Reversible self-organization of a UV-responsive PEG-terminated malachite green derivative: vesicle formation and photoinduced disassembly

Synthetic Biology: Bottom-Up Assembly of Molecular Systems

The bottom-up assembly of biological and chemical components opens exciting opportunities to engineer artificial vesicular systems for applications with previously unmet requirements. The modular combination of scaffolds and functional building blocks enables the engineering of complex systems with biomimetic or new-to-nature functionalities. Inspired by the compartmentalized organization of cells and organelles, lipid or polymer vesicles are widely used as model membrane systems to investigate the translocation of solutes and the transduction of signals by membrane proteins. The bottom-up assembly and functionalization of such artificial compartments enables full control over their composition and can thus provide specifically optimized environments for synthetic biological processes. This review aims to inspire future endeavors by providing a diverse toolbox of molecular modules, engineering methodologies, and different approaches to assemble artificial vesicular systems. Important technical and practical aspects are addressed and selected applications are presented, highlighting particular achievements and limitations of the bottom-up approach. Complementing the cutting-edge technological achievements, fundamental aspects are also discussed to cater to the inherently diverse background of the target audience, which results from the interdisciplinary nature of synthetic biology. The engineering of proteins as functional modules and the use of lipids and block copolymers as scaffold modules for the assembly of functionalized vesicular systems are explored in detail. Particular emphasis is placed on ensuring the controlled assembly of these components into increasingly complex vesicular systems. Finally, all descriptions are presented in the greater context of engineering valuable synthetic biological systems for applications in biocatalysis, biosensing, bioremediation, or targeted drug delivery.

Wavelength-Controlled Light-Responsive Polymer Vesicle Based on Se-S Dynamic Chemistry

Wavelength-controlled Se-S dynamic chemistry was put forward recently as a convenient way to regulate the balance of a selenide sulfide exchange reaction. In this paper, we synthesized an asymmetric polymeric amphiphile linked with a Se-S bond and then induced it to self-assemble into vesicles in water. When the visible light was applied to the assembly solution with addition of toluene, Se-S bonds containing vesicles were ruptured. Thus, the wavelength-controlled light responses of relatively stable polymer assembly were accomplished by introduction of the Se-S dynamic covalent bond, and the response mechanism of the Se-S bond in the vesicle was explored by nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), and X-ray photoelectron spectroscopy (XPS). The results indicated that fracture of the Se-S bond led to the dissociation of assembly. Introduction of Se-S dynamic chemistry into the molecular assembly area enriched the light-responsive polymer systems and would bring many potential applications in the future.

Synthesis and controllable self-assembly of 3D amphiphilic organoplatinum(ii) metallacages in water

A 3D amphiphilic supramolecular coordination metallacycle M1 was designed and fabricated using a new method called “coordination driven self-assembly”. It can self-assemble into well-defined vesicles and further assemble into nanofibres and hybrid vesicles. Importantly, the hybrid vesicles can be applied in photocatalysis in water.

Proton-consumed nanoarchitectures toward sustainable and efficient photophosphorylation

At present, photophosphorylation in natural or artificial systems is accomplished by the production of protons or their pumping across the biomembranes. Herein, different from this strategy above, we demonstrate a designed system which can effectively enhance photophosphorylation by photo-induced proton-scavenging through molecular assembly. Upon the introduction of photobase generators, a (photo-) chemical reaction occurs to produce hydroxyl ions. Accompanying the further extramembranous acid-base neutralization reaction, an outbound flow of protons is generated to drive the reconstituted adenosine triphosphate (ATP) synthase to produce ATP. That is, contrary to biochemistry, the proton gradient to drive photophosphorylation derives from the scavenging of protons present in the external medium by hydroxyl ions, produced by the partially photo-induced splitting of photobase generator. Such assembled system holds great potential in ATP-consuming bioapplications.

