Jonathan M. Voss

A dual cryogenic ion trap spectrometer for the formation and characterization of solvated ionic clusters

A new experimental approach is presented in which two separate cryogenic ion traps are used to reproducibly form weakly bound solvent clusters around electrosprayed ions and messenger-tag them for single-photon infrared photodissociation spectroscopy. This approach thus enables the vibrational characterization of ionic clusters comprised of a solvent network around large and non-volatile ions. We demonstrate the capabilities of the instrument by clustering water, methanol, and acetone around a protonated glycylglycine peptide. For water, cluster sizes with greater than twenty solvent molecules around a single ion are readily formed. We further demonstrate that similar water clusters can be formed around ions having a shielded charge center or those that do not readily form hydrogen bonds. Finally, infrared photodissociation spectra of D2-tagged GlyGlyH(+)⋅(H2O)1-4 are presented. They display well-resolved spectral features and comparisons with calculations reveal detailed information on the solvation structures of this prototypical peptide.

IR-IR Conformation Specific Spectroscopy of Na + (Glucose) Adducts

AbstractWe report an IR-IR double resonance study of the structural landscape present in the Na+(glucose) complex. Our experimental approach involves minimal modifications to a typical IR predissociation setup, and can be carried out via ion-dip or isomer-burning methods, providing additional flexibility to suit different experimental needs. In the current study, the single-laser IR predissociation spectrum of Na+(glucose), which clearly indicates contributions from multiple structures, was experimentally disentangled to reveal the presence of three α-conformers and five β-conformers. Comparisons with calculations show that these eight conformations correspond to the lowest energy gas-phase structures with distinctive Na+ coordination. Graphical Abstractᅟ

Accessing the Vibrational Signatures of Amino Acid Ions Embedded in Water Clusters

We present an infrared predissociation (IRPD) study of microsolvated GlyH+(H2O) n and GlyH+(D2O) n clusters, formed inside of a cryogenic ion trap via condensation of H2O or D2O onto the protonated glycine ions. The resulting IRPD spectra, showing characteristic O-H and O-D stretches, indicate that H/D exchange reactions are quenched when the ion trap is held at 80 K, minimizing the presence of isotopomers. Comparisons of GlyH+(H2O) n and GlyH+(D2O) n spectra clearly highlight and distinguish the vibrational signatures of the water solvent molecules from those of the core GlyH+ ion, allowing for quick assessment of solvation structures. Without the aid of calculations, we can already infer solvation motifs and the presence of multiple conformations. The use of a cryogenic ion trap to cluster solvent molecules around ions of interest and control H/D exchange reactions is broadly applicable and should be extendable to studies of more complex peptidic ions in large solvated clusters.

Probing Solvation-Induced Structural Changes in Conformationally Flexible Peptides: IR Spectroscopy of Gly3H+·(H2O)

IR predissociation spectroscopy of the Gly3H+(H2O) complex formed inside of a cryogenic ion trap reveals how the flexible model peptide structurally responds to solvation by a single water molecule. The resulting one-laser spectrum is quite congested, and the spectral analyses were assisted by both H2O/D2O substitution and IR-IR double resonance spectroscopy, revealing the presence of two contributing isomers and extensive anharmonic features. Comparisons to structures found via a systematic computational search identified the geometries of these two isomers. The major isomer, with all trans amide bonds and protonation on the terminal amine, represents ∼90% of the overall population. It noticeably differs from the unsolvated Gly3H+, which exists in two isomeric forms: one with a cis amide bond and the other with protonation on an amide C═O. These results indicate that interactions with just one water molecule can induce significant structural changes, i.e., cis- trans amide bond rotation and proton migration, even as the clustering occurs within an 80 K cryogenic ion trap. Calculations of the isomerization pathways further reveal that the binding energy of the water molecule provides sufficient internal energy to overcome the barriers for the observed structural changes, and the minor solvation isomer results from a small fraction of the ions being kinetically trapped along one of the pathways.

Vibrational Characterization of Microsolvated Electrocatalytic Water Oxidation Intermediate: [Ru(tpy)(bpy)(OH)]2+(H2O)0–4

The infrared predissociation spectra of the mass-selected electrocatalytic water oxidation intermediate [Ru(tpy)(bpy)(OH)]2+(H2O)0-4 are reported. The [Ru(tpy)(bpy)(OH)]2+ species is generated by passing a solution of [Ru(tpy)(bpy)(H2O)](ClO4)2 through an electrochemical flow cell held at 1.2 V and is immediately introduced into the gas phase via electrospray ionization (ESI). The microsolvated clusters are formed by reconstructing the water network in a cryogenic ion trap. Details of the hydrogen bonding network in these clusters are revealed by the infrared predissociation spectra in the OH stretch region. This improved method for capturing microsolvated clusters yielded colder complexes with much better resolved IR features than previous studies. The analysis of these spectra, supported by electronic structure calculations and compared to previous results on [Ru(tpy)(bpy)(H2O)]2+(H2O)0-4 clusters, reveals the nature of the Ru-OH bond and the effect of hydrogen bonding on facilitating the subsequent oxidation to [Ru(tpy)(bpy)(O)]2+ in the proposed catalytic cycle. Particularly, the hydrogen bonding interaction in [Ru(tpy)(bpy)(OH)]2+(H2O)1 is much weaker than that in the corresponding [Ru(tpy)(bpy)(H2O)]2+(H2O)1 and thus is less effective at activating the hydroxyl ligand for further oxidation via proton coupled electron transfer (PCET). Furthermore, the results here reveal that the Ru-OH bond, though formally described as an Ru3+/OH- interaction, has more covalent bond character than ionic bond character.