Improving the Folding of Supramolecular Copolymers by Controlling the Assembly Pathway Complexity

Effect of Particle Heterogeneity in Catalytic Copper-Containing Single-Chain Polymeric Nanoparticles Revealed by Single-Particle Kinetics

Single-chain polymeric nanoparticles (SCPNs) have been extensively explored as a synthetic alternative to enzymes for catalytic applications. However, the inherent structural heterogeneity of SCPNs, arising from the dispersity of the polymer backbone and stochastic incorporation of different monomers as well as catalytic moieties, is expected to lead to variations in catalytic activity between individual particles. To understand the effect of structural heterogeneities on the catalytic performance of SCPNs, techniques are required that permit researchers to directly monitor SCPN activity at the single-polymer level. In this study, we introduce the use of single-molecule fluorescence microscopy to study the kinetics of Cu(I)-containing SCPNs towards depropargylation reactions. We developed Cu(I)-containing SCPNs that exhibit fast kinetics towards depropargylation and Cu-catalyzed azide-alkyne click reactions, making them suitable for single-particle kinetic studies. SCPNs were then immobilized on the surface of glass coverslips and the catalytic reactions were monitored at a single-particle level using total internal reflection fluorescence (TIRF) microscopy. Our studies revealed the interparticle turnover dispersity for Cu(I)-catalyzed depropargylations. In the future, our approach can be extended to different polymer designs which can give insights into the intrinsic heterogeneity of SCPN catalysis and can further aid in the rational development of SCPN-based catalysts.

Enhanced Efficiency of Pd(0)-Based Single Chain Polymeric Nanoparticles for in Vitro Prodrug Activation by Modulating the Polymer's Microstructure

Bioorthogonal catalysis employing transition metal catalysts is a promising strategy for the in situ synthesis of imaging and therapeutic agents in biological environments. The transition metal Pd has been widely used as a bioorthogonal catalyst, but bare Pd poses challenges in water solubility and catalyst stability in cellular environments. In this work, Pd(0) loaded amphiphilic polymeric nanoparticles are applied to shield Pd in the presence of living cells for the in situ generation of a fluorescent dye and anticancer drugs. Pd(0) loaded polymeric nanoparticles prepared by the reduction of the corresponding Pd(II)-polymeric nanoparticles are highly active in the deprotection of pro-rhodamine dye and anticancer prodrugs, giving significant fluorescence enhancement and toxigenic effects, respectively, in HepG2 cells. In addition, we show that the microstructure of the polymeric nanoparticles for scaffolding Pd plays a critical role in tuning the catalytic efficiency, with the use of the ligand triphenylphosphine as a key factor for improving the catalyst stability in biological environments.

Pentafluorobenzene: Promising Applications in Diagnostics and Therapeutics

Pentafluorobenzene (PFB) represents a class of aromatic fluorine compounds employed exclusively across a spectrum of chemical and biological applications. PFBs are credited with developing various chemical synthesis techniques, networks and biopolymers, bioactive materials, and targeted drug delivery systems. The first part of this review delves into recent developments in PFB-derived molecules for diagnostic purposes. In the latter segment, PFB's role in the domain of theragnostic applications is discussed. The review elucidates different mechanisms and interaction strategies applied in leveraging PFBs to formulate diagnostic and theragnostic tools, substantiated by proper examples. The utilization of PFBs emerges as an enabler, facilitating manifold reactions, improving materials' properties, and even opening avenues for explorative research.

Amphiphilic polymeric nanoparticles enable homogenous rhodium-catalysed NH insertion reactions in living cells

Rh-catalysed NH carbene insertion reactions were exported to living cells with help of amphiphilic polymeric nanoparticles. Hereto, hydrophobic dirhodium carboxylate catalysts were efficiently encapsulated in amphiphilic polymeric nanoparticles comprising dodecyl and Jeffamine as side grafts. The developed catalytic nanoparticles promoted NH carbene insertions between α-keto diazocarbenes and 2,3-diaminonaphthalene, followed by intramolecular cyclisation to form fluorescent or biologically active benzoquinoxalines. These reactions were studied in reaction media of varying complexity. The best-performing catalyst was exported to HeLa cells, where fluorescent and cytotoxic benzoquinoxalines were synthesized in situ at low catalyst loading within a short time. Most of the developed bioorthogonal transition metal catalysts reported to date are easily deactivated by the reactive biomolecules in living cells, limiting their applications. The high catalytic efficiency of the Rh-based polymeric nanoparticles reported here opens the door to expanding the repertoire of bioorthogonal reactions and is therefore promising for biomedical applications.

