Concentration-dependent diffusion of bile salt/phospholipid aggregates

Bile salt/phospholipid aggregates are present as concentrated, anionic colloids in the gallbladder. After mixing with dietary lipids, these aggregates diffuse through the hydrated, negatively charged glycoprotein network at the intestinal surface prior to absorption. Analysis of these complicated processes requires a characterization of the lipid aggregate as well as its diffusion. Moreover, the excluded-volume effects on the lipid aggregate diffusion need to be addressed. Thus, the diffusion coefficients of bile salt, phospholipid, and water were obtained from four series of lipid dilutions by a Fourier transform pulsed-field gradient spin-echo ¹H NMR diffusion method. Dialysis experiments were performed to provide an independent estimate of the intermicellar concentration. Four lipid solutions with different total phospholipid/bile salt ratios were then diluted with bile salt solutions containing the approximate intermicellar concentration. With these four series of solutions, the isopotential specific volume of the micelle, calculated from density measurements, was found to increase with increasing phospholipid/bile salt (PC/BS) molar ratio in the micelle. The hydration of micelle and intermicellar concentration decreased with increasing PC/BS molar ratio in the micelle but was independent of the micellar concentration, as deduced from the NMR measurements. After the theoretical correction for the excluded-volume effect was applied, the size of the micelle was found to increase significantly upon dilution in relation to the corresponding decrease of the PC/BS ratio in the micelle. Series 1 fit well with an excluded volume factor based on hard core spheres. Series 2 yielded physically unreasonable sizes when either spherical or rod corrections were applied. Series 3 and 4 were well fit by rod-shaped excluded volume corrections but only if the flexibility and possible interpenetrability of the micelles were considered.