Fatty acid esters of long-chain hydroxy fatty acids or (O-acyl)-hydroxy fatty acids (OAHFAs) were identified for the first time in vernix caseosa and characterized using chromatography and mass spectrometry. OAHFAs were isolated from the total lipid extract by a two-step semipreparative TLC. The general structure of OAHFAs was established using high-resolution and tandem mass spectrometry of intact lipids and their transesterification and derivatization products. Two isomeric lipid classes were identified: O-acyl esters of ω-hydroxy fatty acids (ωOAHFA) and O-acyl esters of α-hydroxy fatty acids (αOAHFAs). To the best of our knowledge, αOAHFAs have never been detected in any biological sample before. Chromatographic separation and identification of OAHFAs species were achieved using non-aqueous reversed-phase HPLC coupled to electrospray ionization hybrid linear ion trap-Orbitrap mass spectrometry. The lipid species were detected as deprotonated molecules, and their structures were elucidated using data-dependent fragmentation in the negative ion mode. More than 400 OAHFAs were identified in this way. The most abundant ωOAHFAs species were 28:0/ω-18:2, 29:0/ω-18:2, 30:0/ω-18:2, 32:0/ω-18:2, and 30:0/ω-18:3, while αOAHFAs comprised saturated species 21:0/α-24:0, 22:0/α-24:0, 23:0/α-24:0, 24:0/α-24:0, and 26:0/α-24:0. OAHFAs were estimated to account for approximately 0.04% of vernix caseosa lipids. Graphical Abstract.

Department of Analytical Chemistry Faculty of Science Charles University Prague Hlavova 2030 8 128 43 Prague 2 Czech Republic

Department of Obstetrics and Gynaecology General Faculty Hospital and 1st Faculty of Medicine Charles University Prague Apolinářská 18 128 00 Prague 2 Czech Republic

Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences Femingovo nám 2 166 10 Prague 6 Czech Republic

Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences Femingovo nám 2 166 10 Prague 6 Czech Republic Department of Analytical Chemistry Faculty of Science Charles University Prague Hlavova 2030 8 128 43 Prague 2 Czech Republic

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