Charge Inversion of Phospholipids by Dimetal Complexes for Positive Ion-Mode Electrospray Ionization Mass Spectrometry Analysis

Abstract

Phospholipids are vital constituents of living cells, as they are involved in signaling and membrane formation. Mass spectrometry analysis of many phospholipids is preferentially performed in the negative ion-mode because of their acidic nature. Here we have studied the potential of a digallium and dizinc complex to charge-invert a range of different types of phospholipids and measured their ion yield and fragmentation behavior in positive ion-mode tandem mass spectrometry. The dimetal complexes bind specifically the phosphate groups of phospholipids and add an excess of up to three positive charges per phosphate group. Three different phosphoinositide phosphates (mono-, di-, and triphosphorylated inositides), a phosphatidic acid, a phosphatidylcholine, a phosphatidylethanolamine, and a phosphatidylglycerol were investigated. The intensities obtained in positive ion-mode of phosphoinositide phosphates and phosphatidic acid bound to {LGa 2 } 5+ were between 2.5- and 116-fold higher than that of the unmodified lipids in the negative ion-mode. Native phosphoinositide ions yielded upon CID in the negative ion-mode predominantly product ions due to losses of H 3 PO 4, PO 3 - and H 2 O. In comparison, CID spectra of {LGa 2 } 5+ -bound phosphoinositides generally resulted in fragment ions corresponding to loss of the full diglyceride chain as well as the remaining headgroup bound to {LGa 2 } 5+ as the most abundant peaks. A number of signature fragment ions of moderate abundance were observed that allowed for distinction between the three regioisomers of 1,2-di(9Z-octadecenoyl)-sn-glycero-3-[phosphoinositol-x,y-bisphosphate] (PI(3,4)P 2, PI(3,5)P 2, PI(4,5)P 2).