Area Increase and Budding in Giant Vesicles Triggered by Light: Behind the Scene

Photoswitchable Endocytosis of Biomolecular Condensates in Giant Vesicles

Interactions between membranes and biomolecular condensates can give rise to complex phenomena such as wetting transitions, mutual remodeling, and endocytosis. In this study, light-triggered manipulation of condensate engulfment is demonstrated using giant vesicles containing photoswitchable lipids. UV irradiation increases the membrane area, which can be stored in nanotubes. When in contact with a condensate droplet, the UV light triggers rapid condensate endocytosis, which can be reverted by blue light. The affinity of the protein-rich condensates to the membrane and the reversibility of the engulfment processes is quantified from confocal microscopy images. The degree of photo-induced engulfment, whether partial or complete, depends on the vesicle excess area and the relative sizes of vesicles and condensates. Theoretical estimates suggest that utilizing the light-induced excess area to increase the vesicle-condensate adhesion interface is energetically more favorable than the energy gain from folding the membrane into invaginations and tubes. The overall findings demonstrate that membrane-condensate interactions can be easily and quickly modulated via light, providing a versatile system for building platforms to control cellular events and design intelligent drug delivery systems for cell repair.

Light-Activated Synthetic Rotary Motors in Lipid Membranes Induce Shape Changes Through Membrane Expansion

Membranes are the key structures to separate and spatially organize cellular systems. Their rich dynamics and transformations during the cell cycle are orchestrated by specific membrane-targeted molecular machineries, many of which operate through energy dissipation. Likewise, man-made light-activated molecular rotary motors have previously shown drastic effects on cellular systems, but their physical roles on and within lipid membranes remain largely unexplored. Here, the impact of rotary motors on well-defined biological membranes is systematically investigated. Notably, dramatic mechanical transformations are observed in these systems upon motor irradiation, indicative of motor-induced membrane expansion. The influence of several factors on this phenomenon is systematically explored, such as motor concentration and membrane composition., Membrane fluidity is found to play a crucial role in motor-induced deformations, while only minor contributions from local heating and singlet oxygen generation are observed. Most remarkably, the membrane area expansion under the influence of the motors continues as long as irradiation is maintained, and the system stays out-of-equilibrium. Overall, this research contributes to a comprehensive understanding of molecular motors interacting with biological membranes, elucidating the multifaceted factors that govern membrane responses and shape transitions in the presence of these remarkable molecular machines, thereby supporting their future applications in chemical biology.

Engineering pH-Responsive, Self-Healing Vesicle-Type Artificial Tissues with Higher-Order Cooperative Functionalities

Multicellular organisms demonstrate a hierarchical organization where multiple cells collectively form tissues, thereby enabling higher-order cooperative functionalities beyond the capabilities of individual cells. Drawing inspiration from this biological organization, assemblies of multiple protocells are developed to create novel functional materials with emergent higher-order cooperative functionalities. This paper presents new artificial tissues derived from multiple vesicles, which serve as protocellular models. These tissues are formed and manipulated through non-covalent interactions triggered by a salt bridge. Exhibiting pH-sensitive reversible formation and destruction under neutral conditions, these artificial vesicle tissues demonstrate three distinct higher-order cooperative functionalities: transportation of large cargoes, photo-induced contractions, and enhanced survivability against external threats. The rapid assembly and disassembly of these artificial tissues in response to pH variations enable controlled mechanical task performance. Additionally, the self-healing property of these artificial tissues indicates robustness against external mechanical damage. The research suggests that these vesicles can detect specific pH environments and spontaneously assemble into artificial tissues with advanced functionalities. This leads to the possibility of developing intelligent materials with high environmental specificity, particularly for applications in soft robotics.

