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Elucidation of Double Bond Position in Unsaturated Lipids by Ozone Electrospray Ionization Mass Spectrometry
journal contribution
posted on 2007-07-01, 00:00 authored by Michael C. Thomas, Todd W. Mitchell, David G. Harman, Jane M. Deeley, Robert C. Murphy, Stephen J. BlanksbyThe position(s) of carbon−carbon double bonds within
lipids can dramatically affect their structure and reactivity
and thus has a direct bearing on biological function.
Commonly employed mass spectrometric approaches to
the characterization of complex lipids, however, fail to
localize sites of unsaturation within the molecular structure and thus cannot distinguish naturally occurring
regioisomers. In a recent communication [Thomas, M. C.;
Mitchell, T. W.; Blanksby, S. J. J. Am. Chem. Soc. 2006,
128, 58−59], we have presented a new technique for the
elucidation of double bond position in glycerophospholipids using ozone-induced fragmentation within the source
of a conventional electrospray ionization mass spectrometer. Here we report the on-line analysis, using ozone
electrospray mass spectrometry (OzESI-MS), of a broad
range of common unsaturated lipids including acidic and
neutral glycerophospholipids, sphingomyelins, and triacylglycerols. All lipids analyzed are found to form a pair
of chemically induced fragment ions diagnostic of the
position of each double bond(s) regardless of the polarity,
the number of charges, or the adduct ion (e.g., [M − H]-,
[M − 2H]2-, [M + H]+, [M + Na]+, [M + NH4]+). The
ability of OzESI-MS to distinguish lipids that differ only
in the position of the double bonds is demonstrated using
the glycerophosphocholine standards, GPCho(9Z-18:1/9Z-18:1) and GPCho(6Z-18:1/6Z-18:1). While these
regioisomers cannot be differentiated by their conventional tandem mass spectra, the OzESI-MS spectra reveal
abundant fragment ions of distinctive mass-to-charge ratio
(m/z). The approach is found to be sufficiently robust to
be used in conjunction with the m/z 184 precursor ion
scans commonly employed for the identification of phosphocholine-containing lipids in shotgun lipidomic analyses. This tandem OzESI-MS approach was used, in
conjunction with conventional tandem mass spectral
analysis, for the structural characterization of an unknown
sphingolipid in a crude lipid extract obtained from a
human lens. The OzESI-MS data confirm the presence of
two regioisomers, namely, SM(d18:0/15Z-24:1) and SM(d18:0/17Z-24:1), and suggest the possible presence of
a third isomer, SM(d18:0/19Z-24:1), in lower abundance. The data presented herein demonstrate that OzESI-MS is a broadly applicable, on-line approach for
structure determination and, when used in conjunction
with established tandem mass spectrometric methods,
can provide near complete structural characterization of
a range of important lipid classes. As such, OzESI-MS may
provide important new insight into the molecular diversity
of naturally occurring lipids.