ORGANIZATION OF PHOSPHATIDYLCHOLINE AND BILE-SALT IN RODLIKE MIXED MICELLES

Small-angle neutron scattering (SANS) of aqueous mixed colloids of the bile salts glycocholate or glycochenodeoxycholate with egg yolk phosphatidylcholine and mixtures of taurocholate with dipalmatoylphosphatidylcholine (DPPC) shows that all of these systems form particles with rodlike morphology in parts of the concentration-composition phase map. The formation of rodlike mixed micelles thus is a general feature of the phosphatidylcholine-bile salt systems. The orientation of the phosphatidylcholine and bile salts in the rod-shaped mixed micelles is addressed by SANS measurements of the aqueous mixtures of taurocholate with DPPC and with DPPC with the choline methyls deuterated. The results are consistent with the phosphatidylcholine being arranged in the rod such that the long axis is perpendicular to the axis of the rodlike particle. The amount of bile salt in the rods calculated from the small-angle neutron scattering assuming radial arrangement of phosphatidylcholine suggests that the bile salt is inserted with its long axis parallel to the rod axis. Comparison of the cross-sectional radius of gyration and contrast-weighted mass per unit length of the egg yolk phosphatidylcholine-bile salt mixtures with those from the DPPC-bile salt mixtures indicates that this, arrangement of the two components is a general feature of these systems.