Direct measurements of attractive Van der Waals and adhesion forces between uncharged lipid bilayers in aqueous solutions

Abstract

A report is given of the direct measurement of the attractive Van der Waals interaction between lipid bilayers. Investigated are bilayers of l-α-dipalmitoyl-phosphatidyl-choline (DPPC), l-α-dipalmitoyl-phosphatidyl-ethanolamine (DPPE), and bilayers of digalactosyl-diglyceride (DGDG). The dependence of the long-range Van der Waals interaction on the bilayer separation is in agreement with the Hamaker equation. In addition, the adhesion force between the bilayers and the location of the effective Van der Waals plane of origin have been determined independently of each other. Hamaker constants have been determined in two ways: (i) from the (short-range) force of adhesion, and (ii) from the determination of the gradient of the long-range attractive interaction between well-separated bilayers. The latter method yields a value for the Hamaker constant which is independent of the location of the (effective) Van der Waals plane of origin. For DGDG bilayers, the Hamaker constants obtained from the force of adhesion and from the long-range attractive interaction are very similar, viz A = 5.3 ± 1.5 × 10 −21 J and A = 7.5 ± 1.0 × 10 −21 J, respectively. Upon addition of 0.2 M NaCl the Hamaker constant reduces to A = 3.1 ± 0.6 × 10 −21 J as expected theoretically. On the other hand, for the two phospholipids DPPC and DPPE, the Hamaker constant obtained from the long-range attractive interaction between the bilayers, A = 1.3 ± 0.2 × 10 −21 J, is much lower than that obtained from the adhesion force: A = 6.9 ± 1.5 × 10 −21 J. In all lipid systems the Van der Waals plane was clearly found to be a few Ångstroms farther out from the anhydrous bilayer/water interface. The results illustrate the necessity of independent measurements of the attractive and adhesion forces before any inference can be made concerning the nature of electrostatic, hydration, and steric interactions existing between bilayers.