BILAYER STRUCTURE AND STABILITY IN DIHEXADECYL PHOSPHATE DISPERSIONS

The pH-dependent structure and stability of dihexadecyl phosphate (DHP) bilayer dispersions are determined by using fluorescent and spin probes. The hydrocarbon core region is probed by diphenylhexatriene (DPH) while the polar head/water interface is probed by either trans-parinaric acid (TPA) or an iminoxyl derivative of stearamide spin-label (SSL). At 5 mM NaCl, the fluorescence depolarization (FP) of DPH or TPA goes through a maximum as pH increases for vesicles prepared and maintained at a given pH value. On the other hand, if the pH is changed after vesicle preparation, as during titration, FP against pH is a typical sigmoidal curve. SSL in large vesicles (LV) exhibits a higher degree of spectral anisotropy than in small vesicles (SV) which is interpreted as a deeper penetration of SSL in the LV bilayer due to its higher fluidity. The SV bilayer fluidity increases as a function of time after sonication. From electron microscopy, the number of bilayer fragments in the SV dispersion decreases with increasing pH. For LV up to pH 6, however, the bilayer structure remained unchanged as a function of time. The degree of ionization of the headgroups is suggested to be important in determining the nonstationary bilayer structures occurring during titration. Nevertheless, for stationary DHP bilayers, headgroup hydration and hydrogen-bonding ability are important factors in determining the bilayer structure and vesicle size.