G. Briganti et al., STRUCTURAL STUDY OF THE MICELLAR AGGREGATES OF SODIUM AND RUBIDIUM GLYCODEOXYCHOLATE AND TAURODEOXYCHOLATE, Langmuir, 12(5), 1996, pp. 1180-1187
Some salts of bile acids increase the size of their micellar aggregate
s in aqueous solutions by varying parameters such as pH or ionic stren
gth and give rise to transitions: aqueous micellar solution --> gel --
> fiber --> crystal. Helical fibers of sodium and rubidium salts of gl
ycodeoxycholic and taurodeoxycholic acids (NaGDC, RbGDC, NaTDC, and Rb
TDC, respectively) were drawn from aqueous solutions and investigated
by means of X-ray diffraction analysis to clarify the structure of the
ir micellar aggregates and to verify whether the micellar structures o
f the sodium and rubidium salts were similar. In this case, the rubidi
um salts can be studied in place of the sodium ones by extended X-ray
absorption fine structure spectroscopy. The X-ray patterns of the NaGD
C, RbGDC, NaTDC, and RbTDC fibers showed a close resemblance and were
interpreted by means of very similar unit cell parameters and helical
structures, formed by trimers arranged in 7/1 helices. Calculations of
interatomic distances provided a model of the 7/1 helix which satisfa
ctorily packs into the unit cells of the fibers. Quasi-elastic light-s
cattering measurements carried out on aqueous micellar solutions up to
a concentration of NaCl or RbCl of about 0.3 M supported the similari
ty among the structures of the NaGDC, RbGDC, NaTDC, and RbTDC micellar
aggregates. Oligomers of small size were observed without NaCl or RbC
l. The extent of micellar growth increased upon increasing NaCl or RbC
l concentration. The micellar size, which seemed to be dependent on th
e peculiar cation-anion Coulombic interaction starting from a NaCl or
RbCl concentration of about 0.2 M, was in the following order: NaGDC >
NaTDC approximate to RbTDC > RbGDC. Electromotive force measurements
accomplished on NaTDC solutions at constant ionic medium provided the
distribution of micellar sizes. Most of the aggregates have aggregatio
n numbers that are a multiple of 3. The fraction of bound Na+ ions is
high in accordance with an ordered structure like that of the 7/1 heli
x. Further support for the 7/1 helix came from the calculation of mean
hydrodynamic radii using the helical model. The agreement with quasi-
elastic light-scattering measurements at low bile salt concentration a
nd within a wide range of ionic strength was satisfactory. The helical
structure of the anions seems to be similar in the micellar aggregate
s of NaGDC, RbGDC, NaTDC, RbTDC, and tetramethylammonium taurodeoxycho
late.