Occurrence of oleic and 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-Δ9 (oleic) acid, a positional isomer of the well-known and most predominant 18:1 cis-Δ11 (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-Δ12-octadecenoic acid (most likely as trans isomer). The unprecedented observation about 18:1 cis-Δ9 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-Δ7 and 18:1 cis-Δ9 FAs suggested the existence of parallel and/or complementary processes to those invoked for the formation of most common 16:1 cis-Δ9 and 18:1 cis-Δ11 FAs. A further route was hypothesized for the trans FAs biosynthesis in wild-type cells of R. sphaeroides.