Electrical noise from lipid bilayer membranes in the presence of hydrophobic ions

In the presence of the hydrophobic ion dipicrylamine, lipid bilayer membranes exhibit a characteristic type of noise spectrum which is different from other forms of noise described so far. The spectral density of current noise measured in zero voltage increases in proportion to the square of frequency at low frequencies and becomes constant at high frequencies. The observed form of the noise spectrum can be interpreted on the basis of a transport model for hydrophobic ions in which it is assumed that the ions are adsorbed in potential-energy minima at either membrane surface and are able to cross the central energy barrier by thermal activation. Accordingly, current-noise results from random fluctuations in the number of ions jumping over the barrier from right to left and from left to right. On the basis of this model the rate constant ki for the translocation of the hydrophobic ion across the barrier, as well as the mean surface concentration Nt of adsorbed ions may be calculated from the observed spectral intensity of current noise. The values of ki obtained in this way closely agree with the results of previous relaxation experiments. A similar, although less quantitative, agreement is also found for the surface concentration Nt.

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

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 larger than or equal to 4-10(-5). A minimal concentration of about 6-10(11) dipicrylamine ions per cm2 of the membrane could be detected with this instrument. The dipicrylamine concentration in the membrane obtained with the optical method Ntopt is compared with the concentrations Ntel obtained from simultaneous electrical relaxation meausurements. Ntopt and Ntel 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 Ntopt was maximally four times higher than Ntel. The significance of this difference is discussed together with general aspects of the saturation phenomenon.

Spectrophotometric determination of diphenhydramine hydrochloride using dipicrylamine

A spectrophotometric procedure for the determination of diphenhydramine hydrochloride based on the reaction with dipicrylamine was developed. A yellow complex forms and is easily extractable by chloroform at pH 5. The mole ratio of diphenydramine hydrochloride to dipicrylamine in the complex is 1:3. The absorbance of the complex obeys Beer's law over the concentration range of 3-10 mug of diphenhydramine hydrochloride per ml of chloroform. This procedure can be carried out in the presence of other compounds without interference.