Physiology and Physical Chemistry of Bile Acids.

Bile acids (BAs) are facial amphiphiles synthesized in the body of all vertebrates. They undergo the enterohepatic circulation: they are produced in the liver, stored in the gallbladder, released in the intestine, taken into the bloodstream and lastly re-absorbed in the liver. During this pathway, BAs are modified in their molecular structure by the action of enzymes and bacteria. Such transformations allow them to acquire the chemical-physical properties needed for fulling several activities including metabolic regulation, antimicrobial functions and solubilization of lipids in digestion. The versatility of BAs in the physiological functions has inspired their use in many bio-applications, making them important tools for active molecule delivery, metabolic disease treatments and emulsification processes in food and drug industries. Moreover, moving over the borders of the biological field, BAs have been largely investigated as building blocks for the construction of supramolecular aggregates having peculiar structural, mechanical, chemical and optical properties. The review starts with a biological analysis of the BAs functions before progressively switching to a general overview of BAs in pharmacology and medicine applications. Lastly the focus moves to the BAs use in material science.

Bile Salts: Natural Surfactants and Precursors of a Broad Family of Complex Amphiphiles.

Bile salts (BSs) are naturally occurring rigid surfactants with a steroidal skeleton and specific self-assembly and interface behaviors. Using bile salts as precursors, derivatives can be synthesized to obtain molecules with specific functionalities and amphiphilic structure. Modifications on single molecules are normally performed by substituting the least-hindered hydroxyl group on carbon C-3 of the steroidal A ring or at the end of the lateral chain. This leads to monosteroidal rigid building blocks that are often able to self-organize into 1D structures such as tubules, twisted ribbons, and fibrils with helical supramolecular packing. Tubular aggregates are of particular interest, and they are characterized by cross-section inner diameters spanning a wide range of values (3-500 nm). They can form through appealing pH- or temperature-responsive aggregation and in mixtures of bile salt derivatives to provide mixed tubules with tunable charge and size. Other derivatives can be prepared by covalently linking two or more bile salt molecules to provide complex systems such as oligomers, dendrimers, and polymeric materials. The unconventional amphiphilic molecular structure imparts specific features to BSs and derivatives that can be exploited in the formulation of capsules, drug carriers, dispersants, and templates for the synthesis of nanomaterials.

Bioderived, chiral and stable 1-dimensional light-responsive nanostructures: Interconversion between tubules and twisted ribbons.

HYPOTHESIS: Self-assembling molecular structures responding to light stimulus are appealing for applications as sensing and drug delivery. Supramolecular nanotubes have a relevant potential in nanotechnology as they can be used to encapsulate different loads like drugs, biological macromolecules, and nanomaterials. In addition, they are suitable elements for novel supracolloidal materials. Structural responses of supramolecular nanotubes to non-invasive stimuli are very much desired to enable controlled release of the encapsulated guests and to provide these recently developed new materials with an external trigger. Here, we describe the formation of well-defined, single wall tubules that interconvert into twisted ribbons upon UV-light exposure in aqueous environment. The structures are provided by self-assembly of an azobenzene substituted cholic acid, a biological surfactant belonging to the family of bile acids. The azobenzene group allows for the light responsiveness of the molecular packing. Concurrently the steroidal moieties assure both chiral features and extensive hydrophobic interactions for time and temperature resistant aggregates. EXPERIMENTS: The molecular packing interconversion was followed by circular dichroism. Microscopy, small angle X-ray scattering and light scattering measurements demonstrated the drastic morphological variation upon irradiation. A model of the molecular arrangement within the tubular walls was suggested based on the circular dichroism spectra simulation. FINDINGS: Innovatively, the molecular design reported in our work allows for encoding in the same light responsive system multiple desirable features (e.g. bio-origin, temperature resistance and chirality of the aggregates). Such combination of properties, never reported before for a single molecule, might be relevant for the realization of robust, stimuli-responsive bio-vectors.

Revealing the complex self-assembly behaviour of sodium deoxycholate in aqueous solution.

HYPOTHESIS: Sodium deoxycholate is a natural bile salt produced by animals and fulfilling important physiological processes. It is also used as dispersive surfactant and building block for self-assembled architectures in biology and material science. Although long debated, the study of its self-assembly in water is hereto incomplete and the models of the known aggregates are still controversial. This background suggests a complex scenario likely missing of additional mesophases. EXPERIMENTS: Electron and optical microscopy techniques were crossed with SAXS data for the research. FINDINGS: Novel rod, sponge, vesicle, lamellae, nanotube phases and reversible transitions among them arise at conditions (concentration, pH, temperature, ionic strength, ionic composition) fitting the physiological working environment of sodium deoxycholate. These findings enlarge the perspective towards different directions. The integration of the previous literature with this work removes any interpretative contradiction since all the structures cover the entire spectrum of phases expected for surfactants, thus being explained according to the Israelachvili's scheme. It is not trivial that a single molecule can show such a high structural variability. This fact highlights a very versatile system. Probably it is not a coincidence that it occurs in a multitasking biomolecule. These results furnish fundamental knowledge to clarify the bile salts' role in vivo.

Supracolloidal Atomium.

Nature suggests that complex materials result from a hierarchical organization of matter at different length scales. At the nano- and micrometer scale, macromolecules and supramolecular aggregates spontaneously assemble into supracolloidal structures whose complexity is given by the coexistence of various colloidal entities and the specific interactions between them. Here, we demonstrate how such control can be implemented by engineering specially customized bile salt derivative-based supramolecular tubules that exhibit a highly specific interaction with polymeric microgel spheres at their extremities thanks to their scroll-like structure. This design allows for hierarchical supracolloidal self-assembly of microgels and supramolecular scrolls into a regular framework of "nodes" and "linkers". The supramolecular assembly into scrolls can be triggered by pH and temperature, thereby providing the whole supracolloidal system with interesting stimuli-responsive properties. A colloidal smart assembly is embodied with features of center-linker frameworks as those found in molecular metal-organic frameworks and in structures engineered at human scale, masterfully represented by the Atomium in Bruxelles.

C-12 vs C-3 substituted bile salts: An example of the effects of substituent position and orientation on the self-assembly of steroid surfactant isomers.

Biomolecule derivatives are transversally used in nanotechnology. Deciphering their aggregation behavior is a crucial issue for the rational design of functional materials. To this end, it is necessary to build libraries of selectively functionalized analogues and infer general rules. In this work we enrich the highly applicative oriented collection of steroid derivatives, by reporting a rare example of C-12 selectively modified bile salt. While nature often exploits such position to encode functions, it is unusual and not trivial to prepare similar analogues in the laboratory. The introduction of a tert-butyl phenyl residue at C-12 provided a molecule with a self-assembly that remarkably switched from rigid pole-like structures to twisted ribbons at a biologically relevant critical temperature (∼25 °C). The system was characterized by microscopy and spectroscopy techniques and compared with the C-3 functionalized analogue. The twisted ribbons generate samples with a gel texture and a viscoelastic response. The parallel analysis of the two systems suggested that the observed thermoresponsive self-assemblies occur at similar critical temperatures and are probably dictated by the nature of the substituent, but involve aggregates with different structures depending on position and orientation of the substituent. This study highlights the self-assembly properties of two appealing thermoresponsive systems. Moreover, it adds fundamental insights hereto missing in the investigations of the relation between self-assembly and structure of synthetic steroids, which are valuable for the rational design of steroidal amphiphiles.