Electron capture dissociation of complexes of diacylglycerophosphocholine and divalent metal ions: Competition between charge reduction and radical induced phospholipid fragmentation

Abstract

Divalent metal complexes of phosphocholines, [Metal II (L)n] 2ϩ (where Metal ϭ Cu 2ϩ , Co 2ϩ , Mg 2ϩ , and Ca 2ϩ , L ϭ 1,2-dihexanoyl-sn-glycero-3-phosphocholine [6:0/6:0GPCho] and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine [16:0/18:1GPCho] and n ϭ 2-5), were formed upon electrospray ionization mass spectrometry (ESI/MS) of 8 mM solution of phosphocholine (L) with 4 mM metal salt (Metal). The electron capture dissociation (ECD) reactions of these [Metal II (L) n ] 2ϩ complexes were examined via Fourier-transform ioncyclotron resonance mass spectrometry. A rich and complex chemistry was observed, including charge reduction and fragmentation involving losses of a methyl radical, trimethylamine, and the acyl chains. The predominant reaction channel was dependent on the size (n) of the complex, the metal and ligand used, and the size of the acyl chain. Thus charge reduction dominates the ECD spectra of the larger phosphocholine, 16:0/18:1GPCho, but is largely absent in the smaller 6:0/6:0GPCho. For complexes of 16:0/18:1GPCho, n ϭ 4 -5, fragmentation from the head group mainly occurs via loss of the methyl radical and trimethylamine. At n ϭ 3, the relative abundance of fragments due to loss of acyl chain radicals increases. The abundances of ions arising from these radical losses increase further for the n ϭ 2 complexes, thereby providing information on the composition and position of the 16:0 and 18:1 acyl groups. Thus ECD of metal complexes provides structurally useful information on the phosphocholine, including the nature of the head group, the acyl chains, and the positions of the acyl chains. complexes of L ϭ 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, which allows distinction of acyl chains [9]. We were thus intrigued by the possibility of using electron capture dissociation (ECD) on divalent metal complexes of phospholipids to gain additional structural information, since recent studies suggest that divalent metal ions not only can direct the fragmentation of peptides under conditions of ECD [10 -13], but also act as charge tags to allow the use of ECD on other biomolecules that may not undergo multiple protonation under electrospray ionization (ESI) conditions [13].

Collision-induced dissociation of doubly-charged barium-cationized lipids generated from liquid samples by atmospheric pressure matrix-assisted laser desorption/ionization provides structurally diagnostic product ions

“…The dominant product ions were recorded for the fatty acids at the sn2 site, which is in agreement with previous research that indicates the sn2 bond as being more labile in CID of [L+H] + and [L+Li] + [41]. In contrast, ECD of PC (16:0/18:1) has been reported exhibiting almost equal signal intensities for ions resulting from the neutral loss of fatty acids from each site [5]. A preference for the most unsaturated fatty acid was not detected although this had been suggested with Ag + adduct ions previously [15].…”

“…Complexes with a high coordination number (>10) were produced with a biphasic ESI (BESI) source but again no detection of [L+M] 2+ was reported in this study either [19]. Another study investigated ECD MS/MS of complexes [5]. While double bond location was again not possible, the stereochemistry of the fatty acids could be inferred through a preference for neutral loss at sn1.…”

“…Typically, ETD and electron capture dissociation (ECD) rely on the reduction of the positive charge state for fragmentation to occur, so at least two charges are required on precursor ions for the MS detection of charged product ions [4,5].…”

Formation of Peptide Radical Cations (M+·) in Electron Capture Dissociation of Peptides Adducted with Group IIB Metal Ions

“…Several researchers have studied the impact of replacing the mobile protons with fixed charges using functionalities such as quaternary amino groups [20][21][22][23]. Our group and other researchers have focused on the use of divalent and trivalent metal ions as charge carriers [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. Among different charge carriers, metal ions are of particular interest.…”

“…Second, the diverse reactivities of metal ions in gas phase might provide additional structural information. ECD with metal ions as charge carriers have been used to characterize O-sulfated tyrosine [24], petrobacin [25], metabolites [26], oligosaccharides [27], phosphocholines [28], supramolecular complexes [29], and dendrimers [30].…”