Photolipid excitation triggers depolarizing optocapacitive currents and action potentials

Optically-induced changes in membrane capacitance may regulate neuronal activity without requiring genetic modifications. Previously, they mainly relied on sudden temperature jumps due to light absorption by membrane-associated nanomaterials or water. Yet, nanomaterial targeting or the required high infrared light intensities obstruct broad applicability. Now, we propose a very versatile approach: photolipids (azobenzene-containing diacylglycerols) mediate light-triggered cellular de- or hyperpolarization. As planar bilayer experiments show, the respective currents emerge from millisecond-timescale changes in bilayer capacitance. UV light changes photolipid conformation, which awards embedding plasma membranes with increased capacitance and evokes depolarizing currents. They open voltage-gated sodium channels in cells, generating action potentials. Blue light reduces the area per photolipid, decreasing membrane capacitance and eliciting hyperpolarization. If present, mechanosensitive channels respond to the increased mechanical membrane tension, generating large depolarizing currents that elicit action potentials. Membrane self-insertion of administered photolipids and focused illumination allows cell excitation with high spatiotemporal control.

Flavonoid-Labeled Biopolymer in the Structure of Lipid Membranes to Improve the Applicability of Antioxidant Nanovesicles

Nanovesicles produced with lipids and polymers are promising devices for drug and bioactive delivery and are of great interest in pharmaceutical applications. These nanovesicles can be engineered for improvement in bioavailability, patient compliance or to provide modified release or enhanced delivery. However, their applicability strongly depends on the safety and low immunogenicity of the components. Despite this, the use of unsaturated lipids in nanovesicles, which degrade following oxidation processes during storage and especially during the proper routes of administration in the human body, may yield toxic degradation products. In this study, we used a biopolymer (chitosan) labeled with flavonoid (catechin) as a component over a lipid bilayer for micro- and nanovesicles and characterized the structure of these vesicles in oxidation media. The purpose of this was to evaluate the in situ effect of the antioxidant in three different vesicular systems of medium, low and high membrane curvature. Liposomes and giant vesicles were produced with the phospholipids DOPC and POPC, and crystalline cubic phase with monoolein/DOPC. Concentrations of chitosan-catechin (CHCa) were included in all the vesicles and they were challenged in oxidant media. The cytotoxicity analysis using the MTT assay (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide) revealed that concentrations of CHCa below 6.67 µM are non-toxic to HeLa cells. The size and zeta potential of the liposomes evidenced the degradation of their structures, which was minimized by CHCa. Similarly, the membrane of the giant vesicle, which rapidly deteriorated in oxidative solution, was protected in the presence of CHCa. The production of a lipid/CHCa composite cubic phase revealed a specific cubic topology in small-angle X-ray scattering, which was preserved in strong oxidative media. This study demonstrates the specific physicochemical characteristics introduced in the vesicular systems related to the antioxidant CHCa biopolymer, representing a platform for the improvement of composite nanovesicle applicability.

Structural and Mechanical Response of Two-Component Photoswitchable Lipid Bilayer Vesicles

Optical control of phospholipids is an attractive option for the rapid, reversible, and tunable manipulation of membrane structure and dynamics. Azo-PC, a lipid with an azobenzene group within one acyl chain, undergoes a light-induced trans-to-cis isomerization and thus arises as a powerful tool for manipulating lipid order and dynamics. Here, we report on vesicle-scale micropipette measurements and atomistic simulations to probe the elastic stretching modulus, water permeability, toughness, thickness, and membrane area upon isomerization. We investigated both dynamics and steady-state properties. In pure azo-PC membranes, we found that the molecular area in trans was 16% smaller than that in cis, the membrane's stretching modulus kA was 2.5 ± 0.3 times greater, and the water permeability PW was 3.5 ± 0.5 times smaller. We also studied mixtures of azo-PC with the miscible, unsaturated lipid DOPC. Atomistic molecular dynamics simulations show how the membrane thickness, chain order, and correlations across membrane leaflets explain the experimental data. Together, these data show how one rotating bond changes the molecular- and membrane-scale properties. These results will be useful for photopharmacology and for developing new materials whose permeability, elasticity, and toughness may be switched on demand.

