Aqueous Emulsion Droplets Stabilized by Lipid Vesicles as Microcompartments for Biomimetic Mineralization

Abstract

Mineral deposition within living cells relies on control over the distribution and availability of precursors as well as the location and rates of nucleation and growth. This control is provided in large part by biomolecular chelators, which bind precursors and regulate their availability, and compartmentalization within specialized mineralizing vesicles. Biomimetic mineralization in self-assembled lipid vesicles is an attractive means of studying the mineralization process, but has proven challenging due to vesicle heterogeneity in lamellarity, contents, and size across a population, difficulties encapsulating high and uniform precursor concentrations, and the need to transport reagents across an intact lipid bilayer membrane. Here, we report the use of liposome-stabilized all-aqueous emulsion droplets as simple artificial mineralizing vesicles (AMVs). These biomimetic microreactors allow the entry of precursors while retaining a protein catalyst by equilibrium partitioning between internal and external polymer-rich phases. Small molecule chelators with intermediate binding affinity were employed to control Ca(2+) availability during CaCO3 mineralization, providing protection against liposome aggregation while allowing CaCO3 formation. Mineral deposition was limited to the AMV interior, due to localized production of CO3(2-) by compartmentalized urease. Particle formation was uniform across the entire population of AMVs, with multiple submicrometer amorphous CaCO3 particles produced in each one. The all-aqueous emulsion-based approach to biomimetic giant mineral deposition vesicles introduced here should be adaptable for enzyme-catalyzed synthesis of a wide variety of materials, by varying the metal ion, enzyme, and/or chelator.

Combining Catalytic Microparticles with Droplets Formed by Phase Coexistence: Adsorption and Activity of Natural Clays at the Aqueous/Aqueous Interface

“…ATPS volume ratios 1:1 and 20:1 of PEG-rich to Dx-rich phase volumes were compared. The 1:1 volume ratio was used for simplicity and the 20:1 volume ratio was investigated because most hydrophilic molecules such as proteins, peptides, RNAs or nucleotides partition into the Dx-rich phase and hence for protocells or microreactors applications, a Dx-rich droplet phase would be desirable 18, 56 . Transmitted light and fluorescence micrographs for each of the four clay samples in each of the volume ratios are shown in Figs 2 and 3.…”

Particle stabilized water in water emulsions

“…In a follow-up investigation the same authors used the same system to produce CaCO3 within the dextran droplets (Cacace, Rowland, Stapleton, Dewey and Keating, 2015) . They exploited the fact that the enzyme urease partitioned strongly within the dextran phase where it reacted with urea in the medium that could penetrate through the liposome layer.…”

Dynamic Synthetic Cells Based on Liquid–Liquid Phase Separation

“…This has led severalg roups to describe such droplets as water-in-water (w/w) emulsions. Spherical colloidsw ith diameters typicallyg reater than 100 nm, including nanoparticles, [85,86] lipid vesicles [87,88] and protein clusters, [89] as well as high-aspect-ratio colloids, such as clays, [90] nanorods [91] or protein fibrils, [92] adsorb more effectively to interfaces with lows urface tension [92,93] and have therefore been used to stabiliseP EG/dextran aqueous two-phase systems (Figure 2D,E ), as well as complex coacervate droplets. Spherical colloidsw ith diameters typicallyg reater than 100 nm, including nanoparticles, [85,86] lipid vesicles [87,88] and protein clusters, [89] as well as high-aspect-ratio colloids, such as clays, [90] nanorods [91] or protein fibrils, [92] adsorb more effectively to interfaces with lows urface tension [92,93] and have therefore been used to stabiliseP EG/dextran aqueous two-phase systems (Figure 2D,E ), as well as complex coacervate droplets.…”

“…Spherical colloidsw ith diameters typicallyg reater than 100 nm, including nanoparticles, [85,86] lipid vesicles [87,88] and protein clusters, [89] as well as high-aspect-ratio colloids, such as clays, [90] nanorods [91] or protein fibrils, [92] adsorb more effectively to interfaces with lows urface tension [92,93] and have therefore been used to stabiliseP EG/dextran aqueous two-phase systems (Figure 2D,E ), as well as complex coacervate droplets. [87,88] Liposomestabilised dextrand roplets, for instance, were shown to take up ribozymes spontaneously from the continuous solution, due to the preferential partitioning of the polynucleotides within the droplets, resulting in localised cleavage of short oligonucleotide substrates partitioned into the dextran-rich droplets. Thisn on-selectivep ermeability is advantageous for the formation of discreteb ioreactors (stabilised against coalescence) that retain the capacity to sequester and accumulate biomolecules spontaneously from the continuous phase (due to their equilibriump artitioning within the droplets).…”