Homocyclic peptide libraries were constructed by self-assembly of amino acids with the assistance of inorganic phosphorus reagents. The construction of homocyclic peptide library increased the diversity of the peptide libraries, which will provide new selectivity for the medicine screening. By electrospray ionization multistage mass. spectrometry (ESI-MS), the fragmentation pathways of [M+H](+) and [M+Na](+) ions of homocyclic peptides were studied in detail. The fragmentation pathways of [M+H](+) ions were different from those of [M+Na](+) ions and their dissociation mechanisms were proposed, respectively. The results enriched the ESI-MS study of cyclic peptide compounds. It was observed that the ESI-MS data of [M+Na](+) ions were easily used to determine the sequence of cyclic peptides. Therefore, the study is helpful for the identification of the structures of analogous cyclic peptide compounds and for the determination of the sequence of cyclic peptides.
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