| Abstract
| - Previously, fiber and crystal structural models were proposed for bile acid salt micellar aggregates andverified in aqueous solutions. Electromotive force (emf) measurements on sodium salts versus ionic strength,pH, and bile salt concentration provided micellar aggregate compositions that supported the models. Ionicstrength was varied by adding tetramethylammonium chloride (TMACl), although tetramethylammonium(TMA+) and Na+ ions could interfere in the aggregate formation and structure. In this case emf resultscannot be used for sodium salts. Tetramethylammonium (TMAGDC) and sodium glycodeoxycholate(NaGDC), which forms helical aggregates constituted by trimers, are studied to clarify this point. TMAGDCcrystal structure is solved, and circular dichroism (CD), quasi-elastic light-scattering (QELS), electrolyticconductance, and dielectric measurements on TMAGDC and NaGDC aqueous solutions are compared fordetermining similarities and dissimilarities in their behavior. TMA+ and Na+ coordinations in TMAGDCand NaGDC crystals show that Na+ potential energy is lower than that of TMA+, thus suggesting astronger Na+ binding to glycodeoxycholate anion (GDC-) aggregates. Bilirubin-IXα (BR) chiral recognitionis sensitive to aggregate structures. BR CD spectra suggest similar structures for TMAGDC and NaGDCanion aggregates. QELS measurements indicate that GDC- aggregates have a greater affinity for Na+ions than for TMA+ ions and that TMA+ ions form TMAGDC aggregates that are smaller than those formedby NaGDC and could interrupt NaGDC aggregate growth. From conductance data TMA+ seems to bebound to anion aggregates less than Na+, enhances its interactions when micellar size increases, couldbe included together with Cl-, coming from TMACl, into micellar aggregates, and could be bound throughhydrophobic forces with the apolar lateral surface of anion aggregates. High TMAGDC values of theaverage electric dipole moment per monomer μ can be justified by cation and anion hydration. Probably,aggregate composition, population, and structure change slightly or do not change at all within the range15−45 °C, where μ is nearly constant. The high single monomer μ (more than 70 D) suggests that TMA+is anchored to GDC- in dilute solution, thus forming an ion pair. TMAGDC and NaGDC μ trends are bothinterpreted assuming a two-structure model and an equilibrium between dimeric and trimeric species. Inconclusion, TMAGDC and NaGDC bigger aggregates have similar structures, even though the TMAGDCmicellar size is smaller than that of NaGDC.
|
