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Accueil > Séminaires > Archive des séminaires d’Utinam > Archive des séminaires de physique (jusqu’en 2011) > 2009

Aggregation behavior of bile acid salts in aqueous environment

par Edith Burgey -

Pal Jedlovszky

ELTE University of Budapest (Hungary)

Mardi 03 février 2009

Résumé : The aggregation behaviour of two bile acid salts, i.e., sodium cholate and
sodium deoxycholate has been studied in their aqueous solutions of three different
concentrations, i.e., 30 mM, 90 mM and 300 mM by means of molecular dynamics
computer simulation. In order to let the systems reach thermodynamic equilibrium
rather long simulations have been performed : the equilibration period, lasting for
20-50 ns, has been followed by a 10-20 ns long production phase, during which the
average size of the bile aggregates (regarded to be the slowest varying observable) has
already fluctuated around a constant value. The production phase of the runs has been
about an order of magnitude longer than the average lifetime of both the monomeric bile
ions and of the bonds that stick two neighbouring bile ions together to be part of the
same aggregate. This has allowed the bile ions belonging to various aggregates to be in
a dynamic equilibrium with the isolated monomers.
The observed aggregation behaviour of the studied bile ions has been found to
be in a good qualitative agreement with experimental findings. The analysis of the
results have revealed that, due to their molecular structure, which is markedly different
from that of the ordinary aliphatic surfactants, the bile ions form rather different
aggregates than the usual spherical micelles. In the lowest concentration solution
studied the bile ions are only forming small oligomers. In the case of deoxycholate these
oligomers, such as the ordinary micelles, are kept together by hydrophobic interactions,
whereas in the sodium cholate system small hydrogen bonded aggregates (mostly
dimers) are also present. In the highest concentration systems the bile ions are forming
large secondary micelles, which are kept together both by hydrophobic interactions and
hydrogen bonds. Namely, in these secondary micelles small, hydrophobic primary
micelles are linked together via formation of hydrogen bonds between their hydrophilic
outer surfaces.
Besides the mechanism of the aggregation we also analyzed the shape of the
aggregates and counterion binding of the micelles. We have found that the primary
micelles are of somewhat flattened, disk-like shapes, whereas the secondary micelles
might have rather irregular shapes, as well. In order to resolve the apparent
contradiction between the experimental values of the counterion binding measured in
different way, we have calculated its value (i) regarding only the contact Na+ions, and
(ii) regarding also the solvent separated ions as bound ones. We found that the
contradicting experimental results are originated in the fact that solvent separated ions
are seen as bound ones by some of the experimental methods, and not seen as bound
ones by some other methods.