Dipole potential measurements in asymmetric membranes

Abstract

WE report here detection of two kinds of asymmetry in planar lipid bilayers which may bear on the problem of asymmetry in biological membranes. One kind of asymmetry arises as a consequence of an asymmetric distribution in the bilayer of lipids with charged polar head groups, and the other from an asymmetric distribution of lipids with different but neutral polar head groups. Several studies indicate that the lipids of the red cell membrane are asymmetrically distributed. In the erythrocyte membrane the outer half of the bilayer consists predominantly of neutral lipids, whereas the inner half contains all the negatively charged phospholipid phosphatidylserine^1–3. In nerve axons such lipid asymmetry has not been demonstrated directly but can be inferred from surface charge measurements. These measurements indicate that at least in the neighbourhood of the sodium channel, the negative charge density is greater in the outer monolayer of the membrane^4,5.