Abstract
This paper reviews applications of gas chromatography-mass spectrometry techniques for the characterization of photooxidation and autoxidation products of lipids of senescent phototrophic organisms. Particular attention is given to: (i) the selection of oxidation products that are sufficiently stable under environmental conditions and specific to each lipid class and degradation route; (ii) the description of electron ionization mass fragmentation of trimethylsilyl derivatives of these compounds; and (iii) the use of specific fragment ions for monitoring the oxidation of the main unsaturated lipid components of phototrophs. The techniques best geared for this task were gas chromatography-quadrupole-time of flight to monitor fragment ions with very high resolution and accuracy, and gas chromatography-tandem mass spectrometry to monitor very selective transitions in multiple reaction monitoring mode. The extent of the degradation processes can only be estimated if the oxidation products are unaffected by fast secondary oxidation reactions, as it is notably the case of ∆5-sterols, monounsaturated fatty acids, chlorophyll phytyl side-chain, and di- and triterpenoids. In contrast, the primary degradation products of highly branched isoprenoid alkenes possessing more than one trisubstituted double bond, alkenones, carotenoids and polyunsaturated fatty acids, appear to be too unstable with respect to secondary oxidation or other reactions to serve for quantification in environmental samples.
Highlights
Phototrophic organisms carry out photosynthesis that is, conversion of sunlight energy, carbon dioxide and water into organic materials
The primary degradation products of highly branched isoprenoid alkenes possessing more than one trisubstituted double bond, alkenones, carotenoids and polyunsaturated fatty acids, appear to be too unstable with respect to secondary oxidation or other reactions to serve for quantification in environmental samples
gas chromatography (GC)-based analytical procedures require analytes that are volatile and thermally stable. This means that GC-based analysis of the oxidation products of mixtures of complex and simple lipids with such techniques demands a chemical pre-treatment of the samples, including: (i) NaBH4 reduction of thermally-labile hydroperoxides to the corresponding alcohols [10], (ii) alkaline hydrolysis of complex lipids into their constituent fatty acids, plus glycerol, phosphate, sterol or sugar groups [5], and (iii) conversion of polar compounds to volatile derivatives
Summary
Phototrophic organisms (mainly green plants, algae, cyanobacteria and some protists) carry out photosynthesis that is, conversion of sunlight energy, carbon dioxide and water into organic materials. GC-based analytical procedures require analytes that are volatile and thermally stable This means that GC-based analysis of the oxidation products of mixtures of complex and simple lipids with such techniques demands a chemical pre-treatment of the samples, including: (i) NaBH4 reduction of thermally-labile hydroperoxides to the corresponding alcohols [10], (ii) alkaline hydrolysis of complex lipids into their constituent fatty acids, plus glycerol, phosphate, sterol or sugar groups [5], and (iii) conversion of polar compounds to volatile derivatives (derivatization). Type II photosensitized oxidation of unsaturated lipids affords allylic hydroperoxides (for reviews see [8,9])
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