Nanoscale particles, of 200 to 300 nm in diameter, have been developed to sequester loaded cargo until a specific target, recognized by the targeting surface ligands, is reached. The delivery of therapeutic agents including drugs, imaging agents, and macromolecules using nanoscale particles has advantages over conventional small molecule treatment. Delivery using nanoscale particles prevents premature degradation of the therapeutic agents, concentrates the agents at a specific target tissue or cell type, and aids agents in crossing through biological barriers such as the epithelium, endothelium, and plasma membrane [1
]. Nanoparticle design must be precisely tailored to deliver each particular agent to its appropriate destination. To achieve efficient delivery to subcellular organelles, the scientific community must understand the molecular mechanisms of how nanoparticles interact with plasma membranes.
Phagocytosis and endocytosis are the most common internalization mechanisms for nanoparticles such as liposomes and polymer-based nanoparticles [1
]. However, these internalization mechanisms involve lysosomal degradation that attenuates therapeutic efficiency and as such are not suited for the delivery of therapeutic biomolecules. Alternative strategies have been developed where cargo molecules are directly inserted into the cytoplasmic space through transient pores in the plasma membrane. Direct insertion methods include the conjugation of therapeutic cargo to cell-penetrating peptides [2
], electroporation [4
], and therapeutic ultrasound with microbubbles [6
]. However, all of these methods have the potential to cause cell damage by disrupting the plasma membrane [3
Another delivery mechanism is provided by perfluorocarbon-based nanoemulsion particles (PFC-NEPs) which are stabilized by an emulsifying phospholipid monolayer. The so called “contact-facilitated” delivery mechanism of PFC-NEPs involves neither lysosomal pathways nor substantial perturbations of the membrane. This delivery mechanism is hypothesized to start with the formation of a hemifusion complex between the monolayer of PFC-NEP and the outer monolayer of target cell plasma membrane. Cargo molecules then diffuse to the plasma membrane through the hemifusion complex and are finally internalized by lipid raft mediated endocytosis [11
]. The contact-facilitated delivery mechanism is particularly useful for the delivery of biomolecules that are highly susceptible to enzymatic reactions. However, the molecular details of this mechanism are as yet undetermined due to experimental difficulties of structure determination of small, fluid, and highly heterogeneous systems.
Molecular dynamics simulations have been widely used to determine membrane structures at both atomistic [15
] and coarse-grained levels [19
]. Atomistic simulations are useful for collecting accurate structural details but are often too costly for examining larger-scale membrane behavior such as self-assembly of lipids into bilayers or vesicles, bilayer phase changes, domain formation, pore formation, and membrane fusion. Therefore, coarse-grained models have been extensively used to simulate these mesoscopic phenomena [19
]. Proposed membrane fusion mechanisms involving a hemifusion stalk [23
] have been demonstrated by the appearance of proposed intermediate structures in coarse-grained simulations [24
]. Furthermore, such simulations have identified important molecular-scale structural motifs, such as splayed lipids in inducing membrane fusion [26
]. Over the last few years, coarse-grained simulations have been adopted to model atomistic structures of biological emulsions such as low density lipoproteins and high density lipoproteins that support these simulations to model nanoemulsion particles and their interaction with bilayer [28
This research focuses on an initial step of the contact-facilitated delivery mechanism [14
], where two monolayers form a hemifusion complex which resembles the hemifusion stalk [31
]. Coarse-grained molecular dynamics simulations have been used to directly observe hemifusion complex formation between a PFC based nanoemulsion particle and a liposome that models a target cell. These simulations were also used to test the dependence of nanoparticle-liposome fusion on particle size and lipid composition in order to identify important structural (physico-chemical) features of the particles required for optimization of fusion.