Supramolecular Tubules of Bile Salt Derivatives

In the past few years it has been greater and greater the interest towards the fabrication of micro or nano structures, for several applicative purposes. These preparations are often based on the formation of supramolecular structures, obtained by self assembly of organic molecules in solution. In particular the self assembly properties of amphiphilic molecules have been exploited because of their ability to generate aggregates with different morphologies, depending on the molecular shape and solution conditions. Among the various surfactants, the bile salts (BS) and some of their derivatives (DBS) seem to be particularly interesting for two main reasons: i) their ability of generating a large variety of supramolecular structures; ii) the fact that, being the bile salts biological surfactant and their derivatives obtained by slight modification, they are expected to be biocompatible and, therefore, potentially useful in biomedical applications. Among the various supramolecular morphologies, tubular structures are especially important since nanotubes can be involved in the preparation of several outstanding nanostructured systems such as membranes, sensors, optoelectronic devices and interconnected liposomes networks. Among the tubule forming surfactants the BS and the DBS have focused the attention of many researchers because of their ability of generating tubules in a wide range of diameter values. In this contribution we report on some of tubule forming DBS obtained by increasing the hydrophobic moiety of sodium cholate. In particular, we will focus on a derivative of sodium cholate. By starting from a viscous aqueous solution of this surfactant, in bicarbonate buffer (pH~10) and at room temperature, it was observed that the tubule formation is induced if the temperature is raised to a value around 40°C. The final tubules have diameters of about 450 nm and a length as large as 7 microns We reported a deep characterization of the tubule formation kinetics by using static light scattering, circular dichroism, small angle X-ray scattering along with transmission electron and optical microscopies. In such a way we tried to provide a well characterized example of self assembling kinetic in the formation of surfactant tubules.