Occurrence of oleic acid 18:1 methyl-branched acyl chains in lipids of Rhodobacter sphaeroides 2.4.1

The fatty acids (FAs) composition of lipids extracted from Rhodobacter sphaeroides 2.4.1 was investigated by gas chromatography–mass spectrometry (GC–MS) analysis of the corresponding FA methyl esters (FAMEs), obtained through trans-esterification of the original lipid species. A GC stationary phase based on a highly polar ionic liquid (IL) was selected, aimed to enhance the separation of isomeric FAMEs with particular emphasis on positional and geometrical isomers of monounsaturated 16:1 and 18:1 fatty acyl chains. The occurrence of 18:1 cis-D9 (oleic) acid, a positional isomer of the well-known and most predominant 18:1 cis-D11 (cis-vaccenic) acid, has been demonstrated here for the first time. Furthermore a methyl branched 18:1 FA was also identified and its structure tentatively assigned as 11-methyl-D12-octadecenoic acid (most likely as trans isomer). The unprecedented observation about 18:1 cis-D9 FA occurrence in R. sphaeroides 2.4.1 is, even indirectly, supported by a biosynthetic pathway postulated with the aid of the Kyoto Encyclopedia of Genes and Genomes (KEGG) database. The concurrent presence of 16:1 cis-D7 and 18:1 cis-D9 FAs suggested the existence of parallel and/or complementary processes to those invoked for the formation of most common 16:1 cis-D9 and 18:1 cis-D11 FAs. A further route was hypothesized for the trans FAs biosynthesis in wild-type cells of R. sphaeroides.