Tandem mass spectrometry of model peptides modified with trans-2-hexenal, a product of lipid peroxidation.

Small molecules formed during lipid peroxidation can react with the basic groups in proteins through different mechanisms. Recently, substituted pyridinium moieties were observed during in vitro incubations of lysine-containing peptides with 2-alkenals. To explore the dissociation behavior of peptides with pyridinium-derivatized lysine residues, the peptide ions created through either matrix-assisted laser desorption/ionization or electrospray ionization were studied with tandem mass spectrometry. The permanently charged pyridinium ions fragment primarily through the charge-remote processes. Under high energy collision-induced dissociation, a number of diagnostic ions were observed that could potentially be used to identify modified residues in proteins. The origins of these ions were studied using deuterium exchange and higher-order mass spectrometry experiments using an ion trap instrument. Rational structures for these ions are proposed.