STRUCTURE OF SODIUM AND RUBIDIUM TAURODEOXYCHOLATE MICELLAR AGGREGATES AND THEIR INTERACTION COMPLEXES WITH BILIRUBIN-IX-ALPHA

Previously, a structural model, inferred from a crystal structure, was proposed for the micellar aggregates of sodium taurodeoxycholate (NaT DC). This paper described the crystal structure of rubidium taurodeoxy cholate (RbTDC), solved both to obtain a model for its micellar aggreg ates in aqueous solutions and to check whether the micellar structures of NaTDC and RbTDC were similar. In this case, the rubidium salt can be investigated by resorting to the extended X-ray absorption fine str ucture spectroscopy, which is able to distinguish the nearest-neighbor atoms surrounding the Rb+ ions and allows one to severely test a prop osed structural model. Although the NaTDC and RbTDC crystal structures were different, the structural similarity of the NaTDC and RbTDC mice llar aggregates was verified by studying their interaction complexes w ith bilirubin-IXalpha (BR) as a function of pH and BR concentration by circular dichroism measurements, the bile salts being in both submice llar and micellar form. In order to get more enlightening information about the structures of the NaTDC and RbTDC micellar aggregates, fiber s were drawn from their aqueous solutions. The X-ray patterns of the t wo fibers were characteristic helical patterns and showed a very close resemblance. They were interpreted by means of the same unit cell and helical structure. A comparison with the interaction complexes of the sodium deoxycholate, glycocholate, and taurocholate micellar aggregat es with BR, previously investigated, was accomplished. The enantiosele ctive complexation of BR to dihydroxy or trihydroxy bile salts was rem arkably different and seemed to be mainly dependent on the typical str ucture of the micellar aggregates.