Plasmalogen dimethylacetal

The analysis of essential fatty acids involves hydrolysis of the ester bonds and subsequent formation of the fatty acid methyl esters, which can be separated by gas chromatography (GC) [10]. By accident, the plasmalogens are hydrolysed in the same reaction and the methylation reaction transforms them into dimethylacetals, which appear in the GC run of the fatty acid methyl esters [4]. 

The fatty acid glycerol esters and the plasmalogens are transmethylated by adding methanolic HC1 to the sample and heating the mixture in a closed vial at 90°C for 4 h. After cooling the sample, the fatty acid methyl esters and the dimethylacetals are extracted with hexane. The concentrated hexane solution is ready for analysis by GC. 

Fig. 4.7. Partial GC chromatogram of total beef heart lipids methylated using an acid- (upper) or a base-catalyzed (lower) procedure.The acid-catalyzed methylation product included dimethylacetals (DMA) from plasmalogenic lipids and alk-l-enyl methyl ethers (AME) produced by loss of methanol from the DMA during GC analysis.

This has enabled neutral plasmalogens to be detected in the presence of free aldehydes [183] combined and free aldehydes to be separated [183] and aldehydes, dimethylacetals and methyl esters to be fractionated and isolated [185] (see procedures, p. 373). Neutral plasmalogens can be detected by means of the SRS-technique (p. 88) after chromatographing in the first direction, the alkenyl ethers are hydrolysed on the layer with hydrogen chloride vapour the aldehydes and the diglycerides formed are then separated by chromatography in the second direction [183]. Fig. 134 illustrates the detection of neutral plasmalogens in a shark liver oil by means of TLC. 

The same method is used to prepare dimethylacetals from aliphatic aldehydes or plasmalogens (see Chapter 10), and when it is used on lipid samples containing such compounds, acetals are formed which may contaminate the methyl esters. The two classes of compound can be separated by saponification or better by means of preparative TLC with toluene [582] or dichloroethane [980] as the solvent for development (esters migrate ahead of acetals). Acetals are not formed during base-catalysed transesterification. 

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