Unified Fragmentation Pathways of Lithiated, Longer-Chain Acylglycerols Can Be Identified from Tandem Mass Spectrometry and Density Functional Computations.

We extend our previous work on the energetics and mechanisms of fragmentation in the mass spectrometry of triacylglycerols (TAGs). Previously, we proposed viable mechanisms for the collision-induced fragmentation of lithiated tripropionylglycerol using triple-quadrupole mass spectrometry. In this work, we used a QqLIT mass spectrometer to study both double- and triple-stage spectra from a range of TAGs having acid chains of types AAA (identical acid chains), AAB, ABA, and ABC, with chain lengths of 6-18 carbon atoms; we also studied some TAGs having a single double bond in the Δ-9 position. Detailed computations on fragmentation pathways were carried out on lithiated trihexanoylglycerol and on a limited number of longer chain ions. Second-stage fragmentations led to the formation of a lithiated ion after the loss of a neutral acid. With a further input of energy, this ion could rearrange into two other ions, one being formed more easily than the other. In a triple-stage fragmentation, these three ions give a defined series of product ions, some of which are characteristic of the substitution pattern on the glycerol core of the TAG. Computed reaction energies for product ion formation showed comparable trends to the relative intensities of the observed product ion spectra. These results have enabled us to propose unified mechanisms for the double- and triple-stage fragmentation of lithiated TAGs. The mechanisms have reasonable energetics, and all ions, as well as saddle points, have viable geometries. In addition, the fragmentation mechanisms are in accord with all the published experimental mass spectra.