Surface Oxidation under Ambient Air—Not Only a Fast and Economical Method to Identify Double Bond Positions in Unsaturated Lipids But Also a Reminder of Proper Lipid Processing

Abstract

A simple, fast approach elucidated carbon–carbon double bond positions in unsaturated lipids. Lipids were deposited onto various surfaces and the products from their oxidation in ambient air were observed by electrospray ionization (ESI) mass spectrometry (MS). The most common oxidative products, aldehydes, were detected as transformations at the cleaved double bond positions. Ozonides and carboxylic acids were generated in certain lipids. Investigations of the conditions controlling the appearance of these products indicated that the surface oxidation depends on light and ambient air. Since the lipid oxidation was slower in a high concentration of ozone, singlet oxygen appeared to be a parallel oxidant for unsaturated lipids. The 3-hydroxyl group in the sphingoid base of sulfatides offered some protection from oxidation for the Δ4,5-double bond, slowing its oxidation rate relative to that of the isolated double bond in the N-linked fatty acyl chain. Direct sampling by thin-layer chromatography (TLC)-ESI-MS provides a powerful approach to elucidate detailed structural information on biological samples. Co-localization of the starting lipids and their oxidation products after TLC separation allowed assignment of the native unsaturation sites. Phosphatidylserine and N,N-dimethyl phosphatidylethanolamine isomers in a bovine brain total lipid extract were distinguished on the basis of their oxidation products. Meanwhile, the findings reported herein reveal a potential pitfall in the assignment of structures to lipids extracted from TLC plates because of artifactual oxidation after the plate development.

Structural Analysis of Unsaturated Glycosphingolipids Using Shotgun Ozone-Induced Dissociation Mass Spectrometry

“…Altogether, these explanations suggest that the peculiar attribute of sphingosine double bond towards oxidation cannot be rationalized in terms of electronic effects of the α-hydroxyl group alone but rather in conjunction with steric contribution by the head group as a result of its gas phase conformation. Our observation is in agreement with these previous studies [42, 44, 59–61]. …”

“…1b). The relatively low reactivity of this sphingosine double bond towards oxidation as observed before [42, 44, 59, 60] and in this study, may explain why the unsaturation in the fatty acyl chain is preferentially cleaved off prior to that of the sphingosine. In this respect, Thomas and colleagues performed OzESI-MS of analogous compound sphingomyelin d18:1/18:1(9Z), and showed that only the fatty acyl double bond is prominent [44].…”

“…Zhou and colleagues [42] observed attenuated reactivity of the sphingosine double bond towards oxidation with ozone and singlet oxygen, and rationalized it is due to the electron-withdrawing inductive effect by allylic hydroxyl. This was further supported by recent studies on the free radical-induced oxidation of glycosphingolipids [59, 60].…”

“…More recent approaches include Paterno-Buchi reaction coupled with MS [35], radical-directed dissociation [36, 37] and ultraviolet photodissociation (UVPD) [38–41]. A simple and economical procedure involving air exposure of lipids on a thin layer chromatography plate followed by ESI-MS analysis could also reveal double bond positions, but data analysis is rather intricate when complex mixture is involved [42]. …”

Chemically stable fluorescent anti-counterfeiting labels achieved by UV-induced photolysis of nanocellulose

“…26 Similarly, the unsaturated double bonds (C]C) can also be oxidized by the singlet oxygen during the further UV-induced photolysis progress, resulting in the irreversible quench of the uorescence. 38 This causes the decrease of the uorescent intensity of the degraded cellulose, in consistent with the uorescence results in Fig. 1f.…”