Marine cyclic peptide Axinastatin 1 was synthesized. In this process the peptide was sequenced by tandem mass spectrometry. Based on the b_x－y_z pathway, the linear peptide was sequenced through complementarities of the sequence information supplied by b and y ions in the same MS² spectrum. The cyclopeptide was sequenced based on the b_x→b_x－1 pathway. Two sets of b ions were obtained through sequentially removing one amino acid residue in each stage of MS/MS. According to above information, the cyclopeptide sequence was elucidated and the cyclic structure was concluded. The rearrangement of b ions was also observed. All the mechanisms were discussed and verified by semiempirical PM3 and AM1 quantum-chemical calculations. For the rearrangement of b ions, a hypothesis of translation structure was proposed.

Key words: cyclic peptide, sequencing, tandem mass spectrometry, fragmentation pathway