Photomanipulation of Minimal Synthetic Cells: Area Increase, Softening, and Interleaflet Coupling of Membrane Models Doped with Azobenzene-Lipid Photoswitches

Light can effectively interrogate biological systems in a reversible and physiologically compatible manner with high spatiotemporal precision. Understanding the biophysics of photo-induced processes in bio-systems is crucial for achieving relevant clinical applications. Employing membranes doped with the photolipid azobenzene-phosphatidylcholine (azo-PC), a holistic picture of light-triggered changes in membrane kinetics, morphology, and material properties obtained from correlative studies on cell-sized vesicles, Langmuir monolayers, supported lipid bilayers, and molecular dynamics simulations is provided. Light-induced membrane area increases as high as ≈25% and a ten-fold decrease in the membrane bending rigidity is observed upon trans-to-cis azo-PC isomerization associated with membrane leaflet coupling and molecular curvature changes. Vesicle electrodeformation measurements and atomic force microscopy reveal that trans azo-PC bilayers are thicker than palmitoyl-oleoyl phosphatidylcholine (POPC) bilayers but have higher specific membrane capacitance and dielectric constant suggesting an increased ability to store electric charges across the membrane. Lastly, incubating POPC vesicles with azo-PC solutions results in the insertion of azo-PC in the membrane enabling them to become photoresponsive. All these results demonstrate that light can be used to finely manipulate the shape, mechanical and electric properties of photolipid-doped minimal cell models, and liposomal drug carriers, thus, presenting a promising therapeutic alternative for the repair of cellular disorders.

Rapid and reversible optical switching of cell membrane area by an amphiphilic azobenzene

Cellular membrane area is a key parameter for any living cell that is tightly regulated to avoid membrane damage. Changes in area-to-volume ratio are known to be critical for cell shape, but are mostly investigated by changing the cell volume via osmotic shocks. In turn, many important questions relating to cellular shape, membrane tension homeostasis and local membrane area cannot be easily addressed because experimental tools for controlled modulation of cell membrane area are lacking. Here we show that photoswitching an amphiphilic azobenzene can trigger its intercalation into the plasma membrane of various mammalian cells ranging from erythrocytes to myoblasts and cancer cells. The photoisomerization leads to a rapid (250-500 ms) and highly reversible membrane area change (ca 2 % for erythrocytes) that triggers a dramatic shape modulation of living cells.

Endocytosis-Like Vesicle Fission Mediated by a Membrane-Expanding Molecular Machine Enables Virus Encapsulation for In Vivo Delivery

Biological membranes are functionalized by membrane-associated protein machinery. Membrane-associated transport processes, such as endocytosis, represent a fundamental and universal function mediated by membrane-deforming protein machines, by which small biomolecules and even micrometer-size substances can be transported via encapsulation into membrane vesicles. Although synthetic molecules that induce dynamic membrane deformation have been reported, a molecular approach enabling membrane transport in which membrane deformation is coupled with substance binding and transport remains critically lacking. Here, we developed an amphiphilic molecular machine containing a photoresponsive diazocine core (AzoMEx) that localizes in a phospholipid membrane. Upon photoirradiation, AzoMEx expands the liposomal membrane to bias vesicles toward outside-in fission in the membrane deformation process. Cargo components, including micrometer-size M13 bacteriophages that interact with AzoMEx, are efficiently incorporated into the vesicles through the outside-in fission. Encapsulated M13 bacteriophages are transiently protected from the external environment and therefore retain biological activity during distribution throughout the body via the blood following administration. This research developed a molecular approach using synthetic molecular machinery for membrane functionalization to transport micrometer-size substances and objects via vesicle encapsulation. The molecular design demonstrated in this study to expand the membrane for deformation and binding to a cargo component can lead to the development of drug delivery materials and chemical tools for controlling cellular activities.

