The chemistry of (H2O)n•–, CO2•–(H2O)n, and O2•–(H2O)n with small sulfur-containing molecules was studied in the gas phase by Fourier transform ion cyclotron resonance mass spectrometry. Results of quantum chemical calculations indicate that the conversion of e– and CO2•– to O2•– deactivates highly reactive species and may actually reduce oxidative stress. For reactions of (H2O)n•– with CH3SH as well as CO2•–(H2O)n with CH3SSCH3, the reaction products in the gas phase are different from those reported in the literature from pulse radiolysis studies. This observation is rationalized with the reduced cage effect in reactions of gas-phase clusters.
We have used model tripeptides GXW (with X being one of the amino acid residues glycine (G), alanine (A), leucine (L), phenylalanine (F), glutamic acid (E), histidine (H), lysine (K), or arginine (R)) to study the effects of the basicity of the amino acid residue on the radical migrations and dissociations of odd-electron molecular peptide radical cations M.+ in the gas phase. Low-energy collision-induced dissociation (CID) experiments revealed that the interconvertibility of the isomers [G.XW]+ (radical centered on the N-terminal α-carbon atom) and [GXW].+ (radical centered on the π system of the indolyl ring) generally increased upon increasing the proton affinity of residue X. When X was arginine, the most basic amino acid, the two isomers were fully interconvertible and produced almost identical CID spectra despite the different locations of their initial radical sites. The presence of the very basic arginine residue allowed radical migrations to proceed readily among the [G.RW]+ and [GRW].+ isomers prior to their dissociations. Density functional theory calculations revealed that the energy barriers for isomerizations among the α-carbon-centered radical [G.RW]+, the π-centered radical [GRW].+, and the β-carbon-centered radical [GRWβ.]+ (ca. 32–36kcal mol−1) were comparable with those for their dissociations (ca. 32–34 kcal mol−1). The arginine residue in these GRW radical cations tightly sequesters the proton, thereby resulting in minimal changes in the chemical environment during the radical migrations, in contrast to the situation for the analogous GGW system, in which the proton is inefficiently stabilized during the course of radical migration.
Ion–molecule reactions of Mg+(H2O)n, n ≈ 20–60, with O2 and CO2 are studied by Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry. O2 and CO2 are taken up by the clusters. Both reactions correspond to the chemistry of hydrated electrons (H2O)n–. Density functional theory calculations predicted that the solvation structures of Mg+(H2O)16 contain a hydrated electron that is solvated remotely from a hexa-coordinated Mg2+. Ion–molecule reactions between Mg+(H2O)16 and O2 or CO2 are calculated to be highly exothermic. Initially, a solvent-separated ion pair is formed, with the hexa-coordinated Mg2+ ionic core being well separated from the O2•– or CO2•–. Rearrangements of the solvation structure are possible and produce a contact-ion pair in which one water molecule in the first solvation shell of Mg2+ is replaced by O2•– or CO2•–.
Electrochemical activation of carbon dioxide molecules in tailor-made nanoscale water droplets in a mass spectrometer can produce hydrated carbon dioxide radical anions, which can then be utilized as one-carbon building blocks in organic synthesis. This picture illustrates an example of such nanoscale electrochemistry of carbon dioxide for the selective formic acid synthesis.
Theoretical calculations demonstrated that a hydrogen-atom transfer from a thiol group to the carbon dioxide radical anion in the nanoscale environment is a fast and energetically favorable process.