Self-aggregation mechanism of a naphthylamide cationic derivative of cholic acid. From fibers to tubules

The aggregation behavior of a cationic derivative of cholic acid {[3 beta, 5 beta, 7 alpha, 12 alpha]-3-(2-naphthoylamino)-7,12-dihydroxycholan- 24-triethylamonium iodide} has been studied by surface tension measurements, fluorescence spectroscopy, transmission electronic microscopy (TEM), and circular dichroism. The critical aggregation concentration, the fraction of bound counterions and thermodynamic parameters for the formation of aggregates have been determined, as well as the morphology of the aggregates. TEM images support a consecutive transformation mechanism from fibers to tubules, these having a well-defined geometry and being the only structure observed at the end of the process. Intermediate observed structures are helical ribbons.