THERMODYNAMIC STUDIES OF THE BINDING INTERACTIONS OF SURFACTIN ANALOGUES TO LIPID VESICLES Application of isothermal titration calorimetry

Isothermal titration calorimetry was applied for studying the binding interactions of cyclic and linear surfactins with different ionic charge (z=-2 and -3) and lipid chain length (n=14 and 18) to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidyl-choline ( POPC) vesicles in 10 mM Tris buffer at pH 8.5 with 150 mM NaCl at 25 degrees C. Surfactin analogues interacted spontaneously(Delta G(D)(w -> b) < 0) with POPC vesicles. The binding reactions were endothermic(Delta H(D)(w -> b) 0) and entropy-driven process(Delta S(D)(w -> b) 0) Moreover, significant differences in the binding constant values (K) ranging from 6.6.10(3) to 9.6.10(4) M(-1) show that cyclic structure and the increase of lipid chain length are favourable on the surfactin binding affinity to POPC vesicles, whereas the rise of the number of negative charges has an opposite effect.