Controlling aqueous self-assembly mechanisms by hydrophobic interactions

Solid-Phase Synthesis as a Tool to Create Exactly Defined, Branched Polymer Vectors for Cell Membrane Targeting

Modern drug formulations often require, besides the active drug molecule, auxiliaries to enhance their pharmacological properties. Tailor-made, biocompatible polymers covalently connected to the drug molecule can fulfill this function by increasing its solubility, reducing its toxicity, and guiding it to a specific target. If targeting membrane-bound proteins, localization of the drug close to the cell membrane and its target is beneficial to increase drug efficiency and residence time. In this study, we present the synthesis of highly defined, branched polymeric structures with membrane-binding properties. One to three hydrophilic poly(ethylene oxide) or poly(2-ethyloxazoline) side chains were connected via a peptoid backbone using a two-step iterative protocol for solid-phase peptoid synthesis. Additional groups, e.g., a hydrophobic anchor for membrane attachment, were introduced. Due to the nature of solid-phase synthesis, the number and order of the side chains and additional units can be precisely defined. The method proved to be versatile for the generation of multifunctional, branched polymeric structures of molecular weights up to approximately 7000 g mol-1. The behavior of all compounds towards biological membranes and cells was investigated using liposomes as cell membrane models, HEK293 and U251-MG cell lines, and red blood cells, thereby demonstrating their potential value as drug auxiliaries with cell membrane affinity.

Expanding the Scope of Metastable Species in Hydrogen Bonding‐Directed Supramolecular Polymerization

We reveal unique hydrogen (H‐) bonding patterns and exploit them to control the kinetics, pathways and length of supramolecular polymers (SPs). New bisamide‐containing monomers were designed to elucidate the role of competing intra‐ vs. intermolecular H‐bonding interactions on the kinetics of supramolecular polymerization (SP). Remarkably, two polymerization‐inactive metastable states were discovered. Contrary to previous examples, the commonly assumed intramolecularly H‐bonded monomer does not evolve into intermolecularly H‐bonded SPs via ring opening, but rather forms a metastable dimer. In this dimer, all H‐bonding sites are saturated, either intra‐ or intermolecularly, hampering elongation. The dimers exhibit an advantageous preorganization, which upon opening of the intramolecular portion of the H‐bonding motif facilitates SP in a consecutive process. The retardation of spontaneous self‐assembly as a result of two metastable states enables length control in SP by seed‐mediated growth.

A unique amphiphilic triblock copolymer, nontoxic to human blood and potential supramolecular drug delivery system for dexamethasone

The drug delivery system (DDS) often causes toxicity, triggering undesired cellular injuries. Thus, developing supramolecules used as DDS with tunable self-assembly and nontoxic behavior is highly desired. To address this, we aimed to develop a tunable amphiphilic ABA-type triblock copolymer that is nontoxic to human blood cells but also capable of self-assembling, binding and releasing the clinically used drug dexamethasone. We synthesized an ABA-type amphiphilic triblock copolymer (P2L) by incorporating tetra(aniline) TANI as a hydrophobic and redox active segment along with monomethoxy end-capped polyethylene glycol (mPEG2k; Mw = 2000 g mol-1) as biocompatible, flexible and hydrophilic part. Cell cytotoxicity was measured in whole human blood in vitro and lung cancer cells. Polymer-drug interactions were investigated by UV-Vis spectroscopy and computational analysis. Our synthesized copolymer P2L exhibited tuned self-assembly behavior with and without external stimuli and showed no toxicity in human blood samples. Computational analysis showed that P2L can encapsulate the clinically used drug dexamethasone and that drug uptake or release can also be triggered under oxidation or low pH conditions. In conclusion, copolymer P2L is nontoxic to human blood cells with the potential to carry and release anticancer/anti-inflammatory drug dexamethasone. These findings may open up further investigations into implantable drug delivery systems/devices with precise drug administration and controlled release at specific locations.

