<p>Lipids are important cellular
biomolecules that perform essential functional and biological roles. For
instance, lipids in the cell are the compartmentalizer for the cytoplasm and an
energy storage unit. The knowledge surrounding lipids is abundant, yet there is
still so much to uncover. There are many categories of lipids and within each
category the structural composition is extremely diverse. In turn, the dramatic
structural complexity of lipids demands analytical methods capable of providing
in-depth structural characterization of individual molecular structures. However,
lipid structural elucidation has remained challenging, namely due to the
presence of isomeric and isobaric species with a complex mixture. In
particular, isomeric/isobaric lipid structures arise from variations in class,
headgroup, fatty acyl chain, <i>sn</i>-position,
and/or carbon-carbon double bond (C=C) position(s). Recently, recent research
suggests C=C composition impacts lipid physical properties, metabolic fate, and
intermolecular interactions. Thus, analytical strategies capable of localizing
sites of unsaturation are of interest in the lipidomics community.</p>
<p>Mass spectrometry (MS) is a
leading tool for lipid analysis. Electrospray ionization (ESI), a soft
ionization method, is the most commonly used method for lipid ionization as a
means of taking the ions from liquid-phase to gas-phase without extensive decomposition
of the species. Utilizing ESI-MS, lipids can be identified at a sum
compositional level via accurate mass measurements. . With tandem mass
spectrometers, lipid ions can be further probed, utilizing tandem-MS (MS/MS) to
generate structurally informative product ion spectra that facilitate the
assignment of lipid molecular structure. More so, gas-phase ion/ion reactions
represent a unique MS-based technique that has improved the analysis of lipids
structures. Gas-phase ion/ion reactions allow for lipid species to be charge
inverted from one polarity to the opposite polarity. This reaction enables
lipids to be ionized in a polarity that is optimal for class identification and
further investigated in the opposite polarity where more structural information
is obtained. All the information provided is captured without the requirement
of multiple solution conditions which is necessary when analyzing in both
polarities. In the case of charge inverted lipids from positive ion mode to
negative ion mode, fatty acyl composition can be obtained; however, C=C
information is lacking.</p>
<p>MS can also be paired with other
analytically technologies to assist with lipid analysis. One of those
technologies is liquid chromatography (LC), which allows for the separation of
lipids based on different characteristic depending on the column type being
used. Reverse-phase LC (RPLC) allows for the separation of lipid molecular
species based on structural composition. RPLC-MS/MS benefits from the ability
to separate lipids and determine their fatty acyl chain composition but it is
difficult to specify C=C location with the use of a synthetic standard that is
identical to each molecular species being analyzed.</p>
<p>Commonality between the
gas-phase ion/ion reactions for charge inversion of lipids and RPLC-MS/MS
approaches is the inability to provide C=C coverage. In-solution and unique ion
activation techniques have been developed for seeking such information. The
Paternò–Büchi reaction is a UV-initiated [2 + 2]-cycloaddition of an excited carbonyl
containing compound onto an olefin group. This reaction can be initiated onto
the alkene group within an unsaturated lipid aliphatic chain to form an oxetane
ring modification. There are two product ions that can be formed upon each
unsaturation site due to a lack of regioselectivity the reagent can attach at
either side of the C=C. The modified lipids can be taken into gas-phase and
collisionally activated via low-energy collision induced dissociation,
generating product ions indictive of C=C position(s). The work herein shows the
incorporation of the PB reaction into the gas-phases ion/ion reaction and
RPLC-MS/MS apparatuses for C=C localization. The methods have been applied to
the lipid extracts of bovine liver and human plasma for confident molecule
species determination.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/11327954 |
| Date | 10 December 2019 |
| Creators | Elissia T Franklin (8087996) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Incorporation_of_the_Patern_B_chi_reaction_into_mass_spectrometry-based_systems_for_lipid_structural_characterization/11327954 |
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