Modifying Fatty Acid Structure

In addition to unsaturated fatty acids, several other modified fatty acids are found in nature. Microorganisms, for example, often contain branched-chain fatty acids, such as tuberculostearic acid (Figure 8.2). When these fatty acids are incorporated in membranes, the methyl group constitutes a local structural perturbation in a manner similar to the double bonds in unsaturated fatty acids (see Chapter 9). Some bacteria also synthesize fatty acids containing cyclic structures such as cyclopropane, cyclopropene, and even cyclopentane rings. 

Various fatty acids are used in the production of structured lipids these may include both n-3 and n-6 fatty acids. Structured lipids containing MCFA and LCFA may modify the absorption rates because MCFA are rapidly oxidized for energy, whereas LCFA are oxidized very slowly. These specialty lipids are strucmrally and metabolically different from simple physical mixtures of medium-chain and long-chain triacylglycerols. 

Contado MJ, Adams J. Collision-induced dissociations and B/E linked scans for structural determination of modified fatty acid esters. Anal Chim Acta. 1991 246 187-97. 

A few years ago, we began a research program to develop methods of analysis which would involve the use of FAB and a high performance tandem mass spectrometer. The tandem instrument was the first triple sector mass spectrometer to be designed and built by a commercial instrument company (Kratos of Manchester, U.K.). The first mass spectrometer of the combination is a double focussing Kratos MS-50 which is coupled to a low resolution electrostatic analyzer, which serves as the second mass spectrometer U). This FAB MS-MS combination has been used to verify the structures of an unknown cyclic peptide (2), a new amino acid modified by diphtheria toxin (3), and an ornithine-containing lipid (4). A number of methods have also been worked out which rely on this instrumentation. They Include the structural determination of cyclic peptides (5), nucleosides and nucleotides (6), and unsaturated fatty acids (7) and the analysis of mixtures of both anionic (8) and cationic surfactants (9). 

Numerous proteins are covalently modified by fatty acids (see Topic E2). Myristate (04 0) and palmitate (06 0) are directly attached to some proteins, while phosphatidylinositol is covalently linked to the C terminus of other proteins via a complex glycosylated structure ... 

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