Protein-Inspired Control over Synthetic Polymer Folding for Structured Functional Nanoparticles in Water

The folding of proteins into functional nanoparticles with defined 3D structures has inspired chemists to create simple synthetic systems mimicking protein properties. The folding of polymers into nanoparticles in water proceeds via different strategies, resulting in the global compaction of the polymer chain. Herein, we review the different methods available to control the conformation of synthetic polymers and collapse/fold them into structured, functional nanoparticles, such as hydrophobic collapse, supramolecular self-assembly, and covalent cross-linking. A comparison is made between the design principles of protein folding to synthetic polymer folding and the formation of structured nanocompartments in water, highlighting similarities and differences in design and function. We also focus on the importance of structure for functional stability and diverse applications in complex media and cellular environments.

Spectrally PAINTing a Single Chain Polymeric Nanoparticle at Super-Resolution

Folding a polymer chain into a well-defined single-chain polymeric nanoparticle (SCPN) is a fascinating approach to obtaining structured and functional nanoparticles. Like all polymeric materials, SCPNs are heterogeneous in their nature due to the polydispersity of their synthesis: the stochastic synthesis of polymer backbone length and stochastic functionalization with hydrophobic and hydrophilic pendant groups make structural diversity inevitable. Therefore, in a single batch of SCPNs, nanoparticles with different physicochemical properties are present, posing a great challenge to their characterization at a single-particle level. The development of techniques that can elucidate differences between SCPNs at a single-particle level is imperative to capture their potential applications in different fields such as catalysis and drug delivery. Here, a Nile Red based spectral point accumulation for imaging in nanoscale topography (NR-sPAINT) super-resolution fluorescence technique was implemented for the study of SCPNs at a single-particle level. This innovative method allowed us to (i) map the small-molecule binding rates on individual SCPNs and (ii) map the polarity of individual SCPNs for the first time. The SCPN designs used here have the same polymeric backbone but differ in the number of hydrophobic groups. The experimental results show notable interparticle differences in the binding rates within the same polymer design. Moreover, a marked polarity shift between the different designs is observed. Interestingly, interparticle polarity heterogeneity was unveiled, as well as an intraparticle diversity, information which has thus far remained hidden by ensemble techniques. The results indicate that the addition of hydrophobic pendant groups is vital to determine binding properties and induces single-particle polarity diversity. Overall, NR-sPAINT represents a powerful approach to quantifying the single-particle polarity of SCPNs and paves the way to relate the structural heterogeneity to functionality at the single-particle level. This provides an important step toward the aim of rationally designing SCPNs for the desired application.

Direct C−H Trifluoromethylation of (Hetero)Arenes in Water Enabled by Organic Photoredox‐Active Amphiphilic Nanoparticles

Photoredox‐catalyzed chemical conversions are predominantly operated in organic media to ensure good compatibility between substrates and catalysts. Yet, when conducted in aqueous media, they are an attractive, mild, and green way to introduce functional groups into organic molecules. We here show that trifluoromethyl groups can be readily installed into a broad range of organic compounds by using water as the reaction medium and light as the energy source. To bypass solubility obstacles, we developed robust water‐soluble polymeric nanoparticles that accommodate reagents and photocatalysts within their hydrophobic interior under high local concentrations. By taking advantage of the high excited state reduction potential of N‐phenylphenothiazine (PTH) through UV light illumination, the direct C−H trifluoromethylation of a wide array of small organic molecules is achieved selectively with high substrate conversion. Key to our approach is slowing down the production of CF₃ radicals during the chemical process by reducing the catalyst loading as well as the light intensity, thereby improving effectiveness and selectivity of this aqueous photocatalytic method. Furthermore, the catalyst system shows excellent recyclability and can be fueled by sunlight. The method we propose here is versatile, widely applicable, energy efficient, and attractive for late‐stage introduction of trifluoromethyl groups into biologically active molecules.

