Influence of dilution on the physical state of model bile systems: NMR and quasi-elastic light-scattering investigations

Abstract

Multinuclear (1H and 31P) nuclear magnetic resonance (NMR) spectroscopy and quasi-elastic light scattering have been used to characterize molecular aggregates formed in dilute sodium taurocholate--egg lecithin solutions. When mixed micelles (1.25 g/dL) are diluted with 150 mM aqueous sodium chloride, light-scattering measurements suggest a transformation from mixed micelles to unilamellar vesicle species. Decreased 1H NMR line widths for bile salt resonances are consistent with predominance of a monomer form. The concurrent appearance of a second phospholipid choline methyl resonance indicates two types of phospholipid environment in slow chemical exchange: this behavior is consistent with small unilamellar vesicles. The appearance of bilayer vesicles in dilute model bile solutions is confirmed by addition of a lanthanide shift reagent (Pr3+), which splits the 1H or 31P head-group peak into two components with distinct chemical shift sensitivities. These mixed micelle and vesicle aggregates are also distinguished by their susceptibility to the lipolytic enzyme phospholipase A2 from cobra venom.

Structural Evaluation of Phospholipid Bicelles for Solution-State Studies of Membrane-Associated Biomolecules

“…Our hypothesis is that micellar DHPC can solubilize DMPC MLVs, but that upon dilution, the DHPC reverts to monomers and leaves behind DMPC vesicles. There is precedent for such vesicle formation in a system of egg PC solubilized by the bile salt taurocholate (Stark et al, 1985). A similar phenomenon has also been reported in conjunction with the preparation of vesicles using the alcohol injection method (Kensil and Dennis, 1985;Kremer and Wiersema, 1977).…”

Supra-molecular Association and Polymorphic Behaviour In Systems Containing Bile Acid Salts

“…Micelle growth in BS is documented by a lot of experimental evidence, based on viscosity [60-64], DLS and QELS [65][66][67][68][69], NMR self-diffusion [70,71] and so forth [72][73][74]. Such data point out to the existence of an almost continuous association process, driven by surfactant uptake onto micelles and/or by the screening of the electrostatic repulsions occurring between them.…”

Shape Transformations in the Lecithin-Bile Salt System: From Cylinders to Vesicles

“…(A) Static Light Scattering (SLS). The excess Rayleigh ratio A3l(Q) as determined in a static light scattering experiment is calculated by using28 A *(0 = KcM0P(Q)S(Q) (3) where M0 is the molar mass of the particle in solution, P(Q) the particle form factor, and S(Q) the time-averaged structure factor.…”