Salt Effects in the Formation of Self-Assembled Lithocholate Helical Ribbons and Tubes

Abstract

The formation of self-assembled nanotubes is usually accounted for by anisotropic elastic properties of membranelike precursors. We present experimental data as evidence of the role played by electrostatics in the formation of self-assembled tubes in alkaline aqueous suspensions of lithocholic acid (LCA). Striking salt effects are characterized by comparing the rheological, dynamical, and scattering properties of systems prepared either in stoichiometric neutralization conditions (SC) of LCA or in a large excess of sodium hydroxide (EOC, experimentally optimized conditions) and finally, in two steps: stoichiometric neutralization followed by an appropriate addition of NaCl (AISC). The SC liquid system is originally made up of loose helical ribbons (previous transmission electron microscopy data), and upon aging they exhibit both intra- and interordering processes. Initially, the helical ribbons are loose and progressively wind around a cylinder (R = 330 Å) with their edges exposed to the solvent. They can be temporarily organized in a centered rectangular two-dimensional lattice (pgg, a = 224 Å, b = 687 Å). Upon further aging, the ribbons wind into more compact helical ribbons (or tubes with helical grooves): their edges are less-exposed and their ordering vanishes. Upon addition of NaCl salt (as in the AISC systems), the specific screening of the intra-aggregate electrostatic repulsions induces the closure of the ribbons into tubes (R(ext) = 260 Å, R(int) = 245 Å as in the EOC systems). Simultaneously with the closure of the ribbons into plain tubes, a drastic enhancement of their interconnectivity through van der Waals attractions develops. Eventually, gels are obtained with networks having hexagonal bundles of tubes.

Gel Formation in Protein Amyloid Aggregation: A Physical Mechanism for Cytotoxicity

“…Terech [19], working with aggregation of lithocholate, found that increasing salt concentration led to a change from helical to cylindrical fiber morphology similar to the differences we observed in lysozyme. He ascribed the difference in fiber morphology to shielding of coulomb repulsion between like-charged structures in the fiber, resulting in a change in the conformation of the fiber helix from an open spiral to a closed tube.…”

Shape selection and multi-stability in helical ribbons

“…Our results are also consistent with the transition between twisted and helical shapes in self-assembled amphiphiles. 35 We also applied our theory to the shape transitions in the central hydrophobic nucleating core of the Ab peptide of Alzheimer's disease, Ab (16)(17)(18)(19)(20)(21)(22). 39,51 In solution, this peptide assembles into purely twisted ribbons or cylindrical helical ribbons.…”

Physiology and Physical Chemistry of Bile Acids

“…The various self-assembly behavior of natural BA includes the formation of tubular aggregates, provided by LCA at strong alkaline conditions (pH = 12) [171,172]. It was shown by Fang and coworkers that pH variation can induce a switch of the tubules from spiral to a straight shape by changing pH [173].…”