En Route to Stabilized Compact Conformations of Single-Chain Polymeric Nanoparticles in Complex Media

Precise control over the folding pathways of polypeptides using a combination of noncovalent and covalent interactions has evolved into a wide range of functional proteins with a perfectly defined 3D conformation. Inspired hereby, we develop a series of amphiphilic copolymers designed to form compact, stable, and structured single-chain polymeric nanoparticles (SCPNs) of defined size, even in competitive conditions. The SCPNs are formed through a combination of noncovalent interactions (hydrophobic and hydrogen-bonding interactions) and covalent intramolecular cross-linking using a light-induced [2 + 2] cycloaddition. By comparing different self-assembly pathways of the nanoparticles, we show that, like for proteins in nature, the order of events matters. When covalent cross-links are formed prior to the folding via hydrophobic and supramolecular interactions, larger particles with less structured interiors are formed. In contrast, when the copolymers first fold via hydrophobic and hydrogen-bonding interactions into compact conformations, followed by covalent cross-links, good control over the size of the SCPNs and microstructure of the hydrophobic interior is achieved. Such a structured SCPN can stabilize the solvatochromic dye benzene-1,3,5-tricarboxamide-Nile Red via molecular recognition for short periods of time in complex media, while showing slow exchange dynamics with the surrounding complex media at longer time scales. The SCPNs show good biocompatibility with cells and can carry cargo into the lysosomal compartments of the cells. Our study highlights the importance of control over the folding pathway in the design of stable SCPNs, which is an important step forward in their application as noncovalent drug or catalyst carriers in biological settings.

Elucidating the Stability of Single-Chain Polymeric Nanoparticles in Biological Media and Living Cells

The controlled folding of synthetic polymer chains into single-chain polymeric nanoparticles (SCPNs) of defined size and shape in water is a viable way to create compartmentalized, nanometer-sized structures for a range of biological applications. Understanding the relationship between the polymer's microstructure and the stability of folded structures is crucial to achieving desired applications. Here, we introduce the solvatochromic dye Nile red into SCPNs and apply a combination of spectroscopic and microscopic techniques to relate polymer microstructure to nanoparticle stability in complex biological media and cellular environments. Our experimental data show that the polymer's microstructure has little effect on the stability of SCPNs in biological media and cytoplasm of living cells, but only SCPNs comprising supramolecular benzene-1,3,5-tricarboxamide (BTA) motifs showed good stability in lysosomes. The results indicate that the polymer's microstructure is vital to ensure nanoparticle stability in highly competitive environments: both hydrophobic collapse and a structured interior are required. Our study provides an accessible way of probing the stability of SCPNs in cellular environments and paves the way for designing highly stable SCPNs for efficient bio-orthogonal catalysis and sensing applications.

Photoredox-Catalyzed Reduction of Halogenated Arenes in Water by Amphiphilic Polymeric Nanoparticles

The use of organic photoredox catalysts provides new ways to perform metal-free reactions controlled by light. While these reactions are usually performed in organic media, the application of these catalysts at ambient temperatures in aqueous media is of considerable interest. We here compare the activity of two established organic photoredox catalysts, one based on 10-phenylphenothiazine (PTH) and one based on an acridinium dye (ACR), in the light-activated dehalogenation of aromatic halides in pure water. Both PTH and ACR were covalently attached to amphiphilic polymers that are designed to form polymeric nanoparticles with hydrodynamic diameter DH ranging between 5 and 11 nm in aqueous solution. Due to the hydrophobic side groups that furnish the interior of these nanoparticles after hydrophobic collapse, water-insoluble reagents can gather within the nanoparticles at high local catalyst and substrate concentrations. We evaluated six different amphiphilic polymeric nanoparticles to assess the effect of polymer length, catalyst loading and nature of the catalyst (PTH or ACR) in the dechlorination of a range of aromatic chlorides. In addition, we investigate the selectivity of both catalysts for reducing different types of aryl-halogen bonds present in one molecule, as well as the activity of the catalysts for C-C cross-coupling reactions. We find that all polymer-based catalysts show high activity for the reduction of electron-poor aromatic compounds. For electron-rich compounds, the ACR-based catalyst is more effective than PTH. In the selective dehalogenation reactions, the order of bond stability is C-Cl > C-Br > C-I irrespective of the catalyst applied. All in all, both water-compatible systems show good activity in water, with ACR-based catalysts being slightly more efficient for more resilient substrates.