From Oligo(Phenyleneethynylene) Monomers to Supramolecular Helices: The Role of Intermolecular Interactions in Aggregation

Supramolecular helices that arise from the self-assembly of small organic molecules via non-covalent interactions play an important role in the structure and properties of the corresponding materials. Here we study the supramolecular helical aggregation of oligo(phenyleneethynylene) monomers from a theoretical point of view, always guiding the studies with experimentally available data. In this way, by systematically increasing the number of monomer units, optimized n-mer geometries are obtained along with the corresponding absorption and circular dichroism spectra. For the geometry optimizations we use density functional theory together with the B3LYP-D3 functional and the 6–31G** basis set. For obtaining the spectra we resort to time-dependent density functional theory using the CAM-B3LYP functional and the 3–21G basis set. These combinations of density functional and basis set were selected after systematic convergence studies. The theoretical results are analyzed and compared to the experimentally available spectra, observing a good agreement.

The Competitive Aggregation Pathway of an Asymmetric Chiral Oligo(p-phenyleneethynylene) Towards the Formation of Individual P and M Supramolecular Helical Polymers

A complex aggregation pathway towards two diastereomeric P and M supramolecular helices arises from the aggregation of a short, chiral, and rigid oligo(phenyleneethynylene) [OPE, (S)-1]. Thus, while Agg_I aggregate is obtained when a DCM solution of (S)-1 is diluted with MCH at room temperature, Agg_II aggregate is generated only after a slow heating (353 K)/cooling (273 K) process. Interestingly, during Agg_I formation (mechanism 1), short P chain oligomers are produced, which have a great tendency to aggregate in plane, yielding brick-like nanostructures that halt the aggregation process. On the other hand, after a heating/cooling cycle, long M type columnar helical aggregates (Agg_II ) are obtained, formed by individual supramolecular polymer chains (mechanism 2) easily visualized by AFM. The two different P/M orientations obtained for Agg_I and Agg_II reveal the dynamic character of the system and its ability to create diastereomeric helical structures under the right conditions. Different experimental protocols were explored to prepare long M type columnar helical aggregates, which are not obtained by using the previous MCH/DCM 99/1 (v/v) solvent mixture. The generation of the desired M oriented supramolecular polymer is achieved when toluene is added to the solvent mixture in a 97/2/1 MCH/Tol/DCM (v/v/v) ratio.

Block copolymers comprising degradable poly(2-ethyl-2-oxazoline) analogues via copper-free click chemistry

We present the synthesis development of amphiphilic, degradable poly(2-ethyl-2-oxazoline) (PEtOx) analogue block copolymers in a modular fashion utilizing the strain-promoted azide–alkyne cycloaddition (SPAAC).

Solvent-controlled E/Z isomerization vs. [2 + 2] photocycloaddition mediated by supramolecular polymerization

Control over the photochemical outcome of photochromic molecules in solution represents a major challenge, as photoexcitation often leads to multiple competing photochemical and/or supramolecular pathways resulting in complex product mixtures. Herein, we demonstrate precise and efficient control over the photochemical behaviour of cyanostilbenes in solution using a straightforward solvent-controlled approach based on supramolecular polymerization. To this end, we designed a π-extended cyanostilbene bolaamphiphile that exhibits tuneable solvent-dependent photochemical behaviour. Photoirradiation of the system in a monomeric state (in organic solvents) exclusively leads to a highly reversible and efficient E/Z photoisomerization, whereas a nearly quantitative [2 + 2] photocycloaddition into a single cyclobutane (anti head-to-tail) occurs in aqueous solutions. These results can be rationalized by a highly regular and preorganized antiparallel J-type arrangement of the cyanostilbene units that is driven by aqueous supramolecular polymerization. The presented concept demonstrates a novel approach towards solvent-selective and environmentally friendly photochemical transformations, which is expected to broaden the scope of supramolecular polymerization.

