Electrospray ionization multiple-stage linear ion-trap mass spectrometry for structural elucidation of triacylglycerols: Assignment of fatty acyl groups on the glycerol backbone and location of double bonds

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Abstract
Linear ion-trap multiple-stage mass spectrometric approach (MSn) towards nearly complete structural elucidation of triacylglycerol (TAG) including (1) assignment the fatty acid substituents on the glycerol backbone and (2) location of the double bond(s) on the unsaturated fatty acyl groups is reported. The characterization is established by the findings that MS2 on the [M+Li]+ ions of TAG yields more abundant ions reflecting losses of the outer fatty acid substituents either as free acids (i.e., [M+Li-R1CO2H]+ and [M+Li-R3CO2H]+ ions) or as lithium salts (i.e., [M+Li-R1CO2Li]+ and [M+Li-R3CO2Li]+ ions) than the ions reflecting the similar losses of the inner fatty acid substituent (i.e., [M+Li-R2CO2Li]+ and [M+Li-R2CO2Li]+ ions). Further dissociation (MS3) of [M+Li-RnCO2H]+ (n=1, 2, or 3) gives rise to the ion series locating the double bonds along the fatty acid chain. These ions arise from charge-remote fragmentations involving β-cleavage with γ-H shift, analogous to those seen for the unsaturated long-chain fatty acids characterized as initiated ions. Significant differences in abundances in the ion pairs reflecting the additional losses of the fatty acid moieties, respectively, were also seen in the MS3 spectra of the [M+Li-RnCO2H]+ and [M+Li-RnCO2Li]+ ions, leading to confirmation of the fatty acid substituents on the glycerol backbone. MSn on the [M+Na]+ and [M+NH4]+ adduct ions also affords location of fatty acid substituents on the glycerol backbone, but not the position of the double bond(s) along the fatty acid chain. Unique ions from internal losses of the glycerol residues were seen in the MS3 spectra of [M+Alk-RnCO2H]+ (n=1, 2, 3) and of [M+Alk-RnCO2Alk]+ (Alk=Li, Na, NH4; n=1, 3). They are signature ions for glycerides and the pathways leading to their formation may involve rearrangements.

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