Self-Assembled Multi- and Single-Chain Glyconanoparticles and Their Lectin Recognition

In this work, we describe the physicochemical characterization of amphiphilic glycopolymers synthesized via copper(0)-mediated reversible-deactivation radical polymerization (Cu-RDRP). Depending on the chemical composition of the polymer, these glycopolymers are able to form multi-chain or single-chain polymeric nanoparticles. The folding of these polymers is first of all driven by the amphiphilicity of the glycopolymers and furthermore by the supramolecular formation of helical supramolecular stacks of benzene-1,3,5-tricarboxamides (BTAs) stabilized by threefold hydrogen bonding. The obtained polymeric nanoparticles were subsequently evaluated for their lectin-binding affinity toward a series of mannose- and galactose-binding lectins via surface plasmon resonance. We found that addition of 2-ethylhexyl acrylate to the polymer composition results in compact particles, which translates to a reduction in binding affinity, whereas with the addition of BTAs, the relation between the nature of the particle and the binding ability system becomes more unpredictable.

Spontaneous Self-Assembly of Single-Chain Amphiphilic Polymeric Nanoparticles in Water

Single-chain polymeric nanoparticles (SCPNs) have great potential as functional nanocarriers for drug delivery and bioimaging, but synthetic challenges in terms of final yield and purification procedures limit their use. A new concept to modify and improve the synthetic procedures used to generate water-soluble SCPNs through amphiphilic interactions has been successfully exploited. We developed a new ultrahigh molecular weight amphiphilic polymer containing a hydrophobic poly(epichlorohydrin) backbone and hydrophilic poly(ethylene glycol) side chains. The polymer spontaneously self-assembles into SCPNs in aqueous solution and does not require subsequent purification. The resulting SCPNs possess a number of distinct physical properties, including a uniform hydrodynamic nanoparticle diameter of 10-15 nm, extremely low viscosity and a desirable spherical-like morphology. Concentration-dependent studies demonstrated that stable SCPNs were formed at high concentrations up to 10 mg/mL in aqueous solution, with no significant increase in solution viscosity. Importantly, the SCPNs exhibited high structural stability in media containing serum or phosphate-buffered saline and showed almost no change in hydrodynamic diameter. The combination of these characteristics within a water-soluble SCPN is highly desirable and could potentially be applied in a wide range of biomedical fields. Thus, these findings provide a path towards a new, innovative route for the development of water-soluble SCPNs.

Amphiphilic Polymeric Nanoparticles for Photoredox Catalysis in Water

Photoredox catalysis has recently emerged as a powerful synthesis tool in organic and polymer chemistry. In contrast to the great achievements realized in organic solvents, performing photocatalytic processes efficiently in aqueous media encounters several challenges. Here, it is presented how amphiphilic single-chain polymeric nanoparticles (SCPNs) can be utilized as small reactors to conduct light-driven chemical reactions in water. By incorporating a phenothiazine (PTH) catalyst into the polymeric scaffold, metal-free reduction and C-C cross-coupling reactions can be carried out upon exposure to UV light under ambient conditions. The versatility of this approach is underlined by a large substrate scope, tolerance towards oxygen, and excellent recyclability. This approach thereby contributes to a sustainable and green way of implementing photoredox catalysis.

