Oxidized cholesterol bilayers. Dependence of electrical properties on degree of oxidation and aging

Black lipid membranes made from oxidized cholesterol were examined for their specific resistance, capacitance, and physical stability, as a function of cholesterol oxidation time and of age. Membranes formed from cholesterol oxidized in n-octane were not physically stable even after 7 h of oxidation unless they were aged for at least a month. Membranes formed from cholesterol oxidized in decane and tetradecane (1 : 1) were stable immediately after 2--6 h of oxidation. Oxidation times outside this range produced unstable membranes. After 1 month storage, membranes from cholesterol solutions oxidized in decane and tetradecane from 0.75--3 h were stable. After 11 months, only the 0.75 oxidation time produced stable membranes. Storage in nitrogen retarded the aging process. After initial forming of the membrane, total membrane area and capacity increased and then stabilized, although specific capacity and resistance did not change, indicating inherent stability in the bilayer's intrinsic electrical properties. Bilayers formed soon after cholesterol oxidation had membrane capacity which ranged from 0.42 to 0.55 muF/cm2. Specific membrane resistance ranged initially from 2 . 10(6) to 37 . 10(6) omega/cm2 in 0.2 M NaCl with lower resistances in the more oxidized membranes. With aging, membrane capacity decreased gradually over 11 months to values approaching 0.1 muF/cm2 indicating membrane thickening. Membrane resistance ordinarily decreases with storage time. The rate of these changes with age is dependent on the extent of initial cholesterol oxidation and subsequent oxidation, with long term stability best in the least oxidized membranes.