Indane Based Molecular Motors: UV-Switching Increases Number of Isomers

We describe azophenylindane based molecular motors (aphin-switches) which have two different rotamers of trans-configuration and four different rotamers of cis-configuration. The behaviors of these motors were investigated both experimentally and computationally. The conversion of aphin-switch does not yield single isomer but a mixture of these. Although the trans to cis conversion leads to the increase of the system entropy some of the cis-rotamers can directly convert to each other while others should convert via trans-configuration. The motion of aphin-switches resembles the work of a mixing machine with indane group serving as a base and phenol group serving as a beater. The aphin-switches presented herein may provide a basis for promising applications in advanced biological systems or particularly in cases where on demand disordering of molecular packing has value, such as lipid bilayers.

Assessing membrane material properties from the response of giant unilamellar vesicles to electric fields

Knowledge of the material properties of membranes is crucial to understanding cell viability and physiology. A number of methods have been developed to probe membranes in vitro, utilizing the response of minimal biomimetic membrane models to an external perturbation. In this review, we focus on techniques employing giant unilamellar vesicles (GUVs), model membrane systems, often referred to as minimal artificial cells because of the potential they offer to mimick certain cellular features. When exposed to electric fields, GUV deformation, dynamic response and poration can be used to deduce properties such as bending rigidity, pore edge tension, membrane capacitance, surface shear viscosity, excess area and membrane stability. We present a succinct overview of these techniques, which require only simple instrumentation, available in many labs, as well as reasonably facile experimental implementation and analysis.

Characterization of DAG Binding to TRPC Channels by Target-Dependent cis-trans Isomerization of OptoDArG

Azobenzene-based photochromic lipids are valuable probes for the analysis of ion channel-lipid interactions. Rapid photoisomerization of these molecules enables the analysis of lipid gating kinetics and provides information on lipid sensing. Thermal relaxation of the metastable cis conformation to the trans conformation of azobenzene photolipids is rather slow in the dark and may be modified by ligand-protein interactions. Cis photolipid-induced changes in pure lipid membranes as visualized from the morphological response of giant unilamellar vesicles indicated that thermal cis-trans isomerization of both PhoDAG-1 and OptoDArG is essentially slow in the lipid bilayer environment. While the currents activated by cis PhoDAG remained stable upon termination of UV light exposure (dark, UV-OFF), cis OptoDArG-induced TRPC3/6/7 activity displayed a striking isoform-dependent exponential decay. The deactivation kinetics of cis OptoDArG-induced currents in the dark was sensitive to mutations in the L2 lipid coordination site of TRPC channels. We conclude that the binding of cis OptoDArG to TRPC channels promotes transition of cis OptoDArG to the trans conformation. This process is suggested to provide valuable information on DAG-ion channel interactions and may enable highly selective photopharmacological interventions.

Optical Switches for Lipid Membranes: Computed Molecular Projection Area as a Switch Selection Criterion

Optical switches in lipid membranes are an emerging tool to tune the properties of the bilayer or membrane protein integrated therein. Here, we use simple geometry and physics considerations to deduce structural criteria to design efficient photoactivated switches for lipid membranes. We compare how the area of projection on the bilayer of various classes of photoswitches changes upon the trans/cis or open/closed transition and show that azobenzene and stilbene should distort the bilayer structure the most. We also conclude that planar-elongated molecules, in which atoms of isomerizable double bond have no additional substituents, while substituents of the fragments adjacent to the double bond prevent formation of the planar molecule in cis configuration, are to be the best photoswitches for lipid membranes.

Optical Membrane Control with Red Light Enabled by Red-Shifted Photolipids

Photoswitchable phospholipids, or "photolipids", that harbor an azobenzene group in their lipid tails are versatile tools to manipulate and control lipid bilayer properties with light. So far, the limited ultraviolet-A/blue spectral range in which the photoisomerization of regular azobenzene operates has been a major obstacle for biophysical or photopharmaceutical applications. Here, we report on the synthesis of nano- and micrometer-sized liposomes from tetra-ortho-chloro azobenzene-substituted phosphatidylcholine (termed red-azo-PC) that undergoes photoisomerization on irradiation with tissue-penetrating red light (≥630 nm). Photoswitching strongly affects the fluidity and mechanical properties of lipid membranes, although small-angle X-ray scattering and dynamic light scattering measurements reveal only a minor influence on the overall bilayer thickness and area expansion. By controlling the photostationary state and the photoswitching efficiency of red-azo-PC for specific wavelengths, we demonstrate that shape transitions such as budding or pearling and the division of cell-sized vesicles can be achieved. These results emphasize the applicability of red-azo-PC as a nanophotonic tool in synthetic biology and for biomedical applications.

