Vacuum ultraviolet action spectroscopy of polysaccharides

UV-vis spectroscopy of gas-phase ions

Photodissociation action spectroscopy has made a great progress in expanding investigations of gas-phase ion structures. This review deals with aspects of gas-phase ion electronic excitations that result in wavelength-dependent dissociation and light emission via fluorescence, chiefly covering the ultraviolet and visible regions of the spectrum. The principles are briefly outlined and a few examples of instrumentation are presented. The main thrust of the review is to collect and selectively present applications of UV-vis action spectroscopy to studies of stable gas-phase ion structures and combinations of spectroscopy with ion mobility, collision-induced dissociation, and ion-ion reactions leading to the generation of reactive intermediates and electronic energy transfer.

Spectroscopy and Photodissociation of the Perfluorooctanoate Anion

Perfluorocarbons, a class of fully fluorinated compounds, are highly persistent and toxic pollutants that are receiving increasing attention due to their widespread environmental distribution. In this study, attention was focused on one compound in particular, namely, perfluorooctanoic acid (PFOA). The first investigation of the UV/VUV photochemistry of the PFOA anion in the gas phase by action spectroscopy of selected ions is reported. Two main relaxation channels were identified: photodissociation and photodetachment. Absolute cross sections for the individual observed processes were measured. DFT calculations and natural transition orbital analysis were carried out to help in the interpretation of the experimental results.

Gas-Phase Intercluster Thiyl-Radical Induced C-H Bond Homolysis Selectively Forms Sugar C2-Radical Cations of Methyl D-Glucopyranoside: Isotopic Labeling Studies and Cleavage Reactions

A suite of isotopologues of methyl D-glucopyranosides is used in conjunction with multistage mass spectrometry experiments to determine the radical site and cleavage reactions of sugar radical cations formed via a recently developed 'bio-inspired' method. In the first stage of CID (MS²), collision-induced dissociation (CID) of a protonated noncovalent complex between the sugar and S-nitrosocysteamine, [H₃NCH₂CH₂SNO + M]⁺, unleashes a thiyl radical via bond homolysis to give the noncovalent radical cation, [H₃NCH₂CH₂S^• + M]⁺. CID (MS³) of this radical cation complex results in dissociation of the noncovalent complex to generate the sugar radical cation. Replacement of all exchangeable OH and NH protons with deuterons reveals that the sugar radical cation is formed in a process involving abstraction of a hydrogen atom from a C-H bond of the sugar coupled with proton transfer to the sugar, to form [M - H^• + D⁺]. Investigation of this process using individual C-D labeled sugars reveals that the main site of H/D abstraction is the C2 position, since only the C2-deuterium labeled sugar yields a dominant [M - D^• + H⁺] product ion. The fragmentation reactions of the distonic sugar radical cation, [M - H^•+ H⁺], were studied by another stage of CID (MS⁴). ¹³C-labeling studies revealed that a series of three related fragment ions each contain the C1-C3 atoms; these arise from cross-ring cleavage reactions of the sugar. Graphical Abstract ᅟ.

Nucleic acid ion structures in the gas phase

Nucleic acids are diverse polymeric macromolecules that are essential for all life forms. These biomolecules possess a functional three-dimensional structure under aqueous physiological conditions. Mass spectrometry-based approaches have on the other hand opened the possibility to gain structural information on nucleic acids from gas-phase measurements. To correlate gas-phase structural probing results with solution structures, it is therefore important to grasp the extent to which nucleic acid structures are preserved, or altered, when transferred from the solution to a fully anhydrous environment. We will review here experimental and theoretical approaches available to characterize the structure of nucleic acids in the gas phase (with a focus on oligonucleotides and higher-order structures), and will summarize the structural features of nucleic acids that can be preserved in the gas phase on the experiment time scale.