STRUCTURAL STUDY OF THE MICELLAR AGGREGATES OF SODIUM TAURODEOXYCHOLATE

Previously, fibers of sodium and rubidium salts of glycodeoxycholic an d taurodeoxycholic acids have been drawn from aqueous micellar solutio ns. Their X-ray patterns have been interpreted by means of very simila r unit cell parameters and helical structures, formed by trimers, havi ng a 3-fold rotation axis, arranged in 7/1 helices. Micellar aggregate s with an aggregation number N less than or equal to 12 and greater th an or equal to 15, having oblate and cylindrical (7/1 helix) shape, re spectively, have been proposed for sodium taurodeoxycholate (NaTDC) an d satisfactorily verified by means of electromotive force and quasi-el astic light-scattering (QELS) measurements. The aim of this paper is t o further check this two-structure model by means of QELS, circular di chroism (CD), and dielectric measurements on NaTDC aqueous solutions w ithin concentration and temperature ranges 10-100 mM and 5-45 degrees C. The average intensity scattered by the NaTDC samples does not depen d on the temperature and can be fitted by two straight lines, which in tersect at a concentration about 40 mM. Two structures seem to be pres ent, one prevailing above and the other below this concentration. The change of structure, which occurs during the growth of the aggregates, has been monitored by means of CD spectra increasing the ionic streng th. The CD data agree with the two-structure model assuming that the c ylindrical aggregates have an enantioselective ability toward bilirubi n-IX alpha lower than that of the oblate aggregates. The average elect ric dipole moment mu of a NaTDC monomer has been calculated from diele ctric data. The mu values, plotted as a function of NaTDC concentratio n within the temperature range 5-45 degrees C, can be fitted by two st raight lines and show a break at a concentration that is, once more, a bout 40 mM. The mu values vary from 33 to 68 D, depending on the tempe rature and NaTDC concentration. These high values can be justified by a remarkable hydration of the NaTDC aggregates. The moderate decrease of mu when the populations of the aggregates with N greater than or eq ual to 15 increase can agree with the formation of 7/1 helices, but di sagrees with the formation of disordered structures, especially when t hey have a pseudo center of symmetry. The temperature dependence of th e relaxation time follows an Arrhenius law. The molar enthalpy of acti vation vs NaTDC concentration for the relaxation process can be fitted by two straight lines, which show a break point at a concentration ab out 40 mM and could be connected with two different structures.