Single‐Chain Folding of Synthetic Polymers: A Critical Update

The current contribution serves as a critical update to a previous feature article from us (Macromol. Rapid Commun. 2012 , 33, 958−971), and highlights the latest advances in the preparation of single chain polymeric nanoparticles and initial—yet promising—attempts towards mimicking the structure of natural biomacromolecules via single‐chain folding of well‐defined linear polymers via so‐called single chain selective point folding and repeat unit folding. The contribution covers selected examples from the literature published up to ca. September 2015. Our aim is not to provide an exhaustive review but rather highlight a selection of new and exciting examples for single‐chain folding based on advanced macromolecular precision chemistry. Initially, the discussion focuses on the synthesis and characterization of single‐chain folded structures via selective point folding. The second part of the feature article addresses the folding of well‐defined single‐chain polymers by means of repeat unit folding. The current state of the art in the field of single‐chain folding indicates that repeat unit folding‐driven nanoparticle preparation is well‐advanced, while initial encouraging steps towards building selective point folding systems have been taken. In addition, a summary of the—in our view—open key questions is provided that may guide future biomimetic design efforts.

     

Single‐Chain Self‐Folding of Synthetic Polymers Induced by Metal–Ligand Complexation

The controlled folding of a single polymer chain is for the first time realized by metal‐ complexation. α,ω‐Bromine functional linear polymers are prepared via activators regenerated by electron transfer (ARGET) ATRP (_,SEC = 5900 g mol⁻¹, Đ = 1.07 and 12 000 g mol⁻¹, Đ = 1.06) and the end groups of the polymers are subsequently converted to azide functionalities. A copper‐catalyzed azide–alkyne cycloaddition (CuAAC) reaction is carried out in the presence of a novel triphenylphosphine ligand and the polymers to afford homotelechelic bis‐triphenylphosphine polymeric‐macroligands (MLs) (_,SEC = 6600 g mol⁻¹, Đ = 1.07, and 12 800 g mol⁻¹, Đ = 1.06). Single‐chain metal complexes (SCMCs) are formed in the presence of Pd(II) ions in highly diluted solution at ambient temperature. The results derived via ¹H and ³¹P{¹H} NMR experiments, SEC, and DLS unambiguously evidence the efficient formation of SCMCs via metal ligand complexation.

     

Consequences of Block Sequence on the Orthogonal Folding of Triblock Copolymers

ABA‐ and BAB‐type triblock copolymers possessing pendant, self‐assembling motifs in the A and B blocks were synthesized, with 2‐ureidopyrimidinone (UPy) and benzene‐1,3,5‐tricarboxamide (BTA) for the A and B block, respectively. They were investigated to assess if and how the polymer’s microstructure influences the self‐assembly behavior of the supramolecular motifs and, as a result, the single‐chain folding process. BAB‐type triblock copolymers were synthesized via atom transfer radical polymerization (ATRP) with molecular weights ranging from 30 to 120 kg mol^?1; the BTA and UPy motifs were attached using a post‐functionalization approach. The ABA‐type triblock copolymers were available from previous work. In highly dilute solutions, both types of triblock copolymers fold into single‐chain polymeric nanoparticles (SCPNs) via thermally induced BTA self‐assembly and photo‐triggered UPy dimerization. Chain collapse induced by intramolecular UPy dimerization was evaluated using size‐exclusion chromatography (SEC). The BTA self‐assembly was monitored by circular dichroism (CD) spectroscopy. The microstructures of SCPNs were visualized by atomic force microscopy (AFM). SEC analysis indicated a more loose packing for the BAB‐type folded nanoparticles than for the ABA‐type ones, which implies that topological differences in the polymer architecture do affect the folding behavior, although only slightly. The facile synthetic protocol developed here provides topologically different triblock architectures and opens up the area for single‐chain folding technology that is applicable in artificial enzymatic systems with compartmentalized domains.     

Supramolecular Nanoparticles via Single‐Chain Folding Driven by Ferrous Ions

Single‐chain nanoparticles can be obtained via single‐chain folding assisted by intramolecular crosslinking reversibly or irreversibly. Single‐chain folding is also an efficient route to simulate biomacromolecules. In present study, poly(N‐hydroxyethylacrylamide‐co‐4′‐(propoxy urethane ethyl acrylate)‐2,2′:6′,2″‐terpyridine) (P(HEAm‐co‐EMA‐Tpy)) is synthesized via reversible addition fragmentation chain transfer polymerization. Single‐chain folding and intramolecular crosslinking of P(HEAm‐co‐EMA‐Tpy) are achieved via metal coordination chemistry. The intramolecular interaction is characterized on ultraviolet/visible spectrophotometer (UV–vis spectroscopy), proton nuclear magnetic resonance (¹H NMR), and differential scanning calorimetry (DSC). The supramolecular crosslinking mediated by Fe²⁺ plays an important role in the intramolecular collapsing of the single‐chain and the formation of the nanoparticles. The size and morphology of the nanoparticles can be controlled reversibly via metal coordination chemistry, which can be characterized by dynamic light scattering (DLS), transmission electron microscope (TEM), and atomic force microscope (AFM).

