Aggregate Properties of Sodium Deoxycholate and Dimyristoylphosphatidylcholine Mixed Micelles

Mixed micelles of the phospholipid dimyristoylphosphatidylcholine (DMPC) and bile salts of sodium deoxycholate (NaDC) were investigated by a combination of techniques, including time-resolved fluorescence quenching (TRFQ), electron spin resonance (ESR), viscometry, pulsed-gradient spin-echo NMR (PGSE-NMR), and surface tensiometry. Aggregation numbers, and bimolecular collision rate constants of guest molecules confined in the micelles (by TRFQ), interfacial hydration index and microviscosity, (by ESR), axial ratio (from solution viscosity), micelle self-diffusion coefficient (by PGSE-NMR), and the critical micelle concentrations (from surface tension) were determined for various molar compositions defined by the ratio R identical with [NaDC]/[DMPC] and concentrations ([NaDC]+[DMPC]). The data interpretation showed the micelles to be polydisperse rods. Aggregate properties depend on the ratio, R and reveal behavior unlike that in micelles of surfactants with aliphatic nonpolar chains. With increase in concentration from [NaDC] = 0.010 M to [NaDC] = 0.200 M, the hydration index and the aggregation number exhibit non-monotonic variations. Formulation of a polar shell model for cylindrical micelles yielded a set of nonlinear equations for the structural features of the micelle. The solutions give the microstructural description of the mixed micelle that includes the length, diameter, number of water molecules in the hydration shell, and the monomer organization in the micelle.