Cytoskeletal‐like Supramolecular Assembly and Nanoparticle‐Based Motors in a Model Protocell

Abstract

Reconstitution of cellular functions within synthetic constructs such as lipid or surfactant vesicles represents a central objective of protocell research. Realizing this goal has important implications for the design and construction of soft, water-based compartmentalized systems with lifelike properties, and should provide unique opportunities in synthetic biology and bionanotechnology, as well as research on the origins of life. Recent studies have developed synthetic protocell models based on vesicles capable of polymerase chain reaction (PCR) induced DNA amplification, gene expression of single components or cascading networks, biochemical transformations, poly(adenylic acid) synthesis, or RNA replication. In contrast, there have been relatively few reports on the reconstitution of dynamically self-assembled cytoskeletal-like structures within protocell models. Whilst these studies represent important steps towards the confirmation and refinement of biological mechanisms of cytoskeletal assembly and organization, the possibility of mimicking the cytoskeleton synthetically, that is, using the reversible noncovalent supramolecular assembly of nonbiological components, has not, to the best of our knowledge been explicitly explored. In contrast to previous studies on the encapsulation of polymer gels in vesicles, herein we report the noncovalent assembly of a supramolecular network within the interior of phospholipid vesicles using the in situ enzymatic dephosphorylation of small-molecule, amino-acid-based components. Moreover, we exploit supramolecular gelation within the vesicles to generate robust, soft microcompartments capable of chemically derived self-propulsion arising from the platinum-nanoparticle-catalyzed decomposition of hydrogen peroxide (H2O2). Aqueous suspensions of 1-palmitoyl-2-oleoyl-sn-glycero3-phosphocholine (POPC) vesicles comprising supramolecular hydrogel interiors were prepared using an inverted emulsion method combined with in situ alkaline phosphatase-mediated dephosphorylation of N-fluorenylmethylcarbonyl–tyrosine-(O)-phosphate (Fmoc-TyrP; see Scheme 1 in the Supporting Information). As described previously, the latter process results in the noncovalent selfassembly of Fmoc-Tyr molecules into bundles of supramolecular nanofilaments that exhibit solid-like viscoelastic properties (see Figure S1 in the Supporting Information), and can be reversibly disassembled by heating the hydrogels above the gel-sol transition temperature (typically around 45 8C). Optical microscopy studies indicated that the amino acid/enzymecontaining hydrogelled vesicles were not aggregated, spherical in shape, 1–25 mm in diameter, and structurally stable when dispersed in water for several months at room temperature, or freeze-dried and investigated by SEM (Figure 1a,b). The hydrogelled vesicles could be sedimented and rehydrated by slow centrifugation at 120 g for 30 minutes, and were relatively stable to changes in osmotic gradients. In contrast, nongelled, water-filled vesicles aggregated after several days