Specific Binding of Different Vesicle Populations by the Hybridization of Membrane-Anchored DNA

Abstract

We demonstrate a method of heterogeneous vesicle binding using membrane-anchored, single-stranded DNA that can be used over several orders of magnitude in vesicle size, as demonstrated for large 100 nm vesicles and giant vesicles several microns in diameter. The aggregation behavior is studied for a range of DNA surface concentrations and solution ionic strengths. Three analogous states of aggregation are observed on both vesicle size scales. We explain the existence of these three regimes by a combination of DNA binding favorability, vesicle collision kinetics, and lateral diffusion of the DNA within the fluid membrane. The reversibility of the DNA hybridization allows dissociation of the structures formed and can be achieved either thermally or by a reduction in the ionic strength of the external aqueous environment. Difficulty is found in fully unbinding giant vesicles by thermal dehybridization, possibly frustrated by the attractive van der Waals minimum in the intermembrane potential when brought into close contact by DNA binding. This obstacle can be overcome by the isothermal reduction of the ionic strength of the solution: this reduces the Debye screening length, coupling the effects of DNA dehybridization and intermembrane repulsion due to the increased electrostatic repulsion between the highly charged DNA backbones.

Application of nucleic acid–lipid conjugates for the programmable organisation of liposomal modules

“…Changes in the conformational entropy of flexible tethers upon binding, combinatorial entropy arising from the multiplicity of states in a multivalent interaction and changes in the translational entropy of laterally mobile linkers can all have significant contributions to the overall binding free energy [71]. The significant physical differences between the DNA-mediated assembly of soft particles, such as liposomes, and hard particles has also been demonstrated using emulsion droplets, where the deformability of the individual particles and the fluidity of their interface also come into play [69].This study shows that the enrichment of DNA functionalities in adhesion contact sites gives the emulsion droplets an effective valency that can be controlled through the DNA surface density on these particles, similar to how the aggregation state of liposomes can also be controlled through tuning the DNA loading within the membranes [30]. Entropic effects of ligand binding are also found to be important in the growth of adhesion plaques between neighbouring droplets: DNA linkers with rigid double-stranded DNA spacers formed contact sites that were 60% larger than DNA linkers that were spaced from the droplet interface by flexible, single-stranded DNA [69].…”

“…Cholesterol-DNA conjugates have been shown to be able to mediate direct assembly interactions between two populations of liposomes functionalised by complementary strands [30].…”

“…For inter-vesicle interactions, in particular, the fluidity of the membrane, which allows receptor lateral mobility across the surface, and the deformability of the membrane during inter-liposome adhesion processes are important physical differences when comparing liposome [30]. Changes in the conformational entropy of flexible tethers upon binding, combinatorial entropy arising from the multiplicity of states in a multivalent interaction and changes in the translational entropy of laterally mobile linkers can all have significant contributions to the overall binding free energy [71].…”

“…Here, the central focus will be on the use of nucleic acid functionalities on the surface of liposomes to mediate their associations. The two key advantages of nucleic acids for this purpose are [30]: (i) DNA (usually) forms an asymmetric interaction with its complement, allowing the controlled assembly of different liposomal compartments, compared with symmetric binding interactions, e.g. biotin-avidin-biotin, which cross-link liposomes from the same population; (ii) the high fidelity, DNA mean that, in principle, an expansive array of specific interactions can be encoded within large multicomponent systems utilising the same chemistry.…”

Partitioning of Membrane-Anchored DNA between Coexisting Lipid Phases

“…This two-faced texture is analogous to that of "Janus particles". 88 We have previously reported a phase diagram for GUV binding mediated by DNA functionalization with varying DNA surface concentration and solution ionic strength, 27 where the precise quantitative location of the phase boundaries will also be dependent on the DNA sequence used. A cartoon of such a phase diagram is illustrated in the bottom right of Figure 7; the phase boundary drawn indicates a transition between no vesicle adhesion and vesicle adhesion mediated by specific binding of cDNA sequences.…”