Modulation of the Self-Assembly of π-Amphiphiles in Water from Enthalpy- to Entropy-Driven by Enwrapping Substituents

Depending on the connectivity of solubilizing oligoethylene glycol (OEG) side chains to the π-cores of amphiphilic naphthalene and perylene bisimide dyes, self-assembly in water occurs either upon heating or cooling. Herein, we show that this effect originates from differences in the enwrapping capability of the π-cores by the OEG chains. Rylene bisimides bearing phenyl substituents with three OEG chains attached directly to the hydrophobic π-cores are strongly sequestered by the OEG chains. These molecules self-assemble at elevated temperatures in an entropy-driven process according to temperature- and concentration-dependent UV/Vis spectroscopy and calorimetric dilution studies. In contrast, for rylene bisimides in which phenyl substituents with three OEG chains are attached via a methylene spacer, leading to much weaker sequestration, self-assembly originates upon cooling in an enthalpy-driven process. Our explanation for this controversial behavior is that the aggregation in the latter case is dictated by the release of "high energy water" from the hydrophobic π-surfaces as well as dispersion interactions between the π-scaffolds which drive the self-assembly in an enthalpically driven process. In contrast, for the former case we suggest that in addition to the conventional explanation of a dehydration of hydrogen-bonded water molecules from OEG units it is in particular the increase in conformational entropy of back-folded OEG side chains upon aggregation that provides the pronounced gain in entropy that drives the aggregation process. Thus, our studies revealed that a subtle change in the attachment of solubilizing substituents can switch the thermodynamic signature for the self-assembly of amphiphilic dyes in water from enthalpy- to entropy-driven.

pH-Responsive Side Chains as a Tool to Control Aqueous Self-Assembly Mechanisms

pH-Tunable nanoscale morphology and self-assembly mechanism of a series of oligo(p-phenyleneethynylene) (OPE)-based bolaamphiphiles featuring poly(ethylene imine) (PEI) side chains of different length and degree of hydrolysis are described. Protonation and deprotonation of the PEI chains by changing the pH alters the hydrophilic/hydrophobic balance of the systems and, in turn, the strength of intermolecular interactions between the hydrophobic OPE moieties. Low pH values (3) lead to weak interaction between the OPEs and result in spherical nanoparticles, in which aggregation follows an isodesmic mechanism. In contrast, higher pH values (11) induce deprotonation of the polymer chains and lead to a stronger, cooperative aggregation into anisotropic nanostructures. Our results demonstrate that pH-responsive chains can be exploited as a tool to tune self-assembly mechanisms, which opens exciting possibilities to develop new stimuli-responsive materials.

Shape modulation of squaramide-based supramolecular polymer nanoparticles

We report the synthesis and self-assembly of a library of squaramide-based bolaamphiphiles with variable hydrophobic and hydrophilic domain sizes to understand their effect on the formation of supramolecular polymer nanoparticles.

Controlling the Miscibility of X-Shaped Bolapolyphiles in Lipid Membranes by Varying the Chemical Structure and Size of the Polyphile Polar Headgroup

