X-Ray, QELS, and CD Studies of the Micellar Aggregates of Calcium Taurodeoxycholate

Calcium taurodeoxycholate (CaTDC) has been studied in order to throw light on the structure of its micellar aggregates and on the effect of the competition between calcium and sodium ions for the formation of aggregates in aqueous solution. A fiber has been drawn from a gel formed in an aqueous micellar solution of CaTDC. Its X-ray pattern shows a close resemblance with those of the fibers drawn from aqueous micellar solutions of the sodium and rubidium salts of glycodeoxycholic and taurodeoxycholic acids (NaGDC, RbGDC, NaTDC, and RbTDC). Therefore, it has been likewise interpreted by means of a structure composed of trimers arranged in 7/1 helices. The helical structures of the anions are similar for CaTDC, NaGDC, RbGDC, NaTDC, and RbTDC. Quasi-elastic light-scattering measurements, carried out on CaTDC aqueous solutions as a function of ionic strength, point out a micellar growth greater or smaller than that of NaTDC or calcium glycodeoxycholate, respectively. The NaTDC micellar size increases much more with CaCl2 than with an amount of NaCl corresponding approximately to the same ionic strength. It is probable that the Ca2+ ions substitute the Na+ ions and join the NaTDC micellar aggregates, forming aggregates of bigger size. Moreover, the addition of NaCl to CaTDC and CaCl2 gives rise to a lower apparent hydrodynamic radius of the micellar aggregates, confirming that the substitution of Ca2+ ions with Na+ ions decreases the micellar size. Circular dichroism (CD) spectra, using bilirubin-1Xa (BR) as probe molecule, have been recorded as a function of the Ca2+ concentration and pH. The enantioselective ability displayed by the CaTDC micellar aggregates, although less than that of NaTDC or RbTDC, is an evidence of their chiral structure and resembles that observed for the dihydroxy salts. However, at low concentration of Ca2+ the CD spectra change with time and show the progressive rise of a band in the region around 490 nm. Two isosbestic points indicate the occurrence of equilibrium between chromophoric species. Their optical activities can depend on changes of the BR conformation and self-aggregated state as well as on the interactions of BR with Ca2+ ions, free anions, small oligomers, and micellar aggregates of CaTDC. Complexes between BR, Ca2+ ions, and taurodeoxycholate anions seem to be responsible for the changes with time of the CD spectra. The CaTDC micellar aggregates decrease their enantioselective ability by increasing the ionic strength and are associated by BR at high ionic strength.