Selective esterification of surfactin : preparation and properties of surfactin methyl esters

The esterification of surfactin by methalonic HCl gave a mixture of two monoesters and a diester. Selective esterification gave surfactin-Glu-γ-methyl ester as a major compound. These compounds were identified by m.s. and by LiAIH 4 reduction of ester groups. With Asp-β-methyl ester and Glu-γ-methyl ester mixture, two association constants with Ca 2+ were found: K=1.3×10 3 M -1 for the aspartic acid residue and K=2.7*10 2 M -1 for the glutamic acid residue. The major monoester, Glu-γ-methyl ester, has a higher surfactant power than surfactin:31 mN/m instead of 34 mN/m and a much higher haemolytic activity: 12 μM instead of 200 μM for 100% haemolysis