Evolution of Supra-Amphiphiles from Amphiphiles

Molecular amphiphiles are molecules that carry both a hydrophilic part and a hydrophobic part, linked by covalent bonds. In contrast with molecular amphiphiles, supramolecular amphiphiles (or supra-amphiphiles) are amphiphiles that are formed on the basis of noncovalent interactions. The dynamic nature of noncovalent interactions may simplify fabrication procedures and facilitate the introduction of stimuli-responsive moieties, thus endowing supra-amphiphiles with dynamic, reversible and adaptive properties. Supra-amphiphiles provide a delicate platform for combining molecular architecture and functional assembly, enriching the molecular engineering of functional supramolecular systems.

A cavity extended water-soluble resorcin[4]arene: synthesis, pH-controlled complexation with paraquat, and application in controllable self-assembly

A novel pH-responsive molecular recognition motif was built between an anionic carboxylate group modified resorcin[4]arene and paraquat in water. We then employed it to fabricate a supra-amphiphile for controllable self-assembly.

Cooperation of Amphiphilicity and Crystallization for Regulating the Self-Assembly of Poly(ethylene glycol)-block-poly(lactic acid) Copolymers

Tuning the amphiphilicity of block copolymers has been extensively exploited to manipulate the morphological transition of aggregates. The introduction of crystallizable moieties into the amphiphilic copolymers also offers increasing possibilities for regulating self-assembled structures. In this work, we demonstrate a detailed investigation of the self-assembly behavior of amphiphilic poly(ethylene glycol)-block-poly(l-lactic acid) (PEG-b-PLLA) diblock copolymers with the assistance of a common solvent in aqueous solution. With a given length of the PEG block, the molecular weight of the PLA block has great effect on the morphologies of self-assembled nanoaggregates as a result of varying molecular amphiphilicity and polymer crystallization. Common solvents including N,N-dimethylformamide, dioxane, and tetrahydrofuran involved in the early stage of self-assembly led to the change in chain configuration, which further influences the self-assembly of block copolymers. This study expanded the scope of PLA-based copolymers and proposed a possible mechanism of the sphere-to-lozenge and platelet-to-cylinder morphological transitions.

A Family of Photolabile Nitroveratryl-Based Surfactants That Self-Assemble into Photodegradable Supramolecular Structures

Here we report the synthesis and characterization of a family of photolabile nitroveratryl-based surfactants that form different types of supramolecular structures depending on the alkyl chain lengths ranging from 8 to 12 carbon atoms. By incorporating a photocleavable α-methyl-o-nitroveratryl moiety, the surfactants can be degraded, along with their corresponding supramolecular structures, by light irradiation in a controlled manner. The self-assembly of the amphiphilic surfactants was characterized by conductometry to determine the critical concentration for the formation of the supramolecular structures, transmission electron microscopy to determine the size and shape of the supramolecular structures, and dynamic light scattering (DLS) to determine the hydrodynamic diameter of the structures in aqueous solutions. The photodegradation of the surfactants and the supramolecular structures was confirmed using UV-vis spectroscopy, mass spectrometry, and DLS. This surfactant family could be potentially useful in drug delivery, organic synthesis, and other applications.

Water soluble stimuli-responsive star copolymers with multiple encapsulation and release properties

Stimuli-responsive, water soluble, nontoxic, star-copolymers showing reversible encapsulation and release of hydrophobic dye/drug molecule with increasing temperature and decreasing pH.

Reversible formation of supramolecular polymer networks via orthogonal pillar[10]arene-based host–guest interactions and metal ion coordinations

Reversible supramolecular polymer networks were constructed by orthogonal pillar[10]arene-based host–guest molecular recognition, terpyridine–Zn²⁺, and carbene–Ag⁺ coordinations.

