An Unconventional Zwitterionic Bolaamphiphile Containing PEG as Spacer Chain: Surface Tension and Self-Assembly Behavior.

Abstract

Monolayer lipid membrane formation based on self-assembly of bolaamphiphiles containing hydrophobic spacer are well-established in the literature, but monolayer vesicle formation by so-called hydrophilic poly(ethylene glycol) (PEG) spacer has not been reported to date. Here, a novel l-cysteine-derived bolaamphiphile with PEG as spacer has been developed and characterized. The interfacial properties and the solution behavior of the amphiphile were investigated in pH 7.0 at 25 °C. The self-assembly properties of the bolaamphiphile in aqueous buffer were investigated by using different techniques, such as surface tensiometry, fluorescence spectroscopy, UV-vis spectroscopy, isothermal titration calorimetry, dynamic light scattering, transmission electron microscopy, and atomic force microscopy. Surprisingly, despite having so-called polar spacer in between two polar head groups, it exhibits formation of microstructures in aqueous buffer as well as in water at 25 °C. The molecule undergoes self-organization leading to the formation of monolayer vesicles with hydrodynamic diameters between 100 and 250 nm in a wide range of concentration. The thermodynamic parameters clearly suggest that the aggregate formation is mainly driven by the hydrophobic effect. The monolayer vesicles were found to form at a very low concentration (≥0.63 mM) and within a wide pH range (2-10). The vesicles exhibit excellent shelf life at physiological temperature.