Lichenysin - A more efficient cation chelator than surfactin

The lipopeptide lichenysin (cyclo-[L-Gln1 -->D-Leu2 -->L-Leu3 -->L-Val4 --> L-Asp5 -->D-Leu6 -->L-Ile7-beta -OH fatty acid]) produced by Bacillus lich eniformis structurally resembles surfactin from Bacillus subtilis. The main difference is the presence of a glutaminyl residue in position 1 of the pe ptide sequence in place of glutamic acid in surfactin. This local variation causes significant changes in the properties of the molecule compared to s urfactin. Lichenysin has a higher surfactant power, the critical micellar c oncentration (c.m.c.) being strongly reduced from 220 to 22 muM and a much higher hemolytic activity because 100% hemolysis was observed with only 15 muM instead of 200 muM Lichenysin is also a better chelating agent because its association constants with Ca2+ and Mg2+ are increased by a factor of 4 and 16, respectively. This effect is assigned to an increase in the access ibility of the carboxyl group to cations owing to a change in the side chai n topology induced by the Glu/Gln exchange. Additionally, the propensity of the lipopeptide for extensive hydrophobic interactions, as illustrated by its low c.m.c., contributes to further stabilization of the cation and an i ncrease in the partitioning of lichenysin into the erythrocyte membrane. Ou r data support the formation of a lichensyin-Ca2+ complex in a molar ratio of 2:1 instead of 1:1 with surfactin, suggesting an intermolecular salt bri dge between two lichenysin molecules. Therefore, when Ca2+ ions are present in the solution, micellization occurs via a dimer assembly, with a possibl e long-range effect on the spatial arrangement of the micelles or other sup ramolecular structures. Finally, among all the surfactin peptidic variants so far known, lichenysin is the one for which the three tested activities a re the most substantially improved.