Properties of bilayer membranes in the presence of dipicrylamine. A comparative study by optical absorption and electrical relaxation measurements

Abstract

In order to test the question if a pool of lipophilic ions may exist in black lipid membranes which cannot be detected by electrical relaxation measurements we have performed simultaneously measurements of the optical absorption of a lipophilic ion. The absorbance of membrane-bound dipicrylamine at 410 nm was measured with a sensitive spectrophotometer which can detect absorbance changes ⩾ 4 · 10 −5 . A minimal concentration of about 6 · 10 11 dipicrylamine ions per cm 2 of the membrane could be detected with this instrument. The dipicrylamine concentration in the membrane obtained with the optical method N t opt is compared with the concentrations N t el obtained from simultaneous electrical relaxation measurements. N t opt and N t el agreed at low dipicrylamine concentrations (10 −8–10 −7 M in the aqueous phase) and showed saturation at higher concentrations (up to 5 · 10 −6 M). In the saturation range N t opt was maximally four times higher than N t el . The significance of this difference is discussed together with general aspects of the saturation phenomenon.