Studies on phosphatidylserine by tandem quadrupole and multiple stage quadrupole ion-trap mass spectrometry with electrospray ionization: Structural characterization and the fragmentation processes

Abstract

Low-energy CAD product-ion spectra of various molecular species of phosphatidylserine (PS) Ϫ ion, followed by loss of fatty acid substituents as acids (R x CO 2 H, x ϭ 1,2) or as alkali salts (e.g., R x CO 2 Li, x ϭ 1,2). These fragmentations result in a greater abundance ofϪ ions gives a preferential formation of the carboxylate anion at sn-1 (R 1 CO 2 Ϫ ) over that at sn-2 (R 2 CO 2 Ϫ ). Other major fragmentation process arises from differential loss of the fatty acid substituents as ketenes (loss of R x =CHϭCO, x ϭ 1,2). This results in a more prominent ϩ ion, which undergoes consecutive dissociation processes that involve differential losses of the two fatty acyl substituents. Because all of the above tandem mass spectra contain several sets of ion pairs involving differential losses of the fatty acid substituents as ketenes or as free fatty acids, the identities of the fatty acyl substituents and their positions on the glycerol backbone can be easily assigned by the drastic differences in the abundances of the ions in each pair. In yeast and prokaryotes, PS is made by a PS synthase that uses CDP-diacylglycerol and serine [4]. However, in mammals, PS is generated by a base-exchange reaction in which L-serine substitutes the choline and ethanolamine moieties of phosphatidylcholine and phosphatidylethanolamine, respectively, with the participation of PS synthase 1 and PS synthase 2, respectively, to yield PS and choline or ethanolamine [5]. The major route for PE synthesis in

Characterization of inositol phosphorylceramides from Leishmania major by tandem mass spectrometry with electrospray ionization

“…The MS 4 product-ion spectrum of the ion at m/z 592 (784 ¡ 622 ¡ 592) (Figure 3c) is dominated by the ion at m/z 504, corresponding to loss of a P(OH) 2 (OLi) (88 Da) residue (Scheme 2a), consistent with the proposed mechanism that involves rearrangement of the phosphate group. Similar rearrangement of phosphate group has been previously observed for phosphatidylinositol phosphate [30], phosphatidylserine [32], and plasmenylethanolamine [33].…”

“…The major ions observed for the [M -H ϩ 2 Alk] ϩ ion of IPC arises from cleavage of the P-OC 1 bond, similar to that previously observed for the [M -H ϩ 2Alk] ϩ of phosphatidylserine [32]. As seen in Figure 6, the MS 2 product-ion spectra of the [M -H ϩ 2 Li] ϩ ion at m/z 792 obtained with an TSQ (Figure 6a) and with an IT (Figure 6b) instruments are dominated by the ions at m/z 544 and 255, corresponding to a lithiated ceramide and a dilithiated inositol-1,2-cyclic phosphate ions, respectively (Scheme 4).…”

“…In contrast, the product-ion spectrum of the corresponding [M -H ϩ 2 Na] ϩ ion at m/z 824 (Figure 6c) is dominated by a disodiated inositol-1,2-cyclic phosphate ion at m/z 287, and the ion at m/z 560, corresponding to a sodiated ceramide cation, is of low abundance. This drastic rise in the abundance of the alkali phosphate cations upon substitution of Li ϩ with Na ϩ was previously seen for the [M -H ϩ 2Alk] ϩ of phosphatidylserine [32].…”

Algorithm for processing raw mass spectrometric data to identify and quantitate complex lipid molecular species in mixtures by data-dependent scanning and fragment ion database searching

“…First, peak list files of both MS and MS/MS spectra are loaded into memory and undergo data pretreatment that includes smoothing (5-point Triangular Smooth)/noise filtering. MS/MS data are searched against a fragment ion database from a library of reference spectra for complex lipids that we have acquired [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] or constructed from established fragmentation rules [29] to identify lipid molecular structures, and deduced chemical formulae are used to calculate theoretical isotope distributions subsequently used in the deisotoping algorithm. The m/z values of those ions in the MS spectra that do not correspond to a recorded MS/MS spectrum are searched against a lipid chemical formula database, and identified formulae are used to calculate theoretical isotope distributions for deisotoping, as described further below.…”

“…Unfractionated lipid extracts are directly infused and analyzed by ESI/MS with data-dependent switching to MS/MS mode. MS/MS data are searched against a library of reference spectra for complex lipids constructed from studies of standard glycerophospholipids of all major head-group classes [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28], which we used to establish fragmentation rules [29] that facilitate identification of glycerophospholipids in biological mixtures [30,31]. Structure assignment permits determination of an elemental formula, and a theoretical isotope profile is then calculated and used in a deisotoping algorithm.…”