Change of purple membranes geometry induced by protein adsorption

Earlier it was an orthodoxy that purple membranes (PMs) in aqueous medium are shaped as flat hard disks. In a few newer articles it has been shown that PMs are bent and their curvature varies with surface charge density. The purpose of this work is to answer which is the dominant factor for PM bending—structural or electrostatic forces. Two positively charged proteins are used: phytohemagglutinin (PhHA) and protamine.

The electrophoretic mobility and electric polarizability of PMs are measured by microelectrophoresis and electric dichroism. The results show that both proteins reduce the mobility because they are adsorbed on PM surface. However, their influence on the electric polarizability is in the opposite direction—protamine reduces it (trivial effect) while PhHA increases the polarizability (non-trivial effect). The last result is explained by a straightening the initially bent PM because of specific bonding of PhHA to asymmetrically disposed glycolipids of PM in contrast to the electrostatic adsorption of protamine.

It has been concluded that PMs in water medium are bent in the same manner as in in vivo—the intracellular surface with a higher negative charge is concave. The results indicate that electrostatic forces play a significant role in PM curvature but the shape of structural elements is the main factor determining the geometry of PM.