On tetramethylammonium ion role in glycodeoxycholate micellar aggregate formation

Previously, fiber and crystal structural models were proposed for bile acid salt micellar aggregates and verified in aqueous solutions. Electromotive force (emf) measurements on sodium salts versus ionic strength, pH, and bil e salt concentration provided micellar aggregate compositions that supporte d tile models. Ionic strength was varied by adding tetramethylammonium chlo ride (TMACl), although tetramethylammonium (TMA(+)) and Na+ ions could inte rfere in the aggregate formation and structure. In this case emf results ca nnot be used for sodium salts. Tetramethylammonium (TMAGDC) and sodium glyc odeoxycholate (NaGDC), which forms helical aggregates constituted by trimer s, are studied to clarify this point. TMAGDC crystal structure is solved, a nd circular dichroism (CD), quasi-elastic light-scattering (QELS), electrol ytic conductance, and dielectric measurements on TMAGDC and NaGDC aqueous s olutions are compared for determining similarities and dissimilarities in t heir behavior. TMA(+) and Na+ coordinations in TMAGDC and NaGDC crystals sh ow that Na+ potential energy is lower than that of TMA(+), thus suggesting a stronger Na+ binding to glycodeoxycholate anion (GDC(-)) aggregates. Bili rubin-Ma (Bn) chiral recognition is sensitive to aggregate structures. BR C D spectra suggest similar structures for TMAGDC and NaGDC anion aggregates. QELS measurements indicate that GDC(-) aggregates have a greater affinity for Na+ ions than for TMA(+) ions and that TMA(+) ions form TMAGDC aggregat es that are smaller than those formed by NaGDC and could interrupt NaGDC ag gregate growth. From conductance data TMA(+) seems to be bound to anion agg regates less than Na+, enhances its interactions when micellar size increas es, could be included together with Cl-, coming from TMACl, into micellar a ggregates, and could be bound through hydrophobic forces with the apolar la teral surface of anion aggregates. High TMAGDC values of the average electr ic dipole moment per monomer mu can be justified by cation and anion hydrat ion. Probably, aggregate composition, population, and structure change slig htly or do not change at all within tile range 15-45 degreesC, where mu is nearly constant. The high single monomer mu (more than 70 D) suggests that TMA(double dagger) is anchored to GDC(-) in dilute solution, thus forming a n ion pair. TMAGDC and NaGDC mu trends are both interpreted assuming a two- structure model and an equilibrium between dimeric and trimeric species. In conclusion, TMAGDC and NaGDC bigger aggregates have similar structures, ev en though the TMAGDC micellar size is smaller than that of NaGDC.