Application of liquid chromatography–tandem mass spectrometry for the characterization of galactosylated and tagatosylated β-lactoglobulin peptides derived from in vitro gastrointestinal digestion

This article describes a comprehensive characterization of bovine β-lactoglobulin peptides glycated with an aldohexose (galactose) or a ketohexose (tagatose), derived from in vitro gastrointestinal digestion, by liquid chromatography coupled to positive electrospray ion trap tandem mass spectrometry. In addition to the dissociation pathway previously described for aldohexoses-derived Amadori compounds, i.e. formation of the pyrylium ([M+H]+-54) and furylium ions ([M+H]+-84) via the oxonium ion ([M+H]+-18), another and more direct fragmentation route involving the formation of the imminium ion ([M+H]+-150) is also reported following extensive glycation rates of β-lactoglobulin with both carbohydrates. These results indicated that the analysis of digested proteins by LC-ESI-MS2 on a three-dimensional ion trap monitoring neutral losses is an efficient and direct method to identify peptides glycated not only through the Amadori rearrangement but also via the Heyns rearrangement. Nevertheless, as the predominating MS2 fragmentation pattern of the glycated peptides is derived from the sugar moiety, the sequence-informative b- and y-ions resulting from peptide backbone cleavage were undetected. To overcome this drawback, and taking advantage of multi-stage fragmentation capabilities of ion traps, the indicative Amadori and Heyns-derived imminium ions were successfully used in MS3 analyses to identify the peptide backbone, as well as the specific glycation site. In addition, further MS4 analyses were needed to carry out the characterization of doubly glycated peptides.