Caught in the Act: Mechanistic Insight into Supramolecular Polymerization-Driven Self-Replication from Real-Time Visualization

Self-assembly features prominently in fields ranging from materials science to biophysical chemistry. Assembly pathways, often passing through transient intermediates, can control the outcome of assembly processes. Yet, the mechanisms of self-assembly remain largely obscure due to a lack of experimental tools for probing these pathways at the molecular level. Here, the self-assembly of self-replicators into fibers is visualized in real-time by high-speed atomic force microscopy (HS-AFM). Fiber growth requires the conversion of precursor molecules into six-membered macrocycles, which constitute the fibers. HS-AFM experiments, supported by molecular dynamics simulations, revealed that aggregates of precursor molecules accumulate at the sides of the fibers, which then diffuse to the fiber ends where growth takes place. This mechanism of precursor reservoir formation, followed by one-dimensional diffusion, which guides the precursor molecules to the sites of growth, reduces the entropic penalty associated with colocalizing precursors and growth sites and constitutes a new mechanism for supramolecular polymerization.

Synthesis of Polymer Single-Chain Nanoparticle with High Compactness in Cosolvent Condition: A Computer Simulation Study

Polymeric single-chain nanoparticles (SCNPs) are soft nano-objects synthesized by intramolecular crosslinking of isolated single polymer chains. Syntheses of such SCNPs usually need to be performed in a dilute solution. In such a condition, the bonding probability of the two active crosslinking units at a short contour distance along the chain backbone is much higher than those which are far away from each other. Such a reaction condition often results in local spheroidization and, therefore, the formation of loosely packed structures. How to inhibit the local spheroidization and improve the compactness of SCNPs is thus a major challenge for the syntheses of SCNPs. In this study, computer simulations are performed and the fact that a precollapse of the polymer chain conformation in a cosolvent condition can largely improve the probability of the crosslinking reactions at large contour distances is demonstrated, favoring the formations of closely packed globular structures. As a result, the formed SCNPs can be more spherical and have higher compactness than those fabricated in ultradilute good solvent solution in a conventional way. It is believed this simulation work can provide a insight into the effective syntheses of SCNPs with spherical conformations and high compactness.

Preparation of metallosupramolecular single-chain polymeric nanoparticles and their characterization by Taylor dispersion

Polymers with pendant ligands furnish single-chain polymeric nanoparticles upon intramolecular metal–ligand complex formation with different metal-ions and Taylor dispersion analysis is employed to reliably characterize the dispersed particles.

Effect of Intra- versus Intermolecular Cross-Linking on the Supramolecular Folding of a Polymer Chain

Anfinsen's famous experiment showed that the restoration of catalytic activity of a completely unfolded ribonuclease A is only possible when the correct order of events is followed during the refolding process. Inspired by this work, the effect of structural constraints induced by covalent cross-links on the folding of a synthetic polymer chain via hydrogen-bonding interactions is investigated. Hereto, methacrylate-based monomers comprising either benzene-1,3,5-tricarboxamide (BTA)-based or coumarin-based pendants are copolymerized with n-butyl methacrylate in various ratios via reversible addition-fragmentation chain-transfer (RAFT) polymerization. To assess whether the folding and single-chain polymeric nanoparticle (SCPN) formation depend on the order of events, we compare two folding pathways. In the one case, we first covalently cross-link the coumarin pendants within the polymers in a solvent that prevents hydrogen bonding, after which hydrogen bonding is activated, inducing folding of the polymer. In the other case, we induce hydrogen-bonding interactions between tethered BTAs prior to covalent cross-linking of the coumarin pendants. A combination of circular dichroism (CD) spectroscopy, UV-vis spectroscopy, size-exclusion chromatography (SEC), and dynamic light scattering (DLS) is employed to understand the effect of the structural constraints on the folding behavior of these synthetic polymers. The results show that like in ribonuclease A, the order of events matters greatly and determines the outcome. Importantly, a hydrogen-bond-promoting solvent prevents the formation of SCPNs upon covalent cross-linking and results in multichain aggregates. In contrast, covalently cross-linking the polymer when no hydrogen bonds are present followed by inducing hydrogen bonding favors the formation of SCPNs above the UCST of the methacrylate-based polymer. To our surprise, the two systems show a fundamentally different response to changes in temperature, indicating that also in synthetic polymers differences in the folding pathway induce differences in the properties of the resultant nanostructures.