     

Metal‐Folded Single‐Chain Nanoparticle: Nanoclusters and Self‐Assembled Reduction‐Responsive Sub‐5‐nm Discrete Subdomains

Easy access to discrete nanoclusters in metal‐folded single‐chain nanoparticles (metal‐SCNPs) and independent ultrafine sudomains in the assemblies via coordination‐driven self‐assembly of hydrophilic copolymer containing 9% imidazole groups is reported herein. ¹H NMR, dynamic light scattering, and NMR diffusion‐ordered spectroscopy results demonstrate self‐assembly into metal‐SCNPs (>70% imidazole‐units folded) by neutralization in the presence of Cu(II) in water to pH 4.6. Further neutralization induces self‐assembly of metal‐SCNPs (pH 4.6–5.0) and shrinkage (pH 5.0–5.6), with concurrent restraining residual imidazole motifs and hydrophilic segment, which organized into constant nanoparticles over pH 5.6–7.5. Atomic force microscopy results evidence discrete 1.2 nm nanoclusters and sub‐5‐nm subdomains in metal‐SCNP and assembled nanoparticle. Reduction of metal center using sodium ascorbate induces structural rearrangement to one order lower than the precursor. Enzyme mimic catalysis required media‐tunable discrete ultrafine interiors in metal‐SCNPs and assemblies have hence been achieved.     

Hydrogen‐Bonding‐Mediated Dynamic Covalent Synthesis of Macrocycles and Capsules: New Receptors for Aliphatic Ammonium Ions and the Formation of Pseudo[3]rotaxanes

Dynamic covalent synthesis! Intramolecular hydrogen‐bonding induces amino‐ and aldehyde‐appended aryl amides to adopt a rigid “V”‐shaped conformation. As a result, stable two‐layered capsules can be assembled quantitatively through the one‐step formation of six imine bonds. The new capsules form complexes with aliphatic diammonium ions to give unique two‐layered pseudo[3]rotaxanes (see figure).

     

Unexpected beauty and diversity in the structures of three homologous 4,5‐dialkoxy‐1‐ethynyl‐2‐nitrobenzenes: the subtle interplay between intermolecular C—H…O hydrogen bonds and alkyl chain length

The synthesis, ¹H and ¹³C NMR spectra, and X‐ray structures are described for three dialkoxy ethynylnitrobenzenes that differ only in the length of the alkoxy chain, namely 1‐ethynyl‐2‐nitro‐4,5‐dipropoxybenzene, C₁₄H₁₇NO₄, 1,2‐dibutoxy‐4‐ethynyl‐5‐nitrobenzene, C₁₆H₂₁NO₄, and 1‐ethynyl‐2‐nitro‐4,5‐dipentoxybenzene, C₁₈H₂₅NO₄. Despite the subtle changes in molecular structure, the crystal structures of the three compounds display great diversity. Thus, 1‐ethynyl‐2‐nitro‐4,5‐dipropoxybenzene crystallizes in the trigonal crystal system in the space group , with Z = 18, 1,2‐dibutoxy‐4‐ethynyl‐5‐nitrobenzene crystallizes in the monoclinic crystal system in the space group P 2₁/c , with Z = 4, and 1‐ethynyl‐2‐nitro‐4,5‐dipentoxybenzene crystallizes in the triclinic crystal system in the space group , with Z = 2. The crystal structure of 1‐ethynyl‐2‐nitro‐4,5‐dipropoxybenzene is dominated by planar hexamers formed by a bifurcated alkoxy sp‐C—H…O,O′ interaction, while the structure of the dibutoxy analogue is dominated by planar ribbons of molecules linked by a similar bifurcated alkoxy sp‐C—H…O,O′ interaction. In contrast, the dipentoxy analogue forms ribbons of molecules alternately connected by a self‐complementary sp‐C—H…O₂N interaction and a self‐complementary sp²‐C—H…O₂N interaction. Disordered solvent was included in the crystals of 1‐ethynyl‐2‐nitro‐4,5‐dipropoxybenzene and its contribution was removed during refinement.     

