X-RAY-STUDIES, QELS-STUDIES, AND CD-STUDIES OF THE MICELLAR AGGREGATES OF CALCIUM TAURODEOXYCHOLATE

Calcium taurodeoxycholate (CaTDC) has been studied in order to throw l ight on the structure of its micellar aggregates and on the effect of the competition between calcium and sodium ions for the formation of a ggregates in aqueous solution. A fiber has been drawn from a gel forme d 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 ta urodeoxycholic acids (NaGDC, RbGDC, NaTDC, and RbTDC). Therefore, it h as been likewise interpreted by means of a structure composed of trime rs arranged in 7/1 helices. The helical structures of the anions are s imilar for CaTDC, NaGDC, RbGDC, NaTDC, and RbTDC. Quasi-elastic light- scattering measurements, carried out on CaTDC aqueous solutions as a f unction of ionic strength, point out a micellar growth greater or smal ler than that of NaTDC or calcium glycodeoxycholate, respectively. The NaTDC micellar size increases much more with CaCl2 than with an amoun t of NaCl corresponding approximately to the same ionic strength. It i s probable that the Ca2+ ions substitute the Na+ ions and join the NaT DC micellar aggregates, forming aggregates of bigger size. Moreover, t he addition of NaCl to CaTDC and CaCl2 gives rise to a lower apparent hydrodynamic radius of the micellar aggregates, confirming that the su bstitution of Ca2+ ions with Na+ ions decreases the micellar size. Cir cular dichroism (CD) spectra, using bilirubin-1Xa (BR) as probe molecu le, have been recorded as a function of the Ca2+ concentration and pH. The enantioselective ability displayed by the CaTDC micellar aggregat es, 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 c hromophoric species. Their optical activities can depend on changes of the BR conformation and self-aggregated state as well as on the inter actions of BR with Ca2+ ions, free anions, small oligomers, and micell ar aggregates of CaTDC. Complexes between BR, Ca2+ ions, and taurodeox ycholate anions seem to be responsible for the changes with time of th e CD spectra. The CaTDC micellar aggregates decrease their enantiosele ctive ability by increasing the ionic strength and are associated by B R at high ionic strength.