Bilayers of neutral lipids bear a small but significant charge

Many experiments done on neutral lipid bilayers in pure water show weak rep ulsions. These weak forces prevent vesicles from adhering and are generally overcome by adding some salt in the aqueous medium. They also appear as st ray repulsions in surface forces measurements made on lipid bilayers. Using a surface forces apparatus in pure water and in salt solution, ive have me asured the forces between two stearoyl-oleoyl-phosphatidyl-choline (SOPC) b ilayers and between two dimiristoyl-phosphatidylethanolamine (DMPE) bilayer s. The results show that the repulsions are due to a small amount of negati ve charges coming from impurities in SOPC. This was quantitatively confirme d by electrophoretic measurements. There are 3 times less charges in the ca se of DMPE layers. The effect of these charges which is negligible at high salt concentration may significantly affect the adhesion energy and behavio ur of neutral lipid bilayers between 0 and approximate to 40 mM salt.