Transition kinetics of mixed lipid:photosurfactant assemblies studied by time-resolved small angle X-ray scattering

HYPOTHESIS

Photoswitchable surfactants are used in the design of many light-responsive colloids and/or self-assemblies. Photo-isomerization enables to control molecular equilibrium, and triggers transient reorganizations with possibly out-of-equilibrium intermediate states that have been overlooked. Here, we address this question by an in depth structural investigation of intermediate lipid-surfactant assemblies that occur during fast isothermal photo-triggered transition in lipid:surfactant mixtures.

EXPERIMENTS

The structural parameters of mixed assemblies of azobenzene-containing cationic surfactant (AzoTMA) and dioleoylphosphatidylcholine (DOPC) lipids were studied by light scattering and time-resolved small angle X-ray scattering. Structural and compositional information about the assemblies and unimers in bulk were determined at the photostationary states, as well as at intermediate kinetic states formed during UV or blue light illumination.

FINDINGS

DOPC:AzoTMA systems form mixed assemblies representative of phospholipid:cationic surfactant mixtures, that evolve from spheroid, to rod-like micelles, and vesicles with increasing DOPC fraction. Transient assemblies detected during the photo-triggered kinetics are similar to the ones found in stationary states. But changes of AzoTMA unimers in bulk can be considerably faster than mass reorganizations of the mixed assemblies, suggesting that out-of-equilibrium conditions are transiently reached. Mass reorganization of the surfactant-enriched assemblies is much faster than in the lipid enriched ones, providing insight into the role of lipids in a slow reorganization of the assemblies.

Sculpting the shapes of giant unilamellar vesicles using isotropic-nematic-isotropic phase cycles

Understanding how soft matter deforms in response to mechanical interactions is central to the design of functional synthetic materials as well as elucidation of the behaviors of biological assemblies. Here we explore how cycles of thermally induced transitions between nematic (N) and isotropic (I) phases can be used to exert cyclical elastic stresses on dispersions of giant unilamellar vesicles (GUVs) and thereby evolve GUV shape and properties. The measurements were enabled by the finding that I-N-I phase transitions of the lyotropic chromonic liquid crystal disodium cromoglycate, when conducted via an intermediate columnar (M) phase, minimized transport of GUVs on phase fronts to confining surfaces. Whereas I to N phase transitions strained spherical GUVs into spindle-like shapes, with an efflux of GUV internal volume, subsequent N to I transitions generated a range of complex GUV shapes, including stomatocyte, pear- and dumbbell-like shapes that depended on the extent of strain in the N phase. The highest strained GUVs were observed to form buds (daughter vesicles) that we show, via a cycle of I-N-I-N phase transitions, are connected via a neck to the parent vesicle. Additional experiments established that changes in elasticity of the phase surrounding the GUVs and not thermal expansion of membranes were responsible for the shape transitions, and that I-N-I transformations that generate stomatocytes can be understood from the Bilayer-Coupling model of GUV shapes. Overall, these observations advance our understanding of how LC elastic stresses can be regulated to evolve the shapes of soft biological assemblies as well as provide new approaches for engineering synthetic soft matter.

