Resolving sphingolipid isomers using cryogenic infrared spectroscopy

1‐Deoxysphingolipids are a recently described class of sphingolipids that have been shown to be associated with several disease states including diabetic and hereditary neuropathy. The identification and characterization of 1‐deoxysphingolipids and their metabolites is therefore highly important. However, exact structure determination requires a combination of sophisticated analytical techniques due to the presence of various isomers, such as ketone/alkenol isomers, carbon–carbon double‐bond (C=C) isomers and hydroxylation regioisomers. Here we demonstrate that cryogenic gas‐phase infrared (IR) spectroscopy of ionized 1‐deoxysphingolipids enables the identification and differentiation of isomers by their unique spectroscopic fingerprints. In particular, C=C bond positions and stereochemical configurations can be distinguished by specific interactions between the charged amine and the double bond. The results demonstrate the power of gas‐phase IR spectroscopy to overcome the challenge of isomer resolution in conventional mass spectrometry and pave the way for deeper analysis of the lipidome

Programming Photodegradability into Vinylic Polymers via Radical Ring-Opening Polymerization

Incorporation of photolabile moieties into the polymer backbone holds promise to remotely-control polymer degradation. However, suitable synthetic avenues are limited, especially for radical polymerizations. Here we report a strategy to program photodegradability into vinylic polymers by exploiting the wavelength selectivity of photocycloadditions for radical ring-opening polymerization (rROP). Irradiation of coumarin terminated allylic sulfides with UVA light initiated intramolecular [2+2] photocycloaddition producing cyclic macromonomers. Subsequent RAFT-mediated rROP with methyl acrylate yielded copolymers that inherited the photoreactivity of the cyclic parent monomer. Irradiation with UVB initiated efficient photocycloreversion of the coumarin dimers, causing polymer degradation within minutes under UVB light or days under sunlight exposure. Our synthetic strategy may pave the way to insert photolabile linkages into vinylic polymers, tuning degradation for specific wavelengths.</p

Wavelength-gated photoreversible polymerization and topology control

We exploit the wavelength dependence of [2 + 2] photocycloadditions and-reversions of styrylpyrene to exert unprecedented control over the photoreversible polymerization and topology of telechelic building blocks. Blue light (λmax = 460 nm) initiates a catalyst-free polymerization yielding high molar mass polymers (Mn = 60 000 g mol-1), which are stable at wavelengths exceeding 430 nm, yet highly responsive to shorter wavelengths. UVB irradiation (λmax = 330 nm) induces a rapid depolymerization affording linear oligomers, whereas violet light (λmax = 410 nm) generates cyclic entities. Thus, different colors of light allow switching between a depolymerization that either proceeds through cyclic or linear topologies. The light-controlled topology formation was evidenced by correlation of mass spectrometry (MS) with size exclusion chromatography (SEC) and ion mobility data. Critically, the color-guided topology control was also possible with ambient laboratory light affording cyclic oligomers, while sunlight activated the linear depolymerization pathway. These findings suggest that light not only induces polymerization and depolymerization but that its color can control the topological outcomes.</p

Accelerating Ozonolysis Reactions Using Supplemental RF-Activation of Ions in a Linear Ion Trap Mass Spectrometer

Gas-phase ion-molecule reactions provide structural insights across a range of analytical applications. A hindrance to the wider use of ion-molecule reactions is that they are relatively slow compared to other ion activation modalities and can thereby impose a bottleneck on the time required to analyze each sample. Here we describe a method for accelerating the rate of ion-molecule reactions involving ozone, implemented by supplementary RF-activation of mass-selected ions within a linear ion trap. Reaction rate accelerations between 15-fold (for ozonolysis of alkenes in ionised lipids) and 90-fold (for ozonation of halide anions) are observed compared to thermal conditions. These enhanced reaction rates with ozone increase sample throughput, aligning the reaction time with the overall duty cycle of the mass spectrometer. We demonstrate that the acceleration is due to the supplementary RF-activation surmounting the activation barrier energy of the entrance channel of the ion-molecule reaction. This rate acceleration is subsequently shown to aid identification of new, low abundance lipid isomers and enables an equivalent increase in the number of lipid species that can be analyzed

