Based on the [M+H]+ ions, the molecular masses of Pelgipeptins A and B were determined to be 1072 and 1100 Da, respectively. In order to characterize the primary structures of these two antibiotics, the [M+H]+ ions were chosen as precursor ions for further CID analysis. As shown in the MS–MS spectra (Figs 1 and 2), sets of fragment ions were observed and the tentative sequences of Pelgipeptin A (Dab–Val–Leu/Ile–X1–Dab–Val–Dab–Phe–Leu/Ile) and Pelgipeptin B (Dab–Val–Leu/Ile–X2–Dab–Leu/Ile–Dab–Phe–Leu/Ile) were revealed, in which X are still undetermined and ambiguity still remained regarding the Leu/Ile
identification. Z-VAD-FMK molecular weight Dab is a nonproteinogenic amino acid, which represents 2,4-diaminobutyric acid. In addition, the amino acid analysis indicated the presence of l-Dab, d-Phe, l-Leu/Ile, d-Val, l-Val and l-Ser in Pelgipeptin A and l-Dab, d-Phe,
l-Leu/Ile, d-Val and l-Ser in Pelgipeptin B, suggesting that l-Ser was present in X. Leu could not be differentiated from Ile due to the same molecular mass and nearly identical retention time. When compared with the public Dab-containing antibiotics, Pelgipeptins were found to be structurally related to the members of the polypeptin family: BMY-28160 and permetin A (Takeuchi et al., 1979; Sugawara et al., 1984). The molecular mass of Pelgipeptin B was identical to that of permetin A, and their partial find more amino acid sequences were very similar (Fig. 2), suggesting that they were probably the SB-3CT same compound.
Furthermore, Pelgipeptin A and BMY-28160 were probably analogues as they shared similar amino acid sequences and differed from each other by a molecular mass of 14 Da (-CH2) (Fig. 1). Thus, Pelgipeptin A was unequivocally characterized as a new antibiotic of the polypeptin family. In order to determine the inhibitory spectra of the purified antibiotics, the MICs of these compounds against a number of fungi, gram-positive and gram-negative bacteria were measured using microdilution methods (Table 1). Both Pelgipeptins A and B showed inhibitory activity against all the indicator strains; however, their antimicrobial potencies were obviously different. Of the five soil-borne fungal pathogens, Fusarium oxysporum CGMCC 3.2830 were shown to be the most sensitive fungal strain tested to Pelgipeptin A with an MIC of 12.5 μg mL−1, while the most sensitive fungi to Pelgipeptin B were F. oxysporum CGMCC 3.2830 and Fusarium moniliforme CGMCC 3.4759, having an MIC of 6.25 μg mL−1. The other fungal strains including Rhizoctonia solani CGMCC 3.2871, Colletotrichum lini CGMCC 3.4486 and Fusarium graminearum CGMCC 3.4598 were highly susceptible to Pelgipeptin B with an MIC value of 12.5 μg mL−1. Of the several bacterial strains, Staphylococcus epidermidis CMCC 26069 showed the highest sensitivity to both Pelgipeptins A and B with MICs of 3.12 and 0.