Micro‐ and Nanometer‐Scale Porous,Fibrous, and Sheet Architectures Constructed by Supramolecular Assemblies of Dipyrrolyldiketones

X‐ray analysis of some 1,3‐dipyrrolyl‐1,3‐propanediones synthesized from pyrroles and malonyl chloride derivatives revealed 1D supramolecular networks formed by N? H???O?C interactions in the solid state. Micro‐ and nanometer‐scale morphologies of porous, fibrous, and sheet structures were fabricated by hydrogen‐bonding interactions and determined by fine‐tuning the substituents and the solvents used. Of the unique polymorphs, ordered 2D lamellar sheet structures of the derivatives with long alkyl chains (C₁₆H₃₃, C₁₄H₂₉, and so on) were constructed by van der Waals hydrophobic effects between aliphatic chains as well as hydrogen bonding.     

Construction and Self‐Assembly of Single‐Chain Polymer Nanoparticles via Coordination Association and Electrostatic Repulsion in Water

Simultaneous coordination‐association and electrostatic‐repulsion interactions play critical roles in the construction and stabilization of enzymatic function metal centers in water media. These interactions are promising for construction and self‐assembly of artificial aqueous polymer single‐chain nanoparticles (SCNPs). Herein, the construction and self‐assembly of dative‐bonded aqueous SCNPs are reported via simultaneous coordination‐association and electrostatic‐repulsion interactions within single chains of histamine‐based hydrophilic block copolymer. The electrostatic‐repulsion interactions are tunable through adjusting the imidazolium/imidazole ratio in response to pH, and in situ Cu(II)‐coordination leads to the intramolecular association and single‐chain collapse in acidic water. SCNPs are stabilized by the electrostatic repulsion of dative‐bonded block and steric shielding of nonionic water‐soluble block, and have a huge specific surface area of function metal centers accessible to substrates in acidic water. Moreover, SCNPs can assemble into micelles, networks, and large particles programmably in response to the solution pH. These unique media‐sensitive phase‐transformation behaviors provide a general, facile, and versatile platform for the fabrication of enzyme‐inspired smart aqueous catalysts.

     

Investigation of Steric Influences on Hydrogen‐Bonding Motifs in Cyclic Ureas by Using X‐Ray,Neutron, and Computational Methods

A series of urea‐derived heterocycles, 5N‐substituted hexahydro‐1,3,5‐triazin‐2‐ones, has been prepared and their structures have been determined for the first time. This family of compounds only differ in their substituent at the 5‐position (which is derived from the corresponding primary amine), that is, methyl ( 1 ), ethyl ( 2 ), isopropyl ( 3 ), tert‐butyl ( 4 ), benzyl ( 5 ), N,N‐(diethyl)ethylamine ( 6 ), and 2‐hydroxyethyl ( 7 ). The common heterocyclic core of these molecules is a cyclic urea, which has the potential to form a hydrogen‐bonding tape motif that consists of self‐associative (8) dimers. The results from X‐ray crystallography and, where possible, Laue neutron crystallography show that the hydrogen‐bonding motifs that are observed and the planarity of the hydrogen bonds appear to depend on the steric hindrance at the α‐carbon atom of the N substituent. With the less‐hindered substituents, methyl and ethyl, the anticipated tape motif is observed. When additional methyl groups are added onto the α‐carbon atom, as in the isopropyl and tert‐butyl derivatives, a different 2D hydrogen‐bonding motif is observed. Despite the bulkiness of the substituents, the benzyl and N,N‐(diethyl)ethylamine derivatives have methylene units at the α‐carbon atom and, therefore, display the tape motif. The introduction of a competing hydrogen‐bond donor/acceptor in the 2‐hydroxyethyl derivative disrupts the tape motif, with a hydroxy group interrupting the N? H???O?C interactions. The geometry around the hydrogen‐bearing nitrogen atoms, whether planar or non‐planar, has been confirmed for compounds 2 and 5 by using Laue neutron diffraction and rationalized by using computational methods, thus demonstrating that distortion of O‐C‐N‐H torsion angles occurs to maintain almost‐linear hydrogen‐bonding interactions.     

Carbohydrate–DNA Interactions at G‐Quadruplexes: Folding and Stability Changes by Attaching Sugars at the 5′‐End

Quadruplex DNA structures are attracting an enormous interest in many areas of chemistry, ranging from chemical biology, supramolecular chemistry to nanoscience. We have prepared carbohydrate–DNA conjugates containing the oligonucleotide sequences of G‐quadruplexes (thrombin binding aptamer (TBA) and human telomere (TEL)), measured their thermal stability and studied their structure in solution by using NMR and molecular dynamics. The solution structure of a fucose–TBA conjugate shows stacking interactions between the carbohydrate and the DNA G‐tetrad in addition to hydrogen bonding and hydrophobic contacts. We have also shown that attaching carbohydrates at the 5′‐end of a quadruplex telomeric sequence can alter its folding topology. These results suggest the possibility of modulating the folding of the G‐quadruplex by linking carbohydrates and have clear implications in molecular recognition and the design of new G‐quadruplex ligands.     

