Sodium taurodeoxycholate structure from solid to liquid phase

A 7/1 helix was previously identified by X-ray diffraction analysis as the structural unit of a sodium taurodeoxycholate (NaTDC) fiber drawn from an aqueous micellar solution and proposed as a model of NaTDC micellar aggregates. The repetitive unit of the 7/1 helix is a trimer formed by three NaTDC molecules related by a 3-fold rotation axis. This model was supported by a study of NaTDC aqueous micellar solutions performed with different experimental techniques. Moreover, the phase behavior of the NaTDC-water system was recently studied as a function of concentration and temperature with emphasis on concentrated regions beyond the isotropic solution phase. In the present paper, X-ray measurements carried out on NaTDC concentrated aqueous solutions and fibrils that grow from these solutions are presented. Experimental results strongly support the following. The NaTDC aggregates are helices formed by trimers both in concentrated aqueous solutions and in fibrils. Fibrils are composed of the same 7/1 helices of the fiber, whereas concentrated aqueous solutions contain helices with a larger cross section and a shorter identity period along the helical axis than the 7/1 helices. The trimer repeat along the helical axis and the radius are, respectively, about 6.4 and 10.2 Angstrom in the fibrils and 3.6 and 15.8(5) Angstrom in the concentrated aqueous solutions. Both of the helices are packed into rectangular unit cells which can be derived from trigonal or hexagonal unit cells. Because the trimer seems to flatten by decreasing the concentration, the radius of the structural unit in the isotropic solution phase should be greater than about 15.85 A. These results permit a reasonable guess about the structural evolution of NaTDC aggregates from the solid to the liquid phase. Some suggestions about the sodium deoxycholate and taurocholate behavior are also provided.