Fig. 4

LC-ESI–MS/MS spectrum of the surfactin precursors. a–c Surfactin precursors ion [M + H]⁺ at m/z 1022 at retention time 8.05, 8.17, 8.11 min, respectively, containing a C14 β-hydroxy fatty acid chain. d Surfactin precursors ion [M + H]⁺ at m/z 1008 at retention time 8.40 min containing a Glu1-Leu/Ile2-Leu3-Val4-Asp5-Leu6-Val7 peptide and a C14 β-hydroxy fatty acid chain. e Surfactin precursors ion [M + H]⁺ at m/z 1022 at retention time 9.35 min containing a C15 β-hydroxy fatty acid chain