Deoxycholic acid and l-Phenylalanine enrich their hydrogel properties when combined in a zwitterionic derivative

J Colloid Interface Sci. 2019 Oct 15:554:453-462. doi: 10.1016/j.jcis.2019.07.019. Epub 2019 Jul 8.


Hypothesis: Sodium Deoxycholate (NaDC) and Phenylalanine (Phe) are important biological hydrogelators. NaDC hydrogels form by lowering the pH or by increasing the ionic strength. Phe gels form from saturated solution by thermal induction and slow kinetics. The resulting gels hold great potential in medicine and biology as drug carriers and models for fundamental self-assembly in pathological conditions. Based on this background it was hypothesized that a Phe substituted NaDC could provide a molecule with expanded gelling ability, merging those of the precursors.

Experiments: We coupled both building blocks in a zwitterionic derivative bearing a Phe residue at the C3 carbon of NaDC. The specific zwitterionic structure, the concurrent use of Ca2+ ions for the carboxyl group coordination and the pH control generate conditions for the formation of hydrogels. The hydrogels were analyzed by combining UV and circular dichroism spectroscopies, rheology, small angle X-ray scattering and atomic force microscopy.

Findings: Hydrogel appearance occurs in conditions that are uncovered in the case of the pure Phe and NaDC: self-standing gels form instantaneously at room temperature, in the 10-12 pH range and down to concentration of 0.17 wt%. Both thixotropic and shake resistant gels can form depending on the derivative concentration. The gels show an uncommon thermal stability in the scanned range of 20-60 °C. The reported system concurrently enriches the hydrogelation properties of two relevant building blocks. We anticipate some potential applications of such gels in materials science where coordination of metal ions can be exploited for templating inorganic nanostructures.

Keywords: Bile salts; Hydrogels; Metallogels; Phenylalanine; Zwitterionic gels.