Fig. 4. Structural variation among the heterologously produced darobactin derivatives. The side chain moieties of the different amino acids in positions 2, 4–6 and 7 (see numbered circles) that were observed in the heterologously produced darobactin derivatives (see also Table 1) are shown in the grey-shaded areas. Cysteine side chains in positions 4 and 6 led to formation of an adduct with a mass shift of +191.026 Da and a potential adduct sum formula of C₆H₉NO₄S based on ChemCalc prediction (discussed in more detail in the text). Amino acid side chain moieties of derivatives, which were not observed upon expression of the respective pNOSO construct with altered darA sequence in E. coli BL21 (DE3), are not shown (darobactins 1, 5 and 7).