Lipid transfer between charged supported lipid bilayers and oppositely charged vesicles

The bidirectional transfer of phospholipids between a charged, supported lipid bilayer (SLB) on SiO(2) and oppositely charged, unilamellar vesicles was studied by means of quartz crystal microbalance with dissipation (QCM-D) and optical reflectometry techniques. SLBs and vesicles were prepared from binary mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) mixed with different fractions of either 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-l-serine] (POPS) (negatively charged) or 1-palmitoyl-2-oleoyl-sn-glycero-3-ethylphosphocholine (POEPC) (positively charged). The interaction process consists of an attachment-transfer-detachment (ATD) sequence, where added vesicles first attach to and interact with the SLB, after which they detach, leaving behind a compositionally modified SLB and ditto vesicles. When the process is complete, there is no net addition or reduction of total lipid mass in the SLB, but lipid exchange has occurred. The time scale of the process varies from a few to many tens of minutes depending on the type of charged lipid molecule and the relative concentration of charged lipids in the two membranes. Electrostatically symmetric cases, where only the charge sign (but not the fraction of charged lipid) was reversed between the SLB and the vesicles, produce qualitatively similar but quantitatively different kinetics. The time scale of the interaction varies significantly between the two cases, which is attributed to a combination of the differences in the molecular structure of the lipid headgroup for the positively and the negatively charged lipids used, and to nonsymmetric distribution of charged lipids in the lipid membranes. The maximum amounts of attached vesicles during the ATD process were estimated to be 25-40% of a full monolayer of vesicles, with the precise amount depending on the actual charge fractions in the vesicles and the SLB. Interrupted vesicle exposure experiments, and experiments where the bulk concentration of vesicles was varied, show that vesicles in some cases may be trapped irreversibly on the SLB, when only partial transfer of lipid molecules has occurred. Additional supply of vesicles and further transfer induces detachment, when a sufficient amount of oppositely charged lipids has been transferred to the SLB, so that the latter becomes repulsive to the attached vesicles. Possible mechanistic scenarios, including monomer insertion and hemifusion models, are discussed. The observed phenomena and the actual SLB preparation process form a platform both for studies of various intermembrane molecular transfer processes and for modifying the composition of SLBs in a controlled way, for example, for biosensor and cell culture applications.