4.3 Article

Energy and entropy at play in competitive dissociations: The case of uneven positional dissociation of ionized triacylglycerides

Journal

INTERNATIONAL JOURNAL OF MASS SPECTROMETRY
Volume 352, Issue -, Pages 77-86

Publisher

ELSEVIER SCIENCE BV
DOI: 10.1016/j.ijms.2013.06.027

Keywords

Triacylglyceride; Ammonium adduct; CID; Fragmentation; RRKM; Mass spectrometry

Funding

  1. Natural Sciences and Engineering Research Council of Canada
  2. NSERC

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MS/MS of triacylglycerides (TAG) complexed to cations such as NH4+ can allow for the positions of the fatty acids (FA) on the glycerol backbone to be determined. Collision-induced dissociation (CID) of an ammoniated TAG results in the neutral loss of NH3 and a FA. It has been reported that the FA at the central position (sn-2) dissociates less favourably than the terminal fatty acids (sn-1 or sn-3) yet no solid energetic explanation for this has been provided. MS/MS of ammoniated POP, OPO and POL triacylglycerides at multiple collision energies and computational analysis are here used to explain the different dissociation rates of the FAs from the glycerol backbone. The loss of sn-1 and sn-3 FAs are found to have lower activation energies than the loss of the sn-2 position FA, however the loss of the FA from the sn-2 position is more entropically favourable. Theoretical MS/MS breakdown curves were fit to experimental values using RRKM theory to estimate critical energy (E-0)) of dissociation of FAs from the three positions. The E-0, values for cleavage from the sn-1 and sn-3 positions were found to be approximately 1.52 eV, while that for the sn-2 position was highly dependent on the identity of the FA at that position. Computational structures and energy analysis suggest that an important step in the dissociation of [TAG+NH4](+) is the loss of ammonia. In a model system, glyceryl tributyrate, the loss of NH3 produced two distinct [TAG+H](+) product structures sitting 148 kJ and 160 kJ in energy above the ammoniated structure. The [TAG+H](+) structure that leads to the loss of the sn-1(3) is 12 kJ lower than the [TAG+H](+) structure that leads to the loss of the sn-2 FA. From this, the loss of a neutral FA that follows sits only an additional 35-48 kJ above the [TAG+H](+) structures. (C) 2013 Elsevier B.V. All rights reserved.

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