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Free keywords:
DMPC-surfactin-aggregates; differential scanning calorimetry; static light scattering; cryo-electron microscopy; small angle neutron scattering; "Poor Man's" temperature-jump-relaxation spectroscopy; bacillus-subtilis; lipopeptide; biosurfactant; vesicles; acid; temperature; microscopy; monolayers; mechanism; membranes
Abstract:
To understand the biological action of surfactin from Bacillus subtilis we investigated its effects on the phase transition of L-alpha-dimyristoyl phosphatidylcholine (DMPC)-vesicles from the crystalline to the fluid state using differential scanning calorimetry; light scattering; small angle neutron scattering and cryo-clectron microscopy. DSC-thermograms revealed two phase transition peaks. Light scattering profiles showed two branches with characteristic hysteresis phenomena. With both techniques the same values of the phase transition temperatures T-m1 and T-m2 of 23.5 and 23 degrees C were obtained indicating two forms of DMPC-surfactin aggregates which could be visualized by cryo-electron microscopy. Until 4 mol % surfactin the vesicular form predominated, but was accompanied by bilayered membrane fragments by increasing the biosurfactant concentrations. At surfactin concentrations higher than 15 mol% smaller DMPC-surfactin micelles of ellipsoidal conformation were formed, as demonstrated by small angle neutron scattering. In addition, by "Poor Man's" temperature-jump-relaxation spectroscopy slow transients in the phase transition of vesicular DMPC-surfactin aggregates with relaxation times of 20-30 s were detected which presumably indicate the slow dissipation of intermediate lipid-and surfactin domains formed after the main phase transition on the way to the fluid state. This process is accelerated by surfactin.