Gas-Phase Phenyl Radical + O2 Reacts via a Submerged Transition State

In the gas-phase chemistry of the atmosphere and automotive fuel combustion, peroxyl radical intermediates are formed following O2 addition to carbon-centered radicals which then initiate a complex network of radical reactions that govern the oxidative processing of hydrocarbons. The rapid association of the phenyl radical-a fundamental radical related to benzene-with O2 has hitherto been modeled as a barrierless process, a common assumption for peroxyl radical formation. Here, we provide an alternate explanation for the kinetics of this reaction by deploying double-hybrid density functional theory (DFT), at the DSD-PBEP86-D3(BJ)/aug-cc-pVTZ level of theory, and locate a submerged adiabatic transition state connected to a prereaction complex along the reaction entrance pathway. Using this potential energy scheme, experimental rate coefficients k(T) for the addition of O2 to the phenyl radical are accurately reproduced within a microcanonical kinetic model. This work highlights that purportedly barrierless radical oxidation reactions may instead be modeled using stationary points, which in turn provides insight into pressure and temperature dependence

Disentangling Lipid Isomers by High-Resolution Differential Ion Mobility Spectrometry/Ozone-Induced Dissociation of Metalated Species

The preponderance and functional importance of isomeric biomolecules have become topical in biochemistry. Therefore, one must distinguish and identify all such forms across compound classes, over a wide dynamic range as minor species often have critical activities. With all the power of modern mass spectrometry for compositional assignments by accurate mass, the identical precursor and often fragment ion masses render this task a steep challenge. This is recognized in proteomics and epigenetics, where proteoforms are disentangled and characterized employing novel separations and non-ergodic dissociation mechanisms. This issue is equally pertinent to lipidomics, where the lack of isomeric depth has thwarted the deciphering of functional networks. Here we introduce a new platform, where the isomeric lipids separated by high-resolution differential ion mobility spectrometry (FAIMS) are identified using ozone-induced dissociation (OzID). Cationization by metals (here K+, Ag+, and especially Cu+) broadly improves the FAIMS resolution of isomers with alternative C═C double bond (DB) positions or stereochemistry, presumably via metal attaching to the DB and reshaping the ion around it. However, the OzID yield diminishes for Ag+ and vanishes for Cu+ adducts. Argentination still strikes the best compromise between efficient separation and diagnostic fragmentation for optimal FAIMS/OzID performance.</p

Actinic Wavelength Action Spectroscopy of the IO\u3csup\u3e–\u3c/sup\u3e Reaction Intermediate

Iodinate anions are important in the chemistry of the atmosphere where they are implicated in ozone depletion and particle formation. The atmospheric chemistry of iodine is a complex overlay of neutral–neutral, ion–neutral, and photochemical processes, where many of the reactions and intermediates remain poorly characterized. This study targets the visible spectroscopy and photostability of the gas-phase hypoiodite anion (IO–), the initial product of the I– + O3 reaction, by mass spectrometry equipped with resonance-enhanced photodissociation and total ion-loss action spectroscopies. It is shown that IO– undergoes photodissociation to I– + O (3P) over 637–459 nm (15700–21800 cm–1) because of excitation to the bound first singlet excited state. Electron photodetachment competes with photodissociation above the electron detachment threshold of IO– at 521 nm (19200 cm–1) with peaks corresponding to resonant autodetachment involving the singlet excited state and the ground state of neutral IO possibly mediated by a dipole-bound state

ZnO Colloid Crystal Facet-Type Determines both Au Photodeposition and Photocatalytic Activity