Dynamic disordering of liposomal cocktails and the spatio-temporal favorable release of cargoes to circumvent drug resistance

Multidrug resistance (MDR) has been a major impediment to the success of cancer chemotherapy. Extensive efforts have been devoted to the development of drug delivery systems using nanotechnology to reverse MDR in cancer. However, the spontaneous release of drug payloads was always a slow process, which leads to the low intracellular drug concentration resulting in consequent drug insensitivity. To circumvent this limitation, we described a liposomal cocktail (LMDHV) constructed by a pH-responsive molecule (i.e., malachite green carbinol base (MG)) and liposome conjugated with Her-2 antibody for codelivery of doxorubicin (DOX) and verapamil (VER) to suppress drug resistance in Her-2 positive breast cancer. MG inserted in the bilayer as pH responders greatly contributed to the destabilization of the vesicle membrane in low pH, followed by the rapid release of the payloads. LMDHV showed 6-fold reversal efficiency in DOX resistant breast cancer owing to the efficient tumor targeting delivery and rapid burst release of drug intracellularly. Compared to tumor inhibition ratio of treated groups by free DOX (32.4 ± 7.4%), our designed kinetically favorable drug release system exhibited significantly (P < 0.01) enhanced tumor inhibition ratio up to 83.9 ± 12.5%, which is attributed to the remarkably increased drug concentration in cells. The spatio-temporal favorable release of drugs resulted in synergistic inhibition of tumor growth in xenografts. We envision that this new type of liposomal cocktail might be potentially utilized to circumvent drug resistance in the future.

Molecular structural transformation regulated dynamic disordering of supramolecular vesicles as pH-responsive drug release systems

The spontaneous release of drug payloads in the whole body always results in the compromised drug bioavailability and ultimate therapeutic efficacy. To achieve enhanced therapeutic efficacy and reduced side effects, pH-responsive targeted drug delivery systems have been studied due to their enhanced tumor accumulation and controllable maximum drug release feature. The present study described a co-assembly constructed by a pH responsive molecule (i.e., malachite green carbinol base (MG)) and liposome for highly efficient doxorubicin (DOX) release in tumor cells (MG-DOX⊂L). The structural transformation of MG effectively regulates the drug release profile in acidic environment. The pH-responsive sensitivity of co-assembly can be fine-tuned by altering the mixing ratios of building blocks with pH responders (i.e., MG molecules). MG-DOX⊂L was beneficial for the DOX release at pH5.0 and showed a higher cytotoxicity in KB cells owing to the pH-responsive drug release in acidic organelles following endocytosis pathway. In vivo tumor targetability and growth inhibition were evaluated in KB cell-xenografted nude mice. We have demonstrated that effective tumor growth inhibition in vivo is attributed to the synergistic contributions from highly efficient cellular entry and responsive intracellular release of DOX from MG-DOX⊂L.

Hierarchical self-assembly: well-defined supramolecular nanostructures and metallohydrogels via amphiphilic discrete organoplatinum(II) metallacycles

Metallacyclic cores provide a scaffold upon which pendant functionalities can be organized to direct the formation of dimensionally controllable nanostructures. Because of the modularity of coordination-driven self-assembly, the properties of a given supramolecular core can be readily tuned, which has a significant effect on the resulting nanostructured material. Herein we report the efficient preparation of two amphiphilic rhomboids that can subsequently order into 0D micelles, 1D nanofibers, or 2D nanoribbons. This structural diversity is enforced by three parameters: the nature of the hydrophilic moieties decorating the parent rhomboids, the concentration of precursors during self-assembly, and the reaction duration. These nanoscopic constructs further interact to generate metallohydrogels at high concentrations, driven by intermolecular hydrophobic and π-π interactions, demonstrating the utility of coordination-driven self-assembly as a first-order structural element for the hierarchical design of functional soft materials.

Reversible assembly and disassembly of amphiphilic assemblies by electropolymerized polyaniline films: effects rendered by varying the electropolymerization potential

Polymer films that respond to a variety of stimuli are attractive candidates for location-specific guest molecule delivery. These systems release the guest molecules by polymer erosion; thus, these are mono-use systems. If a polymer film is used to disassemble amphiphilic assemblies containing sequestered guest molecules, the polymer erosion issue can be circumvented. However, charge-bearing vinyl polymers, upon interaction with amphiphilic assemblies, are known to adapt to a conformation that results in encapsulating guest molecules instead of releasing them. On the contrary, it has earlier been reported that a rigid, charge-bearing, and water-insoluble conjugated polyaniline film can effectively disassemble amphiphilic assemblies without causing much harm to the film. Herein, we demonstrate the effect rendered by varying the electropolymerization potential on the interaction efficiency between the positive charge-bearing polyaniline film and oppositely charged amphiphilic assemblies. In addition, it is also demonstrated that a film of oxidized polyaniline can be regenerated for repetitive disassembly of the amphiphilic assemblies, and concomitant guest molecule delivery.