Bolaamphiphiles are well-known naturally occurring structures that can increase the thermal and mechanical stability of the phospholipid membrane by incorporation in a transmembrane manner. Modifications of bolaamphiphiles to introduce particular structural elements such as a conjugated aromatic backbone and lateral side chains in the hydrophobic region lead to bolapolyphiles (BPs). We investigated the ability of BPs to form lyotropic phases in water. The BPs had an identical backbone and side chains, but different headgroup structures, leading to different abilities to act as hydrogen bond donors and acceptors. BPs with hydrophilic headgroups capable of acting as hydrogen bond donors as well as acceptors did not form lyotropic phases and were insoluble in water, independent of whether the polar groups were small or large. The extended lipophilic core structure and the multiple intermolecular hydrogen bonds between the headgroups prevented the formation of well-hydrated lyotropic aggregates. A BP with two large hydrophilic headgroups of several ethylene oxide moieties terminated by methyl groups formed sheet- and vesicle-like aggregates in water. These headgroups act only as hydrogen bond acceptors and cannot form hydrogen bonds in the absence of water. The miscibility of BPs with vesicles of 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) in water and the resulting aggregate structures were also investigated. For BPs with headgroups acting as donors and acceptors of hydrogen bonds, macroscopic phase separation occurred in the mixed membranes, and two different membrane domains, a DPPC-rich one containing only little polyphile and a BP-rich one containing varying amounts of lipid, were formed. For headgroups without the ability to act as hydrogen bond donors, small BP aggregates were formed that were homogeneously distributed over the membrane. The lateral organization of BPs in lipid membranes can thus be controlled by the nature of the BP headgroup.

A clickable NHC-Au(i)-complex for the preparation of stimulus-responsive metallopeptide amphiphiles

We report the synthesis of an alkyne functionalised NHC-Au(i)-complex which is conjugated with amphiphilic oligopeptides using a copper(i) catalysed cycloaddition. The resulting Au(i)-metalloamphiphiles are shown to self-assemble into charge-regulated stimulus-responsive supramolecular polymers in water via a weakly cooperative polymerisation mechanism.

Air–water interfacial assembly of all-aromatic-substituted double-decker phthalocyanine forms aligned nanoparticles

In this manuscript, unexpected supramolecular assembly of [Formula: see text]-conjugated molecules containing complex aromatic substituents was investigated. The air–water interfacial assembly of double-decker phthalocyanines containing sixteen phenol substituents (Ce(Pc2)[Formula: see text] and Y(Pc2)[Formula: see text] form aligned nanoparticles. Depending on the different surface pressure, the Ce(Pc2)[Formula: see text] self-assembled nanostructures can be regulated thoroughly. Although Ce(Pc2)[Formula: see text] and Y(Pc2)[Formula: see text] have only aromatic substituent groups, no H- or J-aggregation of [Formula: see text]-conjugated systems can be detected from the UV-vis spectra of the assemblies of these double-decker phthalocyanines. When the nanostructures of these assemblies were changed greatly, no corresponding changes of UV-vis spectra and FT-IR spectra could be detected. These unusual results can be understood from the balance between the hydrophilicity of aromatic substituents and the ether linkages of double-decker phthalocyanines and the surface pressure, and open new. approaches for supramolecular assembly of complex [Formula: see text]-conjugated systems.

From dynamic self-assembly to networked chemical systems

Although dynamic self-assembly, DySA, is a relatively new area of research, the past decade has brought numerous demonstrations of how various types of components - on scales from (macro)molecular to macroscopic - can be arranged into ordered structures thriving in non-equilibrium, steady states. At the same time, none of these dynamic assemblies has so far proven practically relevant, prompting questions about the field's prospects and ultimate objectives. The main thesis of this Review is that formation of dynamic assemblies cannot be an end in itself - instead, we should think more ambitiously of using such assemblies as control elements (reconfigurable catalysts, nanomachines, etc.) of larger, networked systems directing sequences of chemical reactions or assembly tasks. Such networked systems would be inspired by biology but intended to operate in environments and conditions incompatible with living matter (e.g., in organic solvents, elevated temperatures, etc.). To realize this vision, we need to start considering not only the interactions mediating dynamic self-assembly of individual components, but also how components of different types could coexist and communicate within larger, multicomponent ensembles. Along these lines, the review starts with the discussion of the conceptual foundations of self-assembly in equilibrium and non-equilibrium regimes. It discusses key examples of interactions and phenomena that can provide the basis for various DySA modalities (e.g., those driven by light, magnetic fields, flows, etc.). It then focuses on the recent examples where organization of components in steady states is coupled to other processes taking place in the system (catalysis, formation of dynamic supramolecular materials, control of chirality, etc.). With these examples of functional DySA, we then look forward and consider conditions that must be fulfilled to allow components of multiple types to coexist, function, and communicate with one another within the networked DySA systems of the future. As the closing examples show, such systems are already appearing heralding new opportunities - and, to be sure, new challenges - for DySA research.