This study investigated the crystal facet-dependence of the photochemical deposition of Au onto four differently shaped ZnO colloids; hexagonal-based nanocones, nanorods, nanobullets, and nanoplates. The different ZnO nanoparticle (NP) shapes were approximately the same size and synthesized without the use of strong-binding capping agents. Direct photoreduction of AuCl2 - onto the ZnO NPs by UV illumination at 370 nm proved to be an effective approach to produce Au-ZnO NP hybrids, where the active Au(I) precursor was photogenerated from AuCl4 - using the same UV light source. Electrospray ionization mass spectrometry confirmed that preirradiation of ethanolic HAuCl4 solutions with a 370 nm UV-diode transforms the AuCl4 - to AuCl2 -, a metastable species that is stable in a range of solvents. The solvent system used, the irradiation exposure time, and the dissolved oxygen content in the solvent were modified to generate changes in the pattern of Au NP photodeposition onto the different ZnO shapes. The relative surface area of exposed high-energy facets of the ZnO NPs were observed to have a dramatic effect on the energy barrier to Au NP nucleation on different ZnO surfaces, where the facet-dependent activity was established to be (0001) > (101̄ 1) ≈ (0001̄) > (101̄ 0). For ZnO nanoplates, typically 4-6 Au NPs deposited where approximately 50% attached to the {0001} ZnO facets and 50% to the {1010} ZnO facets. For ZnO nanocone hybrids, 1 Au NP deposited per particle, with approximately 30% depositing on the {0001} and 70% on the {1010} facet. On ZnO nanobullets, 8 Au NPs deposited, with a distribution of 8% on the {0001} facet, 75% on {1010}, and 17% on the {1011} facets. For the ZnO nanorod samples, 4 Au NPs were deposited per rod, with 20% attached to {0001} facets in the sample and 80% fixed to the {1010} facets. The different Au NP deposition distributions on different ZnO shapes caused major changes to their photocatalytic activity, as tested by degrading an organic dye, toluidine blue, in aqueous conditions. The Au-ZnO hybrid NP photoactivities were greater than pure, ZnO NP photoactivities, attributed to better separation of charge and a reduced electron-hole recombination rate. Small molecule, free-radical scavengers were added to control samples to confirm the mechanism of dye degradation, which was found to be by hydroxyl radicals generated through oxidative reaction pathways.</p

Reactivity Trends in the Gas-Phase Addition of Acetylene to the N-Protonated Aryl Radical Cations of Pyridine, Aniline, and Benzonitrile

A key step in gas-phase polycyclic aromatic hydrocarbon (PAH) formation involves the addition of acetylene (or other alkyne) to σ-type aromatic radicals, with successive additions yielding more complex PAHs. A similar process can happen for N-containing aromatics. In cold diffuse environments, such as the interstellar medium, rates of radical addition may be enhanced when the σ-type radical is charged. This paper investigates the gas-phase ion-molecule reactions of acetylene with nine aromatic distonic σ-type radical cations derived from pyridinium (Pyr), anilinium (Anl), and benzonitrilium (Bzn) ions. Three isomers are studied in each case (radical sites at the ortho, meta, and para positions). Using a room temperature ion trap, second-order rate coefficients, product branching ratios, and reaction efficiencies are measured. The rate coefficients increase from para to ortho positions. The second-order rate coefficients can be sorted into three groups: low, between 1 and 3 × 10-12 cm3 molecule-1 s-1 (3Anl and 4Anl); intermediate, between 5 and 15 × 10-12 cm3 molecule-1 s-1 (2Bzn, 3Bzn, and 4Bzn); and high, between 8 and 31 × 10-11 cm3 molecule-1 s-1 (2Anl, 2Pyr, 3Pyr, and 4Pyr); and 2Anl is the only radical cation with a rate coefficient distinctly different from its isomers. Quantum chemical calculations, using M06-2X-D3(0)/6-31++G(2df,p) geometries and DSD-PBEP86-NL/aug-cc-pVQZ energies, are deployed to rationalize reactivity trends based on the stability of prereactive complexes. The G3X-K method guides the assignment of product ions following adduct formation. The rate coefficient trend can be rationalized by a simple model based on the prereactive complex forward barrier height

Stepwise reduction of interlocked viologen-based complexes in the gas phase

We present the first application of electrochemical reduction in an ion trap mass spectrometer as a dual-function tool to synthesise and probe the reactivity of interlocked viologen-based complexes. Compared with non-complexed archetypes, electron-donating macrocyclic porphyrin ethers retard electron transfer reaction rates and stabilise intact structures in low oxidation states.</p