Photoinduced intermolecular cross-linking of gas phase triacylglycerol lipid ions

Complex mixtures of plant-derived triglycerol (TG) lipids are commonly used as feedstock components for the production of industrial polymers. However, there remains a need for the development of analytical strategies to investigate the intrinsic intermolecular cross-linking reactivity of individual TG molecules within these mixtures as a function of their structures and physicochemical properties, and for the characterization of the resultant products. Here, to address this need, we describe a novel multistage tandem mass spectrometry based method for intermolecular cross-linking and subsequent structural characterization of TG lipid ions in the gas phase. Cross-linking reactions were initiated using 266 nm ultraviolet photodissociation tandem mass spectrometry (UVPD-MS/ MS) of saturated or unsaturated TG dimers introduced via electrospray ionization into a linear ion trap mass spectrometer as noncovalent complexes with protonated 3,4-, 2,4-or 3,5-diiodoaniline (diIA). UVPD resulted in the initial formation of an anilinyl biradical via the sequential loss of two iodine radicals, which underwent further reaction to yield multiple cross-linked TG products along with competing noncross-linking processes. These chemistries are proposed to occur via sequential combinations of hydrogen abstraction (H-abstraction), radical addition, and radical recombination. Multistage collision-induced dissociation tandem mass spectrometry (CID-MSn) was used to obtain evidence for the structures and mechanisms of formation for these products, as a function of both the TG lipid and diIA ion structures. The efficiency of the UVPD reaction was shown to be dependent on the number of unsaturation sites present within the TG lipids. However, when unsaturation sites were present, the formation of the cross-linked and noncross-linked product ions via H-abstraction and radical addition mechanisms was found to be competitive. Finally, the identity of the anilinyl biradical (e.g., 3,4-versus 2,4-substituted) was found to significantly affect the distribution of these two types of product ions. Importantly, owing to the observed propensity for cross-linking to occur via H-abstraction-initiated processes, this novel gas-phase cross-linking reaction provides a convenient method to link two molecules covalently without the requirement of any specific functional group, and therefore could be applied to examine the gas-phase intermolecular interactions and cross-linking of a wide range of biomolecular classes.