ABSTRACT
Matrix_assisted laser desorption ionization_time of flight tandem mass spectrometry ( MALDI_TOF/TOF MS) and electrospray ionization_quadrupole_time of flight mass spectrometry ( ESI_Q_TOF MS) were used to confirm the structure of cyclic lipopeptide daptomycin fastly. First, the relative molecular weight 1916. 7107 of daptomycin was measured by ESI with error 0. 0007. The sample’s doubly charged peak m/z 809. 848 was selected as precursor ion for ESI_MS/MS analysis, and the exocyclic amino acid sequence C9 H19 CO_Trp_Asn_Asp was successfully matched. Second, the experimental conditions of cleaving daptomycin by lithium hydroxide ( LiOH) were optimized and the ring_opened process was monitored by MALDI_TOF/TOF MS. After obtaining ring_opened product with purity of above 95%, the MS/MS measurements by MALDI and ESI were carried out. The b+and y+of ring_opened product were completely matched, which confirmed the amino acid sequence of daptomycin. Finally, ESI_MS/MS conditions of ring_opened product were further optimized to obtain more low mass fragment ions for analyzing the structure of fatty acid chain and the cleavage pattern of fat chain in mass spectrometry was proposed. The method was fast, convenient, accurate and reliable for identifying cyclic lipopeptide compounds.