Structural Study of the Micellar Aggregates of Sodium Taurodeoxycholate

Abstract

Previously, fibers of sodium and rubidium salts of glycodeoxycholic and taurodeoxycholic acids have been drawn from aqueous micellar solutions. Their X-ray patterns have been interpreted by means of very similar unit cell parameters and helical structures, formed by trimers, having a 3-fold rotation axis, arranged in 7/1 helices. Micellar aggregates with an aggregation number N ≤ 12 and ≥ 15, having oblate and cylindrical (7/1 helix) shape, respectively, have been proposed for sodium taurodeoxycholate (NaTDC) and satisfactorily verified by means of electromotive force and quasi-elastic light-scattering (QELS) measurements. The aim of this paper is to further check this two-structure model by means of QELS, circular dichroism (CD), and dielectric measurements on NaTDC aqueous solutions within concentration and temperature ranges 10−100 mM and 5−45 °C. The average intensity scattered by the NaTDC samples does not depend on the temperature and can be fitted by two straight lines, which intersect at a concentration about 40 mM. Two structures seem to be present, 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 strength. The CD data agree with the two-structure model assuming that the cylindrical aggregates have an enantioselective ability toward bilirubin-IXα lower than that of the oblate aggregates. The average electric dipole moment μ of a NaTDC monomer has been calculated from dielectric data. The μ values, plotted as a function of NaTDC concentration within the temperature range 5−45 °C, can be fitted by two straight lines and show a break at a concentration that is, once more, about 40 mM. The μ values vary from 33 to 68 D, depending on the temperature and NaTDC concentration. These high values can be justified by a remarkable hydration of the NaTDC aggregates. The moderate decrease of μ when the populations of the aggregates with N ≥ 15 increase can agree with the formation of 7/1 helices, but disagrees with the formation of disordered structures, especially when they have a pseudo center of symmetry. The temperature dependence of the relaxation time follows an Arrhenius law. The molar enthalpy of activation vs NaTDC concentration for the relaxation process can be fitted by two straight lines, which show a break point at a concentration about 40 mM and could be connected with two different structures.