Working with Lipids:- Mass Spectrometry Reveals Complete Lipid Structure
To establish unambiguously the length of a hydrocarbon chain or the position of double bonds, mass spectral analysis of lipids or their volatile derivatives is invaluable. The chemical properties of similar lipids (for ex ample, two fatty acids of similar length unsaturated at different positions, or two isoprenoids with different numbers of isoprene units) are very much alike, and their positions of elution from the various chromato graphic procedures often do not distinguish between them. When the effluent from a chromatography column is sampled by mass spectrometry, however, the compo nents of a lipid mixture can be simultaneously separated and identified by their unique pattern of fragmentation (Fig. 10–24).
FIGURE 10–24 Determination of the structure of a fatty acid by mass spectrometry. The fatty acid is first converted to a derivative that minimizes migration of the double bonds when the molecule is fragmented by electron bombardment. The derivative shown here is a picolinyl ester of linoleic acid—18:2(Δ9,12) (Mr371)—in which the alcohol is picolinol (red). When bombarded with a stream of electrons, this molecule is volatilized and converted to a parent ion (M ; Mr 371), in which the N atom bears the positive charge, and a series of smaller fragments produced by breakage of C-C bonds in the fatty acid. The mass spectrometer separates these charged fragments according to their mass/charge ratio (m/z). (To review the principles of mass spectrometry, see Box 3–2.)
The prominent ions at m/z=92, 108, 151, and 164 contain the pyridine ring of the picolinol and various fragments of the carboxyl group, showing that the compound is indeed a picolinyl ester. The molecular ion (m/z=371) confirms the presence of a C-18 fatty acid with two double bonds. The uniform series of ions 14atomic mass units (amu) apart represents loss of each successive methyl and methylene group from the right end of the molecule (C-18of the fatty acid), until the ion at m/z=300 is reached. This is followed by a gap of 26 amu for the carbons of the terminal double bond, at m/z=274; a further gap of 14 amu for the C-11 methylene group, at m/z=260, and so forth. By this means the entire structure is determined, although these data alone do not reveal the configuration (cis or trans) of the double bonds.
