Lipid Exchange and Transfer on Nanoparticle Supported Lipid Bilayers: Effect of Defects, Ionic Strength, and Size

Abstract

Lipid exchange/transfer has been compared for zwitterionic 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine (DMPC) small unilamellar vesicles (SUVs) and for the same lipids on silica (SiO2) nanoparticle supported lipid bilayers (NP-SLBs) as a function of ionic strength, temperature, temperature cycling, and NP size, above the main gel-to-liquid crystal phase transition temperature, Tm, using d- and h-DMPC and DPPC. Increasing ionic strength decreases the exchange kinetics for the SUVs, but more so for the NP-SLBs, due to better packing of the lipids and increased attraction between the lipid and support. When the NP-SLBs (or SUVs) are cycled above and below Tm, the exchange rate increases compared with exchange at the same temperature without cycling, for similar total times, suggesting that defects provide sites for more facile removal and thus exchange of lipids. Defects can occur: (i) at the phase boundaries between coexisting gel and fluid phases at Tm; (ii) in bare regions of exposed SiO2 that form during NP-SLB formation due to mismatched surface areas of lipid and NPs; and (iii) during cycling as the result of changes in area of the lipids at Tm and mismatched thermal expansion coefficient between the lipids and SiO2 support. Exchange rates are faster for NP-SLBs prepared with the nominal amount of lipid required to form a NP-SLB compared with NP-SLBs that have been prepared with excess lipids to minimize SiO2 patches. Nanosystems prepared with equimolar mixtures of NP-SLBs composed of d-DMPC (d(DMPC)-NP-SLB) and SUVs composed of h-DMPC (h(DMPC)-SUV) show that the calorimetric transition of the "donor" h(DMPC)-SUV decreases in intensity without an initial shift in Tm, indicating that the "acceptor" d(DMPC)-NP-SLB can accommodate more lipids, through either further fusion or insertion of lipids into the distal monolayer. Exchange for d/h(DMPC)-NP-SLB is in the order 100 nm SiO2 > 45 nm SiO2 > 5 nm SiO2.

Methanol Accelerates DMPC Flip-Flop and Transfer: A SANS Study on Lipid Dynamics

“…Increasing methanol concentrations had a profound effect on the kinetics of DMPC monomers in free-floating large unilamellar vesicles. Despite differences in vesicle size and investigative techniques, our unperturbed DMPC flipflop and transfer rates are in excellent agreement with many values previously found (17,(21)(22)(23)(24). In a closely related study, Gerelli et al used neutron reflectometry to measure DMPC flip-flop and exchange between vesicle dispersions and adsorbed planar bilayers (25).…”

Multicomponent Supported Membrane Microarray for Monitoring Spatially Resolved Cellular Signaling Reactions

“…All membrane fusion‐mediated lipid delivery experiments were performed on a vibration isolating optical table (Thorlabs, Newton, NJ) by simultaneously seeding bilayer‐coated microbeads (DOPC + Ni‐NTA‐DOGS and DOPC + biotin lipid SLB‐coated microbeads in an 1:1 ratio) on the micropatterned substrate for 15–30 min at 37 °C.…”

Breakage of Hydrophobic Contacts Limits the Rate of Passive Lipid Exchange Between Membranes

“…k ex is then determined by monitoring the rate of mixing of labeled lipids between these two populations of vesicles. 17,25,26,[61][62][63] We can obtain an expression for the rate of mixing by first considering how the concentration of labeled lipids in a single vesicle changes over time. The concentration of labeled lipids in each vesicle will adopt a steady state with the concentration in the solution surrounding that vesicle.…”

“…We set U (r) = 0 far away from a vesicle and shift the peak of the free energy barrier to r = 50 nm, a typical radius of large unilamellar vesicles (LUVs) used in experimental studies. 17,25,26,[61][62][63] Assuming that the concentration profile of labeled lipids reaches steady state much faster than n B is varying, the time dependence of the bulk concentration of labeled lipids,ρ, can be regarded as constant while calculating the steady-state profile ρ(r)/ρ. Assuming as well that equilibration within state B is fast, the concentration of labeled lipids within a vesicle obeys equilibrium statistics,…”

“…Our simulations are performed with neat water, but experimental systems are conducted in buffer, which contains significant amounts of monovalent salts, such as NaCl or KCl. 17,[61][62][63] The addition of NaCl or KCl could salt out the free lipids in solution, increasing the difference in free energy between state A and B. Additionally, monovalent cations bind to the carbonyl region of phosphatidylcholine membranes, causing the thickness of the bilayer and order of the tails to increase. [68][69][70] These structural changes are expected to increase the free energy barrier for hydrophobic contact formation.…”