How Photoswitchable Lipids Affect the Order and Dynamics of Lipid Bilayers and Embedded Proteins

Altering the properties of phospholipid membranes by light is an attractive option for the noninvasive manipulation of membrane proteins and cellular functions. Lipids with an azobenzene group within their acyl chains such as AzoPC are suitable tools for manipulating lipid order and dynamics through a light-induced trans-to-cis isomerization. However, the action of these photoswitchable lipids at the atomic level is still poorly understood. Here, liposomes containing AzoPC, POPE, and POPG have been characterized by solid-state NMR through chemical shift and dipolar CH order parameter measurements. Upon UV-light illumination, an efficient trans-to-cis conversion can be achieved resulting in a localized reduction of the CH order parameter within the bulk lipid acyl chains. This effect is even more pronounced in liposomes containing the integral membrane protein E. coli diacylglycerol kinase. The protein responds to the light-induced trans-to-cis isomerization by a site-specific increase in the molecular dynamics as observed by altered cross peak intensities in NCA spectra. This study represents a proof-of-concept demonstration for the use of photoswitchable lipids to modulate membrane properties by light for inducing dynamic changes within an embedded membrane protein.

Fluorinated Azobenzenes Switchable with Red Light

Molecular photoswitches triggered with red or NIR light are optimal for photomodulation of complex biological systems, including efficient penetration of the human body for therapeutic purposes ("therapeutic window"). Yet, they are rarely reported, and even more rarely functional under aqueous conditions. In this work, fluorinated azobenzenes are shown to exhibit efficient E→Z photoisomerization with red light (PSS660nm >75 % Z) upon conjugation with unsaturated substituents. Initially demonstrated for aldehyde groups, this effect was also observed in a more complex structure by incorporating the chromophore into a cyclic dipeptide with propensity for self-assembly. Under physiological conditions, the latter molecule formed a supramolecular material that reversibly changed its viscosity upon irradiation with red light. Our observation can lead to design of new photopharmacology agents or phototriggered materials for in vivo use.

Unraveling topology-induced shape transformations in dendrimersomes

The vital functions of cell membranes require their ability to quickly change shape to perform complex tasks such as motion, division, endocytosis, and apoptosis. Membrane curvature in cells is modulated by very complex processes such as changes in lipid composition, the oligomerization of curvature-scaffolding proteins, and the reversible insertion of protein regions that act like wedges in the membrane. But, could much simpler mechanisms support membrane shape transformation? In this work, we demonstrate how the change of amphiphile topology in the bilayer can drive shape transformations of cell membrane models. To tackle this, we have designed and synthesized new types of amphiphiles-Janus dendrimers-that self-assemble into uni-, multilamellar, or smectic-ordered vesicles, named dendrimersomes. We synthesized Janus dendrimers containing a photo-labile bond that upon UV-Vis irradiation cleavage lose a part of the hydrophilic dendron. This leads to a change from a cylindrically to a wedge-shaped amphiphile. The high mobility of these dendrimers allows for the concentration of the wedge-shaped amphiphiles and the generation of transmembrane asymmetries. The concentration of the wedges and their rate of segregation allowed control of the budding and generation of structures such as tubules and high genus vesicles.

Responsive Nanostructured Polymer Particles

Responsive polymer particles with switchable properties are of great importance for designing smart materials in various applications. Recently, the self-assembly of block copolymers (BCPs) and polymer blends within evaporative emulsions has led to advances in the shape-controlled synthesis of polymer particles. Despite extensive recent progress on BCP particles, the responsive shape tuning of BCP particles and their applications have received little attention. This review provides a brief overview of recent approaches to developing non-spherical polymer particles from soft evaporative emulsions based on the physical principles affecting both particle shape and inner structure. Special attention is paid to the stimuli-responsive, shape-changing nanostructured polymer particles, i.e., design of polymers and surfactant pairs, detailed experimental results, and their applications, including the state-of-the-art progress in this field. Finally, the perspectives on current challenges and future directions in this research field are presented, including the development of surfactants with higher reversibility to multiple stimuli and polymers with unique structural functionality, and diversification of polymer architectures.

Selective release of a potent anticancer agent from a supramolecular hydrogel using green light

Selective green-light triggered release of an anticancer agent under physiological conditions from a supramolecular hydrogel.

