Structure of Sodium Glycodeoxycholate Micellar Aggregates from Small-Angle X-ray Scattering and Light-Scattering Techniques

Small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) measurements were accomplished
on sodium glycodeoxycholate (NaGDC) aqueous electrolyte solutions as a function of NaGDC and NaCl
concentrations with the aim to determine with satisfactory approximation the NaGDC micellar aggregate
structure at a gross molecular level, assuming monodispersity. Different conditions of interparticle interactions
by varying ionic strength (NaCl concentration from 0 to 0.70 M) and NaGDC concentration (from 0.02 to
0.10 M) were studied. Smeared scattering intensities and electron pair distance distribution functions, radii of
gyration, and aggregate heights are in satisfactory agreement with the corresponding functions calculated
using a 21 helix as model. It is formed by trimers, each one composed by three NaGDC and nine H2O molecules
related by a 3-fold rotation axis, and can be described by a hollow cylinder, probably filled by water molecules,
characterized by a conventional radius of 23.7 Å and a trimer repeat along the helical axis of 3.6 Å. The helix
is considerably inhomogeneous since the volume of the cylinder occupied by NaGDC molecules is less than
one-third of the total. On the other hand, calculations performed with the average radial electronic density of
the helix without water molecules or totally filled by water molecules (a NaGDC/H2O molecular ratio of
about 1/50) or by using a three-shell average radial electronic density, independently evaluated on absolute
scale, do not show significant differences, thus supporting the helical model. The aggregate size increases for
all the samples by increasing either the NaCl or NaGDC concentration. The NaGDC low concentration (0.02
M) samples with NaCl within the range 0.30−0.70 M are characterized by short cylindrical aggregates that
do not give rise to sensible interference effects. This assertion is supported by the satisfactory fit between the
observed apparent mean hydrodynamic radii and the calculated ones by means of the method of Ortega and
Garcia de la Torre (J. Chem. Phys. 2003, 119, 9914), valid for rods with a length-to-diameter ratio ≥0.1 in
dilute solution (noninteracting rods). The NaGDC moderate concentration (0.10 M) samples with NaCl within
the range 0.20−0.60 M are characterized by cylindrical aggregates that, in the presence of repulsive Coulombic
interactions progressively more and more screened, produce interference effects, due to the hard-body repulsion
and attractive forces, but the agreement between observed and calculated SAXS data is satisfactory. The
results of the low and moderate NaGDC concentration samples seem to indicate that the aggregation number
increase, produced by adding 0.10 M NaCl, is constant within an ionic strength range and occurs by the
addition of oligomers formed by trimers. The samples with a variable NaGDC concentration (0.02−0.10 M)
at a fixed and high NaCl concentration (0.6 M) contain cylindrical aggregates that give rise to an attractive
term effect prevailing on the hard-body repulsive one. The same situation seems to occur in the NaGDC
moderate concentration samples.