Fragmentation Patterns of Aromatic 2,5-diketopiperazines Using Liquid chromatography/Mass Spectrometry

Diketopiperazines (DKPs) are cyclic dipeptides which have been detected in a variety of natural products, especially in thermally treated or fermented foods and beverages, providing a metallic bitter taste. DKPs, mainly due to their characteristic heterocyclic system, have been reported to exhibit a broad spectrum of biological activities including antimicrobial, antiviral, antitumor, antihyperglycaemic and antimutagenic. DKPs are formed from unprotected linear dipeptides under basic conditions especially if the necessary head-to-tail folding is not prevented by steric constraints. A methodology to identify DKPs in complex matrices is of high importance. Up to now, only a few studies have reported the MS fragmentation patterns on a certain number of DKPs. The purpose of this study was to develop a liquid chromatography method for separation and identification of DKPs in complex matrices (i.e. food and beverages) as well as a high resolution mass spectrometry method providing accurate full scan MS and MSn data in order to investigate the fragmentation pattern and confirm the chemical formula and structure of DKPs containing aromatic amino acids. Our results which were supported by in silico DFT energy calculations, revealed different and common fragmentation pathways as well as a series of characteristic fragment ions which are representative of the amino acid residues. The role of MSn was signified by the results since it provided a clear picture of the fragmentation cascades. The obtained fragments are diagnostic and could be used to distinguish cyclic dipeptides in different matrices.