Novel morphologies of poly(allyamine hydrochloride)-methotrexate nanoassemblies for methotrexate delivery

Poly(allylamine hydrochloride)-methotrexate (PAH-MTX) nanoassemblies with novel morphologies (i.e. nanostrips, nanorolls, nanosheets, and nanospheres) were achieved for the first time via supramolecular self-assembly directed by the synergistic action of various non-covalent interactions between PAH and MTX molecules in aqueous solution. Herein, MTX acted in a versatile manner as both a morphology-regulating agent and a small molecular hydrophobic anticancer drug. Moreover, different morphologies presented diverse drug release profiles, which may be caused by the distinctive interactions between PAH and MTX molecules. Synergistically non-covalent interactions, including electrostatic interactions, van der Waals forces, and hydrogen bonding, favored easier matrix corrosion and more rapid drug release of non-spherical structures (i.e. nanostrips, nanorolls, and nanosheets) through the ligand exchange process. On the other hand, the highly sealed encapsulation mode for hydrophobic MTX molecules made the nanospheres exhibit slower and better controlled release. In addition, in vitro bioassay tests showed that nanostrips displayed the most obvious suppression on the viability of cancer cells among other morphologies, especially after a longer duration. The strategy of using small molecular anticancer drugs not as passively delivered cargoes but as effective molecular building blocks, opens up a new way to develop self-delivering drugs for anticancer therapy.

Effects of Lateral and Terminal Chains of X-Shaped Bolapolyphiles with Oligo(phenylene ethynylene) Cores on Self-Assembly Behaviour. Part 1: Transition between Amphiphilic and Polyphilic Self-Assembly in the Bulk

Polyphilic self-assembly leads to compartmentalization of space and development of complex structures in soft matter on different length scales, reaching from the morphologies of block copolymers to the liquid crystalline (LC) phases of small molecules. Whereas block copolymers are known to form membranes and interact with phospholipid bilayers, liquid crystals have been less investigated in this respect. Here, series of bolapolyphilic X-shaped molecules were synthesized and investigated with respect to the effect of molecular structural parameters on the formation of LC phases (part 1), and on domain formation in phospholipid bilayer membranes (part 2). The investigated bolapolyphiles are based on a rod-like π-conjugated oligo(phenylene ethynylene) (OPE) core with two glycerol groups being either directly attached or separated by additional ethylene oxide (EO) units to both ends. The X-shape is provided by two lateral alkyl chains attached at opposite sides of the OPE core, being either linear, branched, or semiperfluorinated. In this report, the focus is on the transition from polyphilic (triphilic or tetraphilic) to binary amphiphilic self-assembly. Polyphilic self-assembly, i.e., segregation of all three or four incorporated units into separate nano-compartments, leads to the formation of hexagonal columnar LC phases, representing triangular honeycombs. A continuous transition from the well-defined triangular honeycomb structures to simple hexagonal columnar phases, dominated by the arrangement of polar columns on a hexagonal lattice in a mixed continuum formed by the lipophilic chains and the OPE rods, i.e., to amphiphilic self-assembly, was observed by reducing the length and volume of the lateral alkyl chains. A similar transition was found upon increasing the length of the EO units involved in the polar groups. If the lateral alkyl chains are enlarged or replaced by semiperfluorinated chains, then the segregation of lateral chains and rod-like cores is retained, even for enlarged polar groups, i.e., the transition from polyphilic to amphiphilic self-assembly is suppressed.

