Structure-property relationships in metallosurfactants

The morphology of micelles formed by three sub-classes of metallosurfactants—those with macrocyclic, linear and gemini head groups—has been studied by small-angle neutron scattering (SANS) for a series of metal- and counter-ions. All the data may be described by a model that invokes a globular micelle morphology in which the dimensions of the micelle are consistent with the known chemical structure of the constituent groups within the metallosurfactant. For two macrocyclic head group metallosurfactants, viz.1-(2-hydroxy-tetradecyl)-1,4,7-triazacyclonane that forms predominantly spherical micelles and 1-(2-hydroxy-tetradecyl)-1,4,7,10-tetraazacyclononane that forms disc-like micelles, the metal ion and its counter-ion have a negligible effect on the morphology of the micelle. Binary mixtures of surfactants with these two macrocyclic head groups (with homo- or hetero-metal ions/counter-ions) form mixed micelles whose morphology is an average of the two single component micelles. Further, as found for the single surfactant solutions, the metal and counter-ion had no effect on the morphology of the mixed surfactant micelle. Lastly, the micelle morphology of two gemini surfactants was also shown to be insensitive to the number and nature of the metal and counter-ions present, but sensitive to the structure of the head group. These observations considerably extend our understanding of the relationship between chemical structure and micelle morphology for these interesting molecules

Micelles obtained by aggregation of gemini surfactants containing the CCK8 peptide and a gadolinium complex

Two gemini surfactants, [C18CysL5CCK8]2 and [C18CysDTPAGlu]2, containing, respectively, the CCK8 peptide and the DTPAGlu chelating agent or its gadolinium complex have been prepared by linking lipophilic chains through a disulfide bond between two cysteine residues. The two surfactants aggregate in water solution forming pure or mixed micelles, with a critical micellar concentration in the 5 9 10-6–5 9 10-5 mol kg-1 range, as measured by fluorescence spectroscopy. As indicated by small-angle neutron scattering, the shape and size of the micelles are influenced by the temperature: increasing temperature leads to progressive reduction of the size of the supramolecular aggregates. Cylindrical structures found at lower temperatures (10–40 C) evolve into ellipsoidal micelles at 50–80 C. Furthermore, the surface-exposed CCK8 peptide changes its conformation above a transition temperature of approximately 45 C, going from a beta-sheet to a random-coil structure, as indicated by circular dichroism measurements. The mixed aggregate obtained by coaggregation of the two gemini-based amphiphilic compounds, [C18CysDTPAGlu(Gd)]2 and [C18CysL5CCK8]2 in 70:30 molar ratio, represents the first example of a peptide-containing gemini surfactant as a potential target-selective contrast agent in MRI. In fact, it presents a high relaxivity value of the gadolinium complex, 21.5 mM-1 s-1, and the CCK8 bioactive peptide exposed on the external surface is therefore capable of selective targeting of the cholecystokinin receptors