STRUCTURE OF MIXED SHORT-CHAIN LECITHIN LONG-CHAIN LECITHIN AGGREGATES STUDIED BY SMALL-ANGLE NEUTRON-SCATTERING

Small-angle neutron-scattering measurements with extensive internal and external contrast variations have been made to determine the structure of the small aggregates formed spontaneously by mixing 20 mM dipalmitoyl-PC (PC = phosphatidylcholine) with 5 mM diheptanoyl-PC in aqueous solutions at 30-degrees-C. The radii of gyration of the head groups, the hydrocarbon tails of DPPC, and the hydrocarbon tails of diC7PC in the small aggregates are determined to be 81.4, 72.3, and 67.1 angstrom, respectively. The Kratky-Porod plot shows that these small aggregates have dislike structure with a bilayer thickness of about 46 angstrom. The polydisperse unilamellar vesicle model does not fit the measured neutron-scattering spectrum very well. Instead, a polydisperse disk micelle model with a Gaussian size distribution of radii fits the measured neutron-scattering spectrum very well. The mean radius of the disk micelles is found to be 92.3 angstrom, and the half-width at half-maximum of the Gaussian distribution of the radii is 18 angstrom. The thickness of the disk micelle as determined by the fitting is equal to 45 angstrom. It is concluded that these small aggregates are most likely to be disklike mixed micelles.