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Jianbo Liu

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The reaction between methionine (Met) and electronically excited singlet molecular oxygen (O2[a1Δg]) has been investigated in a systematic fashion, using a home-built electrospray ionization (ESI) guided-ion-beam tandem mass spectrometer (MS). The study started from probing the reaction dynamics between the isolated protonated/deprotonated methionine ions with 1O2 in the gas phase, transited through the same systems micro-solvated with explicit water molecules in gaseous hydrated clusters, and concluded with real-time methionine oxidation kinetics determination in aqueous solution. The reaction products, cross sections, and collision energy dependence were measured by ESI-MS. Density functional theory (DFT) calculations, Rice-Ramsperger-Kassel-Marcus (RRKM) statistical modeling and direct dynamics simulations were carried out to construct the potential energy surface (PES) along their reaction coordinates (including reactants, intermediate complexes, transition states, and products), analyze thermodynamics and energy barriers, as well as provide insight into the different types of stabilization of reactive species upon oxidation.

In project 1, in order to elucidate the charge effects on the reaction mechanism, the reaction were explored between 1O2 and gas-phase dehydrated methionine in both protonated (MetH+) and deprotonated ([Met - H]-) ionization states. For the reaction of MetH+ + 1O2, the product channel corresponds to generation of hydrogen peroxide via transfer of two hydrogen atoms from MetH+ to singlet oxygen. The reaction is mediated by a precursor and/or hydroperoxide intermediate, and is sharply orientation-dependent. The reaction cross section shows strong inhibition by collision energy. No oxidation products were observed in the reaction of [Met - H]- + 1O2, albeit the reaction is mediated by similar hydroperoxides. Due to the high energy barriers in the product exit channels, these nascent hydroperoxides cannot evolve to stable end products at the collision energy range in the present study, but decayed back to reactants.

Project 2 explored the reaction between 1O2 and hydrated protonated/deprotonated methionine clusters (MetH+(H2O)1,2/[Met - H]-(H2O)1,2), aiming at probing the effects of charge and hydration states on the reaction mechanism, as well as mimicking the micro-solvation environment in biological systems. For the reaction of MetH+(H2O)1,2 + 1O2, besides producing hydroperoxides (and their hydrates), an H2O2 elimination mechanism was observed. This observation indicates a transition from the gas-phase oxidation pathways to solution-phase reactions. In contrast to the non-reactivity of its dehydrated counterpart, [Met - H]- becomes oxidizable once it is hydrated by water(s); hydroperoxides and their hydrated species were captured as the oxidation products in the reactions of [Met - H]-(H2O)1,2 + 1O2.

In the last project, a solution-phase reaction setup, which couples the 1O2 generation and detection system to our ESI-MS, was developed. This on-line apparatus minimizes the sample transfer time between reaction and mass spectrometry measurements. It enables us to identify the oxidation products of Met in different pH solutions, and follow real-time reaction profiles for determining their reaction rates. Met-O is the dominant oxidation product in acidic and neutral solutions, whereas [Met - H]--O and dimeric product [Met - H]--O-Met dominate in basic solution despite with lower reaction rates. The calculated reaction rate constants are 6 x 109 M-2's-1 in acidic solution and 2 x 109 M-2's-1 in basic solution, respectively.

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