Poly(PS-b-DMA) micelles for reactive oxygen species triggered drug release

A new micelle drug carrier that consists of a diblock polymer of propylene sulfide (PS) and N,N-dimethylacrylamide (poly(PS₇₄-b-DMA₃₁₀)) has been synthesized and characterized for site-specific release of hydrophobic drugs to sites of inflammation. Propylene sulfide was first polymerized using a thioacyl group transfer (TAGT) method with the RAFT chain transfer agent (CTA) 4-cyano-4-(ethylsulfanylthiocarbonylsulfanyl) pentanoic acid (CEP), and the resultant poly(PS₇₄-CEP) macro-CTA was used to polymerize a second polymer block of DMA using reversible addition-fragmentation chain transfer (RAFT). The formation of the poly(PS₇₄-b-DMA₃₁₀) diblock polymer was confirmed by ¹H NMR spectra and gel permeation chromatography (GPC). Poly(PS₇₄-b-DMA₃₁₀) formed 100 nm micelles in aqueous media as confirmed by dynamic light scattering (DLS) and transmission electron microscopy (TEM). Micelles loaded with the model drugs Nile red and DiO were used to demonstrate the ROS-dependent drug release mechanism of these micelles following treatment with hydrogen peroxide (H₂O₂), 3-morpholinosydnonimine (SIN-1), and peroxynitrite. These oxidants were found to oxidize the micelle PPS core, making it more hydrophilic and triggering micelle disassembly and cargo release. Delivery of poly(PS₇₄-b-DMA₃₁₀) micelles dual-loaded with the Förster Resonance Energy Transfer (FRET) fluorophore pair DiI and DiO was used to prove that endogenous oxidants generated by lipopolysaccharide (LPS)-treated RAW 264.7 macrophages significantly increased release of nanocarrier contents relative to macrophages that were not activated. In vitro studies also demonstrated that the poly(PS₇₄-b-DMA₃₁₀) micelles were cytocompatible across a broad range of concentrations. These combined data suggest that the poly(PS₇₄-b-DMA₃₁₀) micelles synthesized using a combination of TAGT and RAFT have significant potential for site-specific drug delivery to tissues with high levels of oxidative stress.

UV-responsive polymeric superamphiphile based on a complex of malachite green derivative and a double hydrophilic block copolymer

We have prepared a UV-responsive polymeric superamphiphile, formed by a malachite green derivative and the double hydrophilic block copolymer methoxy-poly(ethylene glycol)(114)-block-poly(l-lysine hydrochloride)(200) (PEG-b-PLKC) on the basis of electrostatic interactions. The malachite green derivative undergoes photo-ionization upon UV irradiation, which makes it more hydrophilic, resulting in changes in the self-assembly behavior of the polymeric superamphiphile. For this reason, the polymeric superamphiphile originally self-assembles to form sheetlike aggregates, which disassemble after UV irradiation because of the increased solubility of the malachite green derivative. By use of Nile red as a probe, the polarity of the polymeric superamphiphile solution is confirmed to be increased after UV irradiation by fluorescence spectra, which also explains the disassembly of the polymeric superamphiphile.

Multistimuli responsive micelles formed by a tetrathiafulvalene-functionalized amphiphile

An electroactive tetrathiafulvalene (TTF)-functionalized amphiphile 1 was designed and synthesized to investigate its self-assembling behavior in water. Dynamic light scattering (DLS), (1)H NMR, fluorescence spectrum, and cryogenic transmission electron microscopy (cryo-TEM) studies revealed that amphiphile 1 can form micelle-like aggregates via direct dissolution into water, and the micellar architectures could be disrupted either by addition of chemical oxidant Fe(ClO(4))(3) or by complexation with electron-deficient cyclobis(paraquat-p-phenylene) tetracation cyclophane (CBPQT(4+)) to release encapsulated hydrophobic dye Nile Red from the interior of micelles.

