Structural changes and oscillation of electrical resistance in a millipore doph model membrane

Abstract

Calcium ions in KCl solution change the surface structure of a Millipore DOPH model membrane and reduce its hydrophilic area. A stereoscopic microscope equipped with a photomultiplier tube on the eyepiece is able to detect quantitatively the conformational changes as a change in the intensity of the reflected light. The degree of the conformational change depends on the kinds of dissolved salt (KCl, NaCl, LiCl) and the pH of the solution. Higher Ca 2+ ion concentrations are needed to obtain reflectance changes in LiCl solution. From these results it was concluded that the conformational changes are caused by replacement of monovalent K + ions by divalent Ca 2+ ions as conterions of the hydrophilic radicals of the DOPH molecules. The resistance of the Millipore DOPH membrane increases when the conformation is changed by Ca 2+ ions: this resistance oscillates when current or pressure stimuli are applied. These stimuli create a partial K + ion concentration gradient in the membrane which dissociates Ca 2+ ions from the hydrophilic radicals of the DOPH molecules. This dissociation results in decreased membrane resistance, and also in a drop of the potential difference across the membrane produced by the constant current. As this drop decreases the K + ion concentration gradient in the membrane. Ca 2+ ions recombine with the hydrophilic radicals, and the membrane resistance recovers as well. The repetition of this process causes the oscillation of the membrane resistance.