Photoswitchable Conjugated Oligoelectrolytes for a Light-Induced Change of Membrane Morphology

The synthesis of a new conjugated oligoelectrolyte (COE), namely DSAzB, is described, which contains a conjugated core bearing a diazene moiety in the center of its electronically delocalized structure. Similar to structurally related phenylenevinylene-based COEs, DSAzB readily intercalates into model and natural lipid bilayer membranes. Photoinduced isomerization transforms the linear trans COE into a bent or C-shape form. It is thereby possible to introduce DSAzB into the bilayer of a cell and disrupt its integrity by irradiation with light. This leads to controlled permeabilization of membranes, as demonstrated by the release of calcein from DMPG/DMPC vesicles and by propidium iodide influx experiments on S. epidermidis. Both experiments support that the permeabilization is selective for the light stimulus, highly efficient, and repeatable. Target-selective and photoinduced actions demonstrated by DSAzB may have broad applications in biocatalysis and related biotechnologies.

Photodynamic Control of the Chain Length in Supramolecular Polymers: Switching an Intercalator into a Chain Capper

Supramolecular systems are intrinsically dynamic and sensitive to changes in molecular structure and external conditions. Because of these unique properties, strategies to control polymer length, composition, comonomer sequence, and morphology have to be developed for sufficient control over supramolecular copolymerizations. We designed photoresponsive, mono acyl hydrazone functionalized benzene-1,3,5-tricarboxamide (m-BTA) monomers that play a dual role in the coassembly with achiral alkyl BTAs (a-BTA). In the E isomer form, the chiral m-BTA monomers intercalate into stacks of a-BTA and dictate the chirality of the helices. Photoisomerization to the Z isomer transforms the intercalator into a chain capper, allowing dynamic shortening of chain length in the supramolecular aggregates. We combine optical spectroscopy and light-scattering experiments with theoretical modeling to show the reversible decrease in length when switching from the E to Z isomer of m-BTA in the copolymer with inert a-BTA. With a mass-balance thermodynamic model, we gain additional insights into the composition of copolymers and length distributions of the species over a broad range of concentrations and mixing ratios of a-BTA/m-BTA. Moreover, the model was used to predict the impact of an additive (chain capper and intercalator) on the chain length over a range of concentrations, showing a remarkable amplification of efficiency at high concentrations. By employing a stimuli-responsive comonomer in a mostly inert polymer, we can cooperatively amplify the effect of the switching and obtain photocontrol of polymer length. Moreover, this dynamic decrease in chain length causes a macroscopic gel-to-sol phase transformation of the copolymer gel, although 99.4% of the organogel is inert to the light stimulus.

Design of Collective Motions from Synthetic Molecular Switches, Rotors, and Motors

Precise control over molecular movement is of fundamental and practical importance in physics, biology, and chemistry. At nanoscale, the peculiar functioning principles and the synthesis of individual molecular actuators and machines has been the subject of intense investigations and debates over the past 60 years. In this review, we focus on the design of collective motions that are achieved by integrating, in space and time, several or many of these individual mechanical units together. In particular, we provide an in-depth look at the intermolecular couplings used to physically connect a number of artificial mechanically active molecular units such as photochromic molecular switches, nanomachines based on mechanical bonds, molecular rotors, and light-powered rotary motors. We highlight the various functioning principles that can lead to their collective motion at various length scales. We also emphasize how their synchronized, or desynchronized, mechanical behavior can lead to emerging functional properties and to their implementation into new active devices and materials.

Giant Vesicles and Their Use in Assays for Assessing Membrane Phase State, Curvature, Mechanics, and Electrical Properties

Giant unilamellar vesicles represent a promising and extremely useful model biomembrane system for systematic measurements of mechanical, thermodynamic, electrical, and rheological properties of lipid bilayers as a function of membrane composition, surrounding media, and temperature. The most important advantage of giant vesicles over other model membrane systems is that the membrane responses to external factors such as ions, (macro)molecules, hydrodynamic flows, or electromagnetic fields can be directly observed under the microscope. Here, we briefly review approaches for giant vesicle preparation and describe several assays used for deducing the membrane phase state and measuring a number of material properties, with further emphasis on membrane reshaping and curvature.

