Abstract
Small-angle neutron scattering data for a series of five asymmetric short-chain lecithins in aqueous solutions are analyzed in terms of a thermodynamic model for a system of spherocylindrical micelles. This analysis yields five parameters which uniquely characterize the micellar system formed by each lecithin: N/sub 0/, the aggregation number of the minimum size micelle; delta/kT, the free energy change for a monomer when it is inserted into the cylinder section of the micelle; ..delta../kT, the free energy change when N/sub 0/ monomers aggregate to form a minimum size micelle; R/sub c/, the transverse radius of gyration; and N/L, the aggregation number per unit micelle length. Growth from the minimum micelle size is determined by the difference in free energy of monomers in the end caps vs. the same number of monomers in the cylindrical section, (..delta..-N/sub 0/ delta)/kT. This value is related to the total number of carbons in the fatty acyl chains. It is estimated to be 7 for dihexanoylphosphatidylcholine (12 carbons) and increases to 12, 16, and 23 for lecithins with 13, 14, and 15 carbons, respectively. The conformational nonequivalence of the two fatty acyl chains affects the critical micelle concentrations, the average micelle sizes, and hence themore » extracted free energy parameters. For compounds with the same total number of fatty acyl carbons, the longer S/sub N/1 chain species produce smaller, more slowly growing micelles with slightly lower critical micelle concentrations. This can be understood if the short-chain lecithin molecule in micelles assumes a conformation similar to that of the long-chain lecithin in bilayers. /sup 1/H NMR data assessing the magnetic nonequivalence of the terminal methyl groups are consistent with this view.« less
Published Version
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