Cyclic dipeptides are relatively simple compounds that can exhibit a great variety of important biological activities. The fragmentation pathways of protonated cyclic dipeptides have been studied by electrospray ionization multistage mass spectrometry (ESI-MSn). The mass spectra studies of the cyclic dipeptides showed that the cyclic dipeptides with the similar substituents, the side chains of amino acid residues at the diketopiperazine ring, followed the same fragmentation pathway. In the fragmentation spectra of protonated cyclic dipeptides, some characteristic fragment ions were observed and could be used to distinguish the cyclic dipeptides. The hydrogen/deuterium (H/D) exchange experiment and the high-resolution mass spectrometry (Q-TOF) were used to verify and rationalize the proposed fragmentation pathways. These observations may have some potential applications in the structural elucidation and interpretation of the mass spectra of homologous compounds.

In the work described in this paper, chrysin was phosphorylated by a modified Atheron-Todd reaction. The structure of phosphorylated chrysin was determined by elemental analysis, NMR, ESI-MS/MS, and X-ray data. Electrospray ionization results showed that the phosphorylated flavonoids could form noncovalent complexes with many proteins, such as lysozyme, myoglobin, bovine insulin, and cytochrome c, while noncovalent complexes were not detected in the mixed solution of the chrysin and proteins. The research shows that the phosphorylated flavonoids possess relatively stronger affinities and form noncovalent complexes with the proteins more easily than the non-phosphorylated compounds.

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