Directed Growth of Biomimetic Microcompartments

Contemporary biological cells are sophisticated and highly compartmentalized. Compartmentalization is an essential principle of prebiotic life as well as a key feature in bottom-up synthetic biology research. In this review, the dynamic growth of compartments as an essential prerequisite for enabling self-reproduction as a fundamental life process is discussed. The micrometer-sized compartments are focused on due to their cellular dimensions. Two types of compartments are considered, membraneless droplets and membrane-bound microcompartments. Growth mechanisms of aqueous droplets such as protein (condensates) or macromolecule-rich droplets (aqueous two phase systems) and coacervates are discussed, for which growth occurs via Ostwald ripening or coalescence. For membrane-bound compartments, vesicles are considered, which are composed of fatty acids, lipids, or polymers, where directed growth can occur via fusion or uptake of material from the surrounding. The development of novel approaches for growth of biomimetic microcompartments can eventually be utilized to construct new synthetic cells.

Highly Efficient Protein-free Membrane Fusion: A Giant Vesicle Study

Membrane fusion is a ubiquitous process in biology and is a prerequisite for many intracellular delivery protocols relying on the use of liposomes as drug carriers. Here, we investigate in detail the process of membrane fusion and the role of opposite charges in a protein-free lipid system based on cationic liposomes (LUVs, large unilamellar vesicles) and anionic giant unilamellar vesicles (GUVs) composed of different palmitoyloleoylphosphatidylcholine (POPC)/palmitoyloleoylphosphatidylglycerol (POPG) molar ratios. By using a set of optical-microscopy- and microfluidics-based methods, we show that liposomes strongly dock to GUVs of pure POPC or low POPG fraction (up to 10 mol%) in a process mainly associated with hemifusion and membrane tension increase, commonly leading to GUV rupture. On the other hand, docked LUVs quickly and very efficiently fuse with negative GUVs of POPG fractions at or above 20 mol%, resulting in dramatic GUV area increase in a charge-dependent manner; the vesicle area increase is deduced from GUV electrodeformation. Importantly, both hemifusion and full fusion are leakage-free. Fusion efficiency is quantified by the lipid transfer from liposomes to GUVs using fluorescence resonance energy transfer (FRET), which leads to consistent results when compared to fluorescence-lifetime-based FRET. We develop an approach to deduce the final composition of single GUVs after fusion based on the FRET efficiency. The results suggest that fusion is driven by membrane charge and appears to proceed up to charge neutralization of the acceptor GUV.

Light controlled cell-to-cell adhesion and chemical communication in minimal synthetic cells

Light controlled adhesions between sender and receiver GUVs, used as minimal synthetic cells, photoregulates their spatial proximity and chemical communication.

Light-Guided Motility of a Minimal Synthetic Cell

Cell motility is an important but complex process; as cells move, new adhesions form at the front and adhesions disassemble at the back. To replicate this dynamic and spatiotemporally controlled asymmetry of adhesions and achieve motility in a minimal synthetic cell, we controlled the adhesion of a model giant unilamellar vesicle (GUV) to the substrate with light. For this purpose, we immobilized the proteins iLID and Micro, which interact under blue light and dissociate from each other in the dark, on a substrate and a GUV, respectively. Under blue light, the protein interaction leads to adhesion of the vesicle to the substrate, which is reversible in the dark. The high spatiotemporal control provided by light, allowed partly illuminating the GUV and generating an asymmetry in adhesions. Consequently, the GUV moves into the illuminated area, a process that can be repeated over multiple cycles. Thus, our system reproduces the dynamic spatiotemporal distribution of adhesions and establishes mimetic motility of a synthetic cell.