Photocontrolled self-assembly and disassembly of block ionomer complex vesicles: a facile approach toward supramolecular polymer nanocontainers

A new concept of designing a photocontrollable supramolecular polymer nanocontainer through the electrostatic association between an azobenzene-containing surfactant (AzoC10) and a double-hydrophilic block ionomer, poly(ethylene glycol)-b-poly(acrylic acid) (PEG(43)-PAA(153)), is described. Such a block ionomer complex can self-assemble in aqueous solution and form vesicle-like aggregates, which are composed of a poly(ethylene glycol) corona and a poly(acrylic acid) shell associated with azobenzene-containing surfactant. The photoisomerization of azobenzene moieties in the block ionomer complex can reversibly tune the amphiphilicity of the surfactants, inducing the disassembly of the vesicles. Such block ionomer complex vesicles are further evaluated as nanocontainers capable to encapsulate and release guest solutes on demand controlled by light irradiation. For example, the vesicles encapsulating the fluorescein sodium display clear spherical images observed by fluorescence microscopy. However, such fluorescence-marked images disappear after releasing the solute from the vesicles triggered by the UV light. Such novel materials are of both basic and practical significance, especially as prospective nanocontainers for cargo delivery.

Dual drug delivery microcapsules via layer-by-layer self-assembly

The integration of hydrophobic and hydrophilic drugs in the polymer microcapsule offers the possibility of developing a new drug delivery system that combines the best features of these two distinct classes of material. Recently, we have reported the encapsulation of an uncharged water-insoluble drug in the polymer membrane. The hydrophobic drug is deposited using a layer-by-layer (LbL) technique, which is based on the sequential adsorption of oppositely charged polyelectrolytes onto a charged substrate. In this paper, we report the encapsulation of two different drugs, which are invariably different in structure and in their solubility in water. We have characterized these dual drug vehicular capsules by confocal laser scanning microscopy, atomic force microscopy, visible microscopy, and transmission electron microscopy. The growth of a thin film on a flat substrate by LbL was monitored by UV-vis spectra. The desorption kinetics of two drugs from the thin film was modeled by a second-order rate model.

Facile reversible UV-controlled and fast transition from emulsion to gel by using a photoresponsive polymer with a malachite green group

In this paper we describe the facile reversible UV-controlled and fast transition from emulsion to gel by using a photoresponsive polymer with a malachite green group. The photoresponsive polymer with the hydrophobic malachite green group can be used for the formation of an oil-in-water emulsion. However, upon UV irradiation of 5 min, the photochromic malachite green group could be ionized to its corresponding cation, leading to the transformation from emulsion to gel. Upon shaking, such gel can recover the emulsion state, and further UV irradiation can turn the emulsion into gel again. Such transition from emulsion to gel by photochemical reaction and reverse shaking treatment can be repeated several times. It is anticipated greatly that this line of research may provide new insight into the mechanism behind stimuli-responsive systems, facilitating the design and synthesis of new responsive molecules for the fabrication of stimuli-responsive materials with designed functions.

Organogels based on 1H-imidazolecarboxamide amphiphiles

We prepared 1H-imidazolecarboxamide amphiphiles as potential organogelators. Compounds A1-A3, in which an imidazole head was connected to a hydrophobic trialkyloxyphenyl group, showed an ability to gelate nonpolar solvents, including alkanes. The dry gels obtained from compounds A1-A3 had columnar hexagonal structures. Polycatenar 1H-imidazolecarboxamide amphiphile B2, consisting of a 1H-imidazole head connected through a benzene ring to a tridecyloxyphenyl tail, formed an organogel in DMSO. In a concentrated THF solution (30 wt %), compound B2 exhibited a lyotropic liquid-crystalline phase with a columnar hexagonal structure. X-ray diffraction (XRD) results suggested a molecular arrangement consisting of a disk, via hydrogen bonding between successive imidazole moieties, and an assembly of columnar structures.