STRUCTURAL INVESTIGATIONS OF DIHEXADECYL PHOSPHATE SMALL UNILAMELLAR VESICLES

Freeze-fracture electron microscopy and light scattering methods have been used to determine morphologies of dihexadecyl phosphate (DHP) vesicles formed by ultrasonication. Contrary to an earlier report, the data indicate that the vesicles are highly hydrated spherical, or nearly spherical, particles with mean hydrodynamic radii of 10-13 nm. Vesicle sizes were unchanged when formed in the presence of any of a series of strongly binding N-methyl-N'-alkyl-4,4'-bipyridinium ions or when N-methyl-N'-hexyl-4,4'-bipyridinium ion was adsorbed to the outer interface of preformed vesicles, but the vesicles underwent progressive swelling with addition of increasing amounts of N-methyl-N'-hexadecyl-4,4'-bipyridinium chloride. This behavior was interpreted to indicate a bimodal binding pattern whereby the short-chain viologen bound at interfacial sites on the membrane surface, and the long-chain analogue intercalated within the bilayer plane, forming part of the membrane structure. The gel-to-liquid crystalline phase transition determined by differential scanning calorimetry was highly dependent upon the composition of the aqueous buffer used. In particular, phase transition temperature shifts and multiple thermogram peaks were observed with cationic amines, indicative of specific interactions with the DHP phosphate headgroups. The complex phase transition behavior inferred for DHP in tris(hydroxymethyl)aminomethane was confirmed by examination of the temperature-dependent Raman spectra of the vesicles.