Single-chain polymer nanoparticles in controlled drug delivery and targeted imaging

As a relatively new class of materials, single-chain polymer nanoparticles (SCNPs) just entered the field of (biomedical) applications, with recent advances in polymer science enabling the formation of bio-inspired nanosized architectures. Exclusive intramolecular collapse of individual polymer chains results in individual nanoparticles. With sizes an order of magnitude smaller than conventional polymer nanoparticles, SCNPs are in the size regime of many proteins and viruses (1-20 nm). Multifaceted syntheses and design strategies give access to a wide set of highly modular SCNP materials. This review describes how SCNPs have been rendered water-soluble and highlights ongoing research efforts towards biocompatible SCNPs with tunable properties for controlled drug delivery, targeted imaging and protein mimicry.

Efficient sampling of reversible cross-linking polymers: Self-assembly of single-chain polymeric nanoparticles

We present a new simulation technique to study systems of polymers functionalized by reactive sites that bind/unbind forming reversible linkages. Functionalized polymers feature self-assembly and responsive properties that are unmatched by the systems lacking selective interactions. The scales at which the functional properties of these materials emerge are difficult to model, especially in the reversible regime where such properties result from many binding/unbinding events. This difficulty is related to large entropic barriers associated with the formation of intra-molecular loops. In this work, we present a simulation scheme that sidesteps configurational costs by dedicated Monte Carlo moves capable of binding/unbinding reactive sites in a single step. Cross-linking reactions are implemented by trial moves that reconstruct chain sections attempting, at the same time, a dimerization reaction between pairs of reactive sites. The model is parametrized by the reaction equilibrium constant of the reactive species free in solution. This quantity can be obtained by means of experiments or atomistic/quantum simulations. We use the proposed methodology to study the self-assembly of single-chain polymeric nanoparticles, starting from flexible precursors carrying regularly or randomly distributed reactive sites. We focus on understanding differences in the morphology of chain nanoparticles when linkages are reversible as compared to the well-studied case of irreversible reactions. Intriguingly, we find that the size of regularly functionalized chains, in good solvent conditions, is non-monotonous as a function of the degree of functionalization. We clarify how this result follows from excluded volume interactions and is peculiar of reversible linkages and regular functionalizations.

Catalytically Active Single-Chain Polymeric Nanoparticles: Exploring Their Functions in Complex Biological Media

Dynamic single-chain polymeric nanoparticles (SCPNs) are intriguing, bioinspired architectures that result from the collapse or folding of an individual polymer chain into a nanometer-sized particle. Here we present a detailed biophysical study on the behavior of dynamic SCPNs in living cells and an evaluation of their catalytic functionality in such a complex medium. We first developed a number of delivery strategies that allowed the selective localization of SCPNs in different cellular compartments. Live/dead tests showed that the SCPNs were not toxic to cells while spectral imaging revealed that SCPNs provide a structural shielding and reduced the influence from the outer biological media. The ability of SCPNs to act as catalysts in biological media was first assessed by investigating their potential for reactive oxygen species generation. With porphyrins covalently attached to the SCPNs, singlet oxygen was generated upon irradiation with light, inducing spatially controlled cell death. In addition, Cu(I)- and Pd(II)-based SCPNs were prepared and these catalysts were screened in vitro and studied in cellular environments for the carbamate cleavage reaction of rhodamine-based substrates. This is a model reaction for the uncaging of bioactive compounds such as cytotoxic drugs for catalysis-based cancer therapy. We observed that the rate of the deprotection depends on both the organometallic catalysts and the nature of the protective group. The rate reduces from in vitro to the biological environment, indicating a strong influence of biomolecules on catalyst performance. The Cu(I)-based SCPNs in combination with the dimethylpropargyloxycarbonyl protective group showed the best performances both in vitro and in biological environment, making this group promising in biomedical applications.