Using ambient ozone for assignment of double bond position in unsaturated lipids

Unsaturated lipids deposited onto a range of materials are observed to react with the low

concentrations of ozone present in normal laboratory air. Parent lipids and ozonolysis cleavage

products are both detected directly from surfaces by desorption electrospray ionisation mass

spectrometry (DESI-MS) with the resulting mass spectra providing clear evidence of the double bond

position within these molecules. This serendipitous process has been coupled with thin-layer

chromatography (TLC) to provide a simple but powerful approach for the detailed structural

elucidation of lipids present in complex biological extracts. Lipid extracts from human lens were

deposited onto normal phase TLC plates and then developed to separate components according to lipid

class. Exposure of the developed plates to laboratory air for ca. 1 h prior to DESI-MS analysis gave rise

to ozonolysis products allowing for the unambiguous identification of double bond positions in even

low abundant, unsaturated lipids. In particular, the co-localization of intact unsaturated

lactosylceramides (LacCer) with products from their oxidative cleavage provide the first evidence for

the presence of three isomeric LacCer (d18:0/24:1) species in the ocular lens lipidome, i.e., variants with

double bonds at the n-9, n-7 and n-5 positions.