"Installation art"-like hierarchical self-assembly of giant polymeric elliptical platelets

This paper reports the aqueous self-assembly of giant elliptical platelets over 20 μm in axial length, from a novel polyamide. Both the self-assembly pathway and mechanism were studied using morphology and X-ray characterizations. The polymer first self-organizes into small quadrangular frustum pyramid platelets, and then these small platelets can be further installed into giant elliptical platelets through an "installation art"-like hierarchical self-assembly process driven by crystallization. The as-prepared regular giant platelets can further aggregate together into multi-horned or flower-like superstructures.

Tuning supramolecular aurophilic structures: the effect of counterion, positive charge and solvent

The synthesis of different cationic gold(i) complexes gave rise to the formation of unexpected supramolecular assemblies in water going from rod-like structures to vesicles and square-like structures.

Strategies to create hierarchical self-assembled structures via cooperative non-covalent interactions

Cooperative phenomena are common processes involved in the hierarchical self-assembly of multiple systems in nature, such as the tobacco mosaic virus and a cell's cytoskeleton. Motivated by the high degree of order exhibited by these systems, a great deal of effort has been devoted in the past two decades to design hierarchical supramolecular polymers by combining different classes of cooperative interactions. In this review, we have classified the field of supramolecular polymers depending on the cooperative non-covalent forces driving their formation, with particular emphasis on recent examples from literature. We believe that this overview would help scientists in the field to design novel self-assembled systems with improved complexity and functionalities.

Steric Constraints Induced Frustrated Growth of Supramolecular Nanorods in Water

A unique example of supramolecular polymerisation in water based on monomers with nanomolar affinities, which yield rod-like materials with extraordinarily high thermodynamic stability, yet of finite length, is reported. A small library of charge-neutral dendritic peptide amphiphiles was prepared, with a branched nonaphenylalanine-based core that was conjugated to hydrophilic dendrons of variable steric demand. Below a critical size of the dendron, the monomers assemble into nanorod-like polymers, whereas for larger dendritic side chains frustrated growth into near isotropic particles is observed. The supramolecular morphologies observed by electron microscopy, X-ray scattering and diffusion NMR spectroscopy studies are in agreement with the mechanistic insights obtained from fitting polymerisation profiles: non-cooperative isodesmic growth leads to degrees of polymerisation that match the experimentally determined nanorod contour lengths of close to 70 nm. The reported designs for aqueous self-assembly into well-defined anisotropic particles has promising potential for biomedical applications and the development of functional supramolecular biomaterials, with emerging evidence that anisotropic shapes in carrier design outperform conventional isotropic materials for targeted imaging and therapy.

Dendritic domains with hexagonal symmetry formed by x-shaped bolapolyphiles in lipid membranes

A novel class of bolapolyphile (BP) molecules are shown to integrate into phospholipid bilayers and self-assemble into unique sixfold symmetric domains of snowflake-like dendritic shapes. The BPs comprise three philicities: a lipophilic, rigid, π-π stacking core; two flexible lipophilic side chains; and two hydrophilic, hydrogen-bonding head groups. Confocal microscopy, differential scanning calorimetry, XRD, and solid-state NMR spectroscopy confirm BP-rich domains with transmembrane-oriented BPs and three to four lipid molecules per BP. Both species remain well organized even above the main 1,2-dipalmitoyl-sn-glycero-3-phosphocholine transition. The BP molecules only dissolve in the fluid membrane above 70 °C. Structural variations of the BP demonstrate that head-group hydrogen bonding is a prerequisite for domain formation. Independent of the head group, the BPs reduce membrane corrugation. In conclusion, the BPs form nanofilaments by π stacking of aromatic cores, which reduce membrane corrugation and possibly fuse into a hexagonal network in the dendritic domains.

Aqueous self-assembly of a charged BODIPY amphiphile via nucleation–growth mechanism

A new amphiphilic boron-dipyrromethene dye forms fluorescent vesicular aggregates in water through a highly cooperative self-assembly process.