A study on the structure-function relationship of lipopeptide biosurfactants

Arthrofactin (AF) and surfactin (SF) are the most effective cyclic Lipopeptide biosurfactants ever reported. Linear AF and linear SF were prepared by saponification of lactone ring. The oil displacement activities decreased to one third of their respective original values. When residues of both an aspartic acid and a glutamic acid of SF were methylated or amidated, the activity increased by 20%, although their water solubility was lost. When these amino acid residues were modified by aminomethane sulfonic acid, the activity was drastically decreased probably owing to charge repulsion and structural distortion inhibiting micelle formation. Both AF and SF expressed higher activity under alkaline conditions than acidic conditions. AF was more resistant to acidic conditions than SF and it kept high activity even under pH 0.5. Although SF drastically reduced its activity under acidic conditions, surfactin-Asp/Glu-amido ester and surfactin-Asp/Glu-methyl ester retained similar activities irrespective of the pH change. A couple of conformers of SF prepared by reverse-phase HPLC showed the same oil displacement activity but different surface tension-reducing activity. AF was produced as a series of different fatty acid chain lengths (from C8 to C12). Among them, AF with fatty acid chain length of C10, which was the main product of the strain, showed the highest activity.