The Paternò–Büchi
(PB) reaction is a photochemical
reaction involving [2 + 2] cycloaddition between electronically excited
carbonyl and carbon–carbon double bond (CC). It has
been established as a lipid derivatization strategy, leading to confident
assignment of CC locations in lipids when coupled with tandem
mass spectrometry (MS/MS). Although acetone and several aryl-containing
ketones or aldehydes have been explored as PB reagents, the chemical
properties critical to achieving efficient conversion and minimum
side reactions remain unclear. Herein, we investigated a set of acetophenone
(AP) derivatives, aiming to provide insights into the development
of new PB reagents with enhanced performance for lipid analysis. For
AP derivatives, we found that electron-withdrawing groups (e.g., −F
and −CF3) on the benzene ring improved the overall
conversion, while a bulky group at the ortho-position
decreased the conversion. Norrish Type I cleavage was largely diminished;
however, the Norrish Type II side reaction was more competitive, producing
products isomeric to the PB reaction products. Among all AP derivatives
tested, 2′,4′,6′-trifluoroacetophenone (triFAP)
showed the best performance. It offered a relatively high PB yield
(20–30%) for different types of CC, high sensitivity
(sub-nM) for CC identification, and accurate isomer quantitation.
Due to the significantly reduced chemical interferences in shotgun
analysis, triFAP provided better performance than that from acetone
PB-MS/MS. An offline triFAP PB reaction was implemented in a liquid
chromatography analysis workflow, which enabled the large-scale identification
of phospholipids including CC location isomers from a complex
lipid extract.
A new series of hydrogen-bonded helical aromatic hydrazide oligomers and polymer that bear phenylalanine tripeptide chains have been designed and synthesized. It was revealed that the helical structures could insert into lipid bilayers to form unimolecular channels. The longest oligomeric and polymeric helical channels exhibited an NH4(+)/K(+) selectivity that was higher than that of natural gramicidin A, whereas the transport of a short helical channel for Tl(+) could achieve an efficiency as high as that of gramicidin A.
A new apparatus for ion soft landing research was developed and is reported in this contribution. The instrument includes a dual polarity high-flux electrospray ionization (ESI) interface, a tandem electrodynamic ion funnel system, a collisional flatapole, a quadrupole mass filter, and a focusing lens. The instrument enables production of ionic layers by soft landing of mass-selected ions onto surfaces with balanced or imbalanced charge conditions using either layer-by-layer (LBL) or fast polarity switching modes. We present the first evidence of using weakly coordinating stable anions to protect the ionizing protons of soft-landed cations on the surface. The observed proton retention is particularly efficient when fast polarity switching of anions and cations is employed to deposit small quantities of ions in short deposition segments. Furthermore, we observe more efficient charge retention and better ionic complexation in a charge-balanced layer prepared by fast polarity switching deposition. These findings open up new opportunities for the fabrication of novel ionic assemblies using well-defined gaseous ions as building blocks.
Unraveling the complexity of the lipidome requires the development of novel approaches for the structural characterization of lipid species with isomer-level discrimination. Herein, we introduce an online photochemical approach for lipid isomer identification through selective derivatization of double bonds by reaction with singlet oxygen. Lipid hydroperoxide products are generated promptly after laser irradiation. Fragmentation of these species in a mass spectrometer produces diagnostic fragments revealing the C=C locations in the unreacted lipids. This approach uses an inexpensive light source and photosensitizer making it easy to incorporate into any lipidomics workflow. We demonstrate the utility of this approach for the shotgun profiling of C = C locations in different lipid classes present in tissue extracts using electrospray ionization (ESI) and ambient imaging of lipid species differing only by the location of C=C bonds using nanospray desorption electrospray ionization (nano-DESI).
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