Halogen‐ and Hydrogen‐Bonded Salts and Co‐crystals Formed from 4‐Halo‐2,3,5,6‐tetrafluorophenol and Cyclic Secondary and Tertiary Amines: Orthogonal and Non‐orthogonal Halogen and Hydrogen Bonding,and Synthetic Analogues of Halogen‐Bonded Biological Systems

Co‐crystallisation of, in particular, 4‐iodotetrafluorophenol with a series of secondary and tertiary cyclic amines results in deprotonation of the phenol and formation of the corresponding ammonium phenate. Careful examination of the X‐ray single‐crystal structures shows that the phenate anion develops a C?O double bond and that the C?C bond lengths in the ring suggest a Meissenheimer‐like delocalisation. This delocalisation is supported by the geometry of the phenate anion optimised at the MP2(Full) level of theory within the aug‐cc‐pVDZ basis (aug‐cc‐pVDZ‐PP on I) and by natural bond orbital (NBO) analyses. With sp² hybridisation at the phenate oxygen atom, there is strong preference for the formation of two non‐covalent interactions with the oxygen sp² lone pairs and, in the case of secondary amines, this occurs through hydrogen bonding to the ammonium hydrogen atoms. However, where tertiary amines are concerned, there are insufficient hydrogen atoms available and so an electrophilic iodine atom from a neighbouring 4‐iodotetrafluorophenate group forms an I???O halogen bond to give the second interaction. However, in some co‐crystals with secondary amines, it is also found that in addition to the two hydrogen bonds forming with the phenate oxygen sp² lone pairs, there is an additional intermolecular I???O halogen bond in which the electrophilic iodine atom interacts with the C?O π‐system. All attempts to reproduce this behaviour with 4‐bromotetrafluorophenol were unsuccessful. These structural motifs are significant as they reproduce extremely well, in low‐molar‐mass synthetic systems, motifs found by Ho and co‐workers when examining halogen‐bonding interactions in biological systems. The analogy is cemented through the structures of co‐crystals of 1,4‐diiodotetrafluorobenzene with acetamide and with N‐methylbenzamide, which, as designed models, demonstrate the orthogonality of hydrogen and halogen bonding proposed in Ho’s biological study.     

From Nongelating to Gelating: Synthesis and Structural Self‐Assembling Property Relationships of a Homologous Series of Oligo(amide–triazole)s

A homologous series of oligo(amide–triazole)s (OAT) [ OAT‐CO₂H‐2 n and OAT‐COPrg‐(2 n +1) ] with an increasing number of primary amide (CONH) and triazole hydrogen‐bonding functionalities was prepared by an iterative synthetic procedure. It was found that their self‐assembly and thermoreversible gelation strength had a strong correlation to the number of hydrogen‐bonding moieties in the oligomers. There also existed a threshold value of the number of CONH units, above which all the oligomers became organogelators. Hence, oligomers with ≤4 CONH units are devoid of intermolecular hydrogen bonding and also non‐organogelating, whereas those that contain >4 CONH units show intermolecular association and organogelating properties. For the organogelators, the T_gel value increases monotonically with increasing number of CONH units. On the basis of FTIR measurements, both the CONH and triazole C? H groups were involved in the hydrogen‐bonding process. A mixed xerogel that consisted of a 1:1 weight ratio of two oligomers of different lengths ( OAT‐CO₂H‐6 and OAT‐CO₂H‐12 ) was found to show microphase segregation according to differential scanning calorimetry, thus indicating that oligomers that bear a different number of hydrogen‐bonding units exhibited self‐sorting to maximize the extent of intermolecular hydrogen bonding in the xerogel state.     

Single chain self‐assembly of well‐defined heterotelechelic polymers generated by ATRP and click chemistry revisited

Well‐defined heterotelechelic poly(styrene) carrying thymine/diaminopyridine (DAP) (M_n,SEC = 9300, PDI = 1.04) and Hamilton wedge (HW)/cyanuric acid (CA) (M_n,SEC = 8200, PDI = 1.04) bonding motifs are prepared via a combination of controlled/living radical polymerization and copper catalyzed azide/alkyne “click” chemistry and are subsequently self‐assembled as single chains to emulate—on a simple level—the self‐folding behavior of natural biomacromolecules. Hydrogen nuclear magnetic resonance (¹H NMR) in deuterated dichloromethane and dynamic light scattering analyses provides evidence for the hydrogen bonding interactions between the α‐thymine and ω‐DAP as well as α‐CA and ω‐HW chain ends of the heterotelechelic polymers leading to circular entropy driven single chain self‐assembly. This study demonstrates that the choice of NMR solvent is important for obtaining well‐resolved NMR spectra of the self‐assembled structures. In addition, steric effects on the HW can affect the efficiency of the self‐assembly process. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Peptide Conjugates of a Nonsteroidal Anti‐