonated tryptophan was encapsulated.  Selective encapsulation was also observed between arginine and tryptophan.  Arginine has a pI of 10.8.  The mass spectrum of NaAOT (1.0 mM), arginine (0.1 mM) and tryptophan (0.1 mM) showed encapsulation of only protonated arginine in gas-phase RMs.  Preferential encapsulation of tryptophan over aspartic acid, and of arginine over tryptophan, indicate that different amino acids compete for protons in solution and/or during ESI.  The amino acid with a higher pI is more readily protonated, which allows integration into the micellar core through electrostatic interactions with AOT polar heads.  Encapsulation of protonated amino acids makes the micellar structure more rigid and stable, inhibiting further incorporation of neutral amino acids at the interfacial region.  This explains the observed highly exclusive selectivity of NaAOT RMs in the gas phase. 

Our next step is to examine other reactions of reverse micelles, including ¹O₂-induecd oxidation of encapsulated amino acids.  The construction of a ¹O₂ generation and detection apparatus for this experiment is underway. 

In brief, we investigated the generation, structure and applications of RMs in the gas-phase, particularly the driving forces for amino acid solubilization in micelles, and dissociation reactions of micellar assemblies.  The competitive encapsulation of various amino acids demonstrates that gas-phase RMs are able to act as nanometer-sized vehicles for selective transport of amino acids from solution to the gas phase.  These results provide insight into the dynamics of amino acid transport in atmospheric aerosols.  Two papers resulted from this work, one published on IJMS and the other submitted to PCCP for review now.

Other impacts on PI's career and students:  With this grant, the PI was able to upgrade the guided-ion-beam apparatus to a higher mass range.  This machine is currently the only one in CUNY with demonstrated capabilities in studying energy-dependent ion-scattering, and has become a major research tool in the PI's lab.  The upgraded instrument is now also being used for a project featuring reaction dynamics of biomolecular ions recently funded by NSF.  The grant also supported the PI and his students to present results at local and National ACS meetings, making their research more visible to the chemistry community.  This project was central to the training of a talented PhD student in mass spectrometry and reaction dynamics, who received a CUNY Research Excellence Award (2010) in Chemistry.  Three undergraduates also worked on this project.  In addition, two high school students were involved in the research.