Cryogenic Ion Chemistry and Spectroscopy

Abstract

The use of mass spectrometry in macromolecular analysis is an incredibly important technique and has allowed efficient identification of secondary and tertiary protein structures. Over 20 years ago, Chemistry Nobelist John Fenn and co-workers revolutionized mass spectrometry by developing ways to non-destructively extract large molecules directly from solution into the gas phase. This advance, in turn, enabled rapid sequencing of biopolymers through tandem mass spectrometry at the heart of the burgeoning field of proteomics. In this Account, we discuss how cryogenic cooling, mass selection, and reactive processing together provide a powerful way to characterize ion structures as well as rationally synthesize labile reaction intermediates. This is accomplished by first cooling the ions close to 10 K and condensing onto them weakly bound, chemically inert small molecules or rare gas atoms. This assembly can then be used as a medium in which to quench reactive encounters by rapid evaporation of the adducts, as well as provide a universal means for acquiring highly resolved vibrational action spectra of the embedded species by photoinduced mass loss. Moreover, the spectroscopic measurements can be obtained with readily available, broadly tunable pulsed infrared lasers because absorption of a single photon is sufficient to induce evaporation. We discuss the implementation of these methods with a new type of hybrid photofragmentation mass spectrometer involving two stages of mass selection with two laser excitation regions interfaced to the cryogenic ion source. We illustrate several capabilities of the cryogenic ion spectrometer by presenting recent applications to peptides, a biomimetic catalyst, a large antibiotic molecule (vancomycin), and reaction intermediates pertinent to the chemistry of the ionosphere. First, we demonstrate how site-specific isotopic substitution can be used to identify bands due to local functional groups in a protonated tripeptide designed to stereoselectively catalyze bromination of biaryl substrates. This procedure directly reveals the particular H-bond donor and acceptor groups that enforce the folded structure of the bare ion as well as provide contact points for noncovalent interaction with substrates. We then show how photochemical hole-burning involving only vibrational excitations can be used in a double-resonance mode to systematically disentangle overlapping spectra that arise when several conformers of a dipeptide are prepared in the ion source. Finally, we highlight our ability to systematically capture reaction intermediates and spectroscopically characterize their structures. Through this method, we can identify the pathway for water-network-mediated, proton-coupled transformation of nitrosonium, NO(+) to HONO, a key reaction controlling the cations present in the ionosphere. Through this work, we reveal the critical role played by water molecules occupying the second solvation shell around the ion, where they stabilize the emergent product ion in a fashion reminiscent of the s...

A multi-agent quantum Monte Carlo model for charge transport: Application to organic field-effect transistors

“…A number of cryogenic Paul traps have been built recently to conduct spectroscopy on cold ions, [71][72][73][74] in particular, Johnson and co-workers have developed it into a powerful cryogenic ion spectroscopy technique via H 2 tagging. [28][29][30] The current ESI Paul trap system at Brown University has been significantly improved over the first design. 41 The ion optics is much more compact without the 90 • ion bender, as shown in Fig.…”

Sensitivity of Photoelectron Angular Distributions to Molecular Conformations of Anions

“…Gas-phase ion spectroscopy in particular has provided a wealth of insight into the intrinsic interactions leading to specific conformations in biological, chemical and catalytic contexts. [1][2][3][4][5][6][7] Electronic structure is of course closely linked to molecular structure and, generally, specific geometric structures can have very different electronic structures that in turn lead to differing physical or chemical properties. Anion photoelectron (PE) spectroscopy is a powerful and well-established method that probes electronic structure directly and, by comparison with calculations and vibrational analysis of the Franck-Condon profiles of the PE spectra, can also probe geometric structure.…”

Infrared Spectroscopy of Mobility-Selected H+-Gly-Pro-Gly-Gly (GPGG)

“…Although a number of groups have developed experiments that couple mass spectrometry with infrared spectroscopy [21][22][23][24][25], the application of these experiments tends to be focused on relatively small molecules. This is because one cannot deduce a structure from the experimental data as directly as in the cases of nuclear magnetic resonance (NMR) or X-ray crystallography.…”