X-ray, electrolytic conductance, and dielectric studies of bile salt micellar aggregates

Previously, structural models, observed in fibers and crystals, were propos ed for sodium deoxycholate (NaDC), glycodeoxycholate (NaGDC), taurodeoxycho late (NaTDC), and taurocholate (NaTC) micellar aggregates, and were verifie d in aqueous solutions by means of several techniques. Here we report the X -ray analysis of sodium glycocholate (NaGC) fibers, which indicates that Na GC micellar aggregates could be formed by dimers and octamers as in the cas e of NaTC. Moreover, we present electrolytic conductance and dielectric mea surements on NaGDC, NaTC, and NaGC aqueous micellar solutions to verify our micellar aggregate models. Specific conductance values of 0.1 mol dm(-3) N aDC, NaTDC, NaGDC, NaTC, and NaGC solutions containing NaCl at concentratio n ranging from 0 to 0.8 mol dm(-3) practically do not depend on the particu lar bile salt. Comparison with NaCl values shows that bile salt contributio n to conductance decreases by increasing NaCl concentration, is nearly zero around the concentration range 0.5-0.6 mol dm(-3), and becomes negative at higher concentration. This behavior can be explained if Na+ ions strongly interact with bile salt anions and reinforce their interaction when micella r size increases. Even the inclusion of Na+ and Cl- ions, coming from NaCl, into micellar aggregates cannot be excluded, especially at high ionic stre ngth. NaDC, NaTDC, NaGDC, NaTC, and NaGC present high values of the average electric dipole moment per monomer mu that can be justified by a remarkabl e hydration of their micellar aggregates. Reasonably, micellar aggregate co mposition and population change very slightly or do not change at all withi n the temperature range 15-45 degreesC, because mu is nearly constant in th is interval. Results also suggest that Na+ ions are anchored to anions in d ilute solution, thus forming ion pairs in the case of NaTC and NaGC, at lea st. Dihydroxy and trihydroxy bile salts are characterized by very similar c ation-anion interaction strengths, even though their Structures are differe nt. The trend of mu, which moderately decreases by increasing bile salt con centration, agrees with our structural models and can be due to coexistence of two structures, at least.