Current Projects

I.     Study on biochemical and biophysical interactions by real-time gas-phase techniques

This project uses gas-phase ion-molecule reaction techniques, in conjunction with computational chemistry, to probe biophysical and biochemical processes. One of such research efforts is directed toward probing the chemistry of biomolecule photooxidation and oxidative post-translational modifications. Photooxidation of biomolecules by reactive oxygen species is an important biological process associated with aging, disease and photodynamic therapy for cancer, and is also related to the photochemical transformations of atmospheric biomolecule aerosols. Most of photooxidation experiments have been carried out in solution, with various reactive oxygen species generated simultaneously via photosensitization. The inability to unambiguously distinguish the individual roles of various reactive oxygen species in solution, together with the large number of coupled experimental variables (sensitizers, oxygen, solvent, pH, light wavelength, etc.), makes the interpretation of mechanism complicated.

On the other hand, gas-phase techniques offer unique approaches that isolate the interactions, which not possible in solution.  Gas-phase experiments can distinguish between singlet oxygen and radical-mediated processes, identify early intermediates, quantify transient species and products, and focus on intrinsic reactivity of biomolecules.  Resulting insights can then be extrapolated to solution models by incorporating hydration effects at two different levels: by clustering of biomolecules with water in the gas phase, and by comparing with solution experiments.   

Experimental Techniques: Biomolecular ions are generated by electrospray ionization (ESI). Ion-molecule reactions between biomolecular ions and neutral reactive species are carried out using guided-ion-beam scattering methods. Experimental measurement includes reaction cross sections, product branching. Measurement of differential cross sections (i.e., recoil energy and angular distributions of products) is also possible. The main instrument employed for this project is a versatile guided-ion-beam tandem mass spectrometer, useful both for ion-molecule dynamics, and as a general-purpose analytical instrument for solution-phase experiments.

Gas-phase biochemical reactions using guided-ion-beam scattering methods
ESI guided-ion-beam tandem mass spectrometer


On-line spectroscopy and ESI-MS monitoring of solution-phase reactions

Computational Methods: In conjunction with our experimental effort, we use ab initio electronic structure calculations to map out reaction coordinates (i.e., intermediate complexes, transition states, products, etc.). For systems with modest molecular weight, we perform direct dynamics trajectory simulations to examine their dynamical behavior. The combination of the ion-beam scattering experiment with the computational simulation allows us to probe the origins of experimentally measured reaction kinetics/dynamics, and help design experiments to get at particular mechanistic or energetic issues.


II.    Gas-phase reverse micelles ¾ A novel gas-phase nano-reactor

Another area of our research involves the use of aerosol particles and droplets to study biophysical and biochemical processes in the solution phase or heterogeneous environments, with subsequent analysis using mass spectrometry.

        In the atmosphere fatty acids and phospholipids are adsorbed preferentially on aqueous aerosol surfaces, generating gas-phase reverse micelle aerosols. These reverse micelle aerosols provide vehicles for transport of organics to the upper troposphere and lower stratosphere, affect radiative transfer and global temperatures, and could have provided prebiotic chemical reactors and thus are possible precursors to cell-based life.

Inspired by this fascinating chemistry in nature, we are exploring an approach to generate, characterize and utilize gas-phase reverse micelle in the laboratory environment.  Our recent experiments demonstrated the formation of multiply charged, gas-phase NaAOT (sodium bis(2-ethylhexyl) sulfosuccinate) reverse micelles by using ESI mass spectrometry.  These micelles represent nanometric compartmentalized, polar microenvironments, and can encapsulate various species including amino acids and other biomolecules within the micellar core.  A more interesting observation is that amino acids of different isoelectric points could be selectively encapsulated into, and transport by, reverse micelles from solution to the gas phase. The combination of gas-phase reverse micelles, spectroscopy and mass spectrometry brings about a new experimental technique and open up new opportunities for gas-phase studies of biochemistry and biophysics. It allows for the study of solution-phase chemistry (including structure, reaction, kinetics and dynamics) using precisely defined nano-reactors in the gas phase and in vacuo, so that we can monitor what is going on by using sensitive gas-phase techniques.






III.     Dynamics simulations of excited molecules and energetic compounds 

a)      Steps toward Polanyi Rules for polyatomic reactions

   We are interested in using dynamics simulations to probe small polyatomic reactions.  One effort is on devising Polanyi-type Rules for predicting the effects of translational and vibrational energy on polyatomic reactions, in parallel with the state-selected ion-molecule scattering experiments of Scott Anderson's group (Univ of Utah).


b)     Energetic materials
       
    
     Another theoretical project focuses on dynamics simulations and statistical modeling of energetic materials.  In collaboration with the Edwards Air Force Research Laboratory, we are investigating the thermal decomposition mechanisms, kinetics, dynamics and product branching ratios of nitrogen-rich explosive compounds, e.g., dinitrobiuret (DNB), and 1-ethyl-3-methylimidazolium dicyanamide and 1-ethyl-2,3-dimethylimidazolium dicyanamide ionic liquids.


                      DNB