The aqueous surroundings alter the bending rigidity of lipid membranes

Phosphatidylcholine Membrane Fusion Is pH-Dependent

“…What could cause such a change of the energy barrier to fusion? As reported in [ 36 , 37 ], a decrease of environmental pH from 7 to 4 results in an increase of the membrane bending modulus by about 20%. It is intuitively clear that it should cause an increase, rather than a decrease of the fusion energy barrier, since growth of the bending modulus should result in an approximately proportional increase of the membrane deformation energy.…”

“…However, assessment of the possible electrostatic contribution into the membrane interaction energy according to the Derjaguin–Landau–Verwey–Overbeek theory shows that the increase of ζ-potential observed for phosphatidylcholines in the neutral environment [ 36 , 40 ] yields, in our conditions, the electrostatic interaction energy of about 0.1 k B T , which clearly cannot explain the observed experimental dependence on the environmental pH . Thus, it would appear that change of pH should primarily modify the bending modulus [ 36 , 37 ] and spontaneous curvature of lipid monolayers.…”

“…In the course of the calculation of the energy surfaces represented on Figure 4 , we assumed that when pH changes from 7.5 to 4.1, the bending modulus and spontaneous curvature change by 20%. The bending modulus change was determined experimentally in [ 36 , 37 ]. There is but circumstantial evidence of changes of spontaneous curvature: when pH is varied, the dipole potential of the membrane changes considerably [ 36 ].…”

“…Nevertheless, to the best of our knowledge, none of these publications addressed the influence of pH upon the elastic parameters of the lipid bilayer and molecular geometry of lipids, which defines the spontaneous curvature of lipid monolayers [ 24 , 29 , 34 , 35 ]. At the same time, the pH of the environment in the range of transition from neutral conditions (7.0–7.5) to the conditions of ripe endosomes (pH 4.5–5.0) can notably change the parameters of even zwitterionic lipids, such as phosphatidylcholine: membrane bending modulus, dipolar moment, structure of the polar part of the lipid molecule, and hence the spontaneous curvature of the monolayer [ 36 , 37 , 38 ]. In the existing models of membrane fusion based on the elastic properties of lipid bilayers, it has been shown that it is the bending modulus and spontaneous curvature that determine the elastic energy of fusion intermediates and the height of the energy barrier, defining the rate of the fusion process [ 17 , 28 ].…”

Bending rigidity of charged lipid bilayer membranes

“…Membranes containing larger fractions of charged lipids (up to 40%) were studied by Vitkova et al 28 Micropipette aspiration of giant vesicles made of POPC:POPS mixtures showed an increase in membrane bending rigidity by 10 k B T. However, these experiments were conducted in a high sucrose concentration (0.17 M sucrose), at which sugars bind quite strongly to a lipid bilayer, resulting in membrane lateral expansion and thinning. 35 This results in a softer membrane, 36,37 making it difficult to isolate the electrostatic contribution to the membrane bending rigidity. Overall, a comprehensive investigation of the bending rigidity as a function of surface charge over a broad range of charged lipid fraction is lacking.…”