Hydration repulsion effects on the formation of supported lipid bilayers

When zwitterionic lipids fuse onto substrates such as silica (SiO2), the water of hydration between the two approaching surfaces must be removed, giving rise to an effective hydration repulsion. Removal of water around the polar headgroups of the lipid and the silanols (SiOH) of SiO2 allows supported lipid bilayer (SLB) formation, although an interstitial water layer remains between the lipid and surface. The importance of hydration repulsion in SLB formation is demonstrated by monitoring fusion of zwitterionic lipids onto silica (SiO2) nanoparticles heat treated to control the silanol group (SiOH) density and thus the amount of bound water. SLB formation, observed by cryo-TEM and nano-differential scanning calorimetry, was found to be slower for the more hydrated surfaces. Although the SiOH density decreased with increasing heat treatment temperature, zeta-potentials were the same for all the SiO2. This arose since at the pH 8 of the experiments, only isolated silanols, with a pK(a) = 4.9, and not hydrogen bonded silanols, with a pK(a) = 8.5, were dissociated/charged.(1) Since there were no differences in double layer forces between the SUVs and SiO2, which are the largest and most important interactions determining lipid fusion onto surfaces,(2,3) the slower rate of SLB formation of DMPC onto SiO2 nanoparticles with higher silanol densities and more bound water was therefore attributed to greater hydration repulsion of the more hydrated nanoparticles. For SiO2 heated to 1000 degrees C, with only a few isolated silanols, little adsorbed water and many hydrophobic Si-O-Si groups, particle aggregation occurred and lipid sheaths formed around the nanoparticle aggregates.