Structural characterization of molecular phospholipid species in cytoplasmic membranes of the cell wall-less Streptomyces hygroscopicus L form by use of electrospray ionization coupled with collision-induced dissociation mass spectrometry

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Abstract
A comparative analysis of the lipid compositions and fatty acids in the cytoplasmic membranes of Streptomyces hygroscopicus and its stable cell wall-less L form has been carried out to detect the differences which may be involved in the altered properties of the L-form membranes. Because only quantitative differences could be found (8), we analyzed the lipid components at the molecular level. Electrospray ionization (ESI), collision-induced dissociation (CID), and tandem mass spectrometry (MS-MS) were used for qualitative detection and quantitative determination of the molecular lipid species in phosphatidylethanolamine (PE1), lyso-cardiolipin (LCL), and cardiolipin (CL). Each phospholipid, isolated by preparative high-performance liquid chromatography showed several homologous molecular ion groups (PE1, four groups; LCL, six groups; CL, six groups) in the negative ESI-MS spectra. The sizes of the peaks represent their relative amounts in the corresponding phospholipid classes. Structural details about individual components of the molecular ion groups were obtained by mass selection and CID with MS-MS. Product ions derived from CID (daughter ions) give information about the molecular weights of the acyl constituents. The qualitative and quantitative compositions of the molecular species were determined by combining the data from the fatty acid pattern obtained by gas chromatography (GC), the relative quantities of the molecular ion groups, and the acyl constituents detected in these molecular ions. Because the ESI-MS-CID-MS data do not allow us to distinguish between n, iso, and anteiso fatty acids of the same molecular weight, it has been assumed that the ratio of these equal-numbered fatty acids determined by GC analysis of the isolated fatty acids is also present in the CID-MS peaks. In this way, 18 species were found in PE1, 43 species were estimated in LCL, and 59 species were ascertained for CL.

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