Pinpointing Double Bonds in Lipids by Paternò‐Büchi Reactions and Mass Spectrometry

The positions of double bonds in lipids play critical roles in their biochemical and biophysical properties. In this study, by coupling Paternò–Büchi (P‐B) reaction with tandem mass spectrometry, we developed a novel method that can achieve confident, fast, and sensitive determination of double bond locations within various types of lipids. The P‐B reaction is facilitated by UV irradiation of a nanoelectrospray plume entraining lipids and acetone. Tandem mass spectrometry of the on‐line reaction products via collision activation leads to the rupture of oxetane rings and the formation of diagnostic ions specific to the double bond location.     

Entropy‐Driven Diastereoselectivity Improvement in the Paternò–Büchi Reaction of 1‐Naphthyl Aryl Ethenes with a Chiral Cyanobenzoate through Remote Alkylation

The precise stereocontrol of photocycloaddition reactions is still a significant challenge owing to their mechanistic complexity and the involvement of highly reactive and short‐lived intermediates. Attempts have hitherto been made through structural modifications, mostly by introducing steric conflicts, to increase the difference between the enthalpic barriers. Herein, we show that entropy plays a crucial role in influencing the diastereoselectivity of a Paternò–Büchi reaction. Remote meta alkylation of the donor caused nominal changes in its photophysical properties as well as those of the exciplexes derived thereof. Nevertheless, the diastereomeric excess of the oxetane product was greatly improved by about 40 %. This enhancement, which is not accompanied by any significant changes in the photophysical properties, is difficult to rationalize by conventional enthalpic control concepts based on repulsive steric and/or attractive intermolecular interactions as well as electronic perturbations. Differential activation parameters and compensatory enthalpy–entropy relationships revealed that the diastereoselectivity enhancement is not simply enthalpic but also entropic in origin.     

Recent Progress Regarding Regio‐, Site‐, and Stereoselective Formation of Oxetanes in Paternò‐Büchi Reactions

Recently, the demand for synthetic oxetanes has increased because the highly strained structure allows specific biologic functions in vivo and also serves unique purposes in industrial materials. This review article summarizes recent progress in regio‐, site‐, and stereo‐selective formation of oxetanes via a photochemical process.     

Olefinic reagents tested for peptide derivatization with switchable properties: Stable upon collision induced dissociation and cleavable by in‐source Paternò‐Büchi reactions

This contribution is part of our ongoing efforts to develop innovative cross‐linking (XL) reagents and protocols for facilitated peptide mixture analysis and efficient assignment of cross‐linked peptide products. In this report, we combine in‐source Paternò‐Büchi (PB) photo‐chemistry with a tandem mass spectrometry approach to selectively address the fragmentation of a tailor‐made cross‐linking reagent. The PB photochemistry, so far exclusively used for the identification of unsaturation sites in lipids and in lipidomics, is now introduced to the field of chemical cross‐linking. Based on trans‐3‐hexenedioic acid, an olefinic homo bifunctional amine reactive XL reagent was designed and synthesized for this proof‐of‐principle study. Condensation products of the olefinic reagent with a set of exemplary peptides are used to test the feasibility of the concept. Benzophenone is photochemically reacted in the nano‐electrospray ion source and forms oxetane PB reaction products. Subsequent CID‐MS triggered retro‐PB reaction of the respective isobaric oxetane molecular ions and delivers reliably and predictably two sets of characteristic fragment ions of the cross‐linker. Based on these signature ion sets, a straightforward identification of covalently interconnected peptides in complex digests is proposed. Furthermore, CID‐MSⁿ experiments of the retro‐PB reaction products deliver peptide backbone characteristic fragment ions. Additionally, the olefinic XL reagents exhibit a pronounced robustness upon CID‐activation, without previous UV‐excitation. These experiments document that a complete backbone fragmentation is possible, while the linker‐moiety remains intact. This feature renders the new olefinic linkers switchable between a stable, noncleavable cross‐linking mode and an in‐source PB cleavable mode.     

On‐Demand Electrochemical Epoxidation in Nano‐Electrospray Ionization Mass Spectrometry to Locate Carbon–Carbon Double Bonds

Reported here is the first on‐demand electrochemical epoxidation incorporated into the standard nano‐electrospray ionization mass spectrometry (nanoESI‐MS) workflow for double‐bond identification. The capability lies in a novel tunable electro‐epoxidation of double bonds, where onset of the reaction can be controlled by simply tuning the spray voltage. On‐demand formation of mono‐/multiple epoxides is achieved at different voltages. The electro‐epoxidized products are then fragmented by tandem MS to generate diagnostic ions, indicating the double bond position(s). The process is completed within seconds, holding great potential for high‐throughput analysis. The rapid switch‐on/off electro‐epoxidation of a single sample, the low sample consumption, the demonstrated applicability to complex lipids containing multiple double bonds, and the advantage of not requiring extra apparatus make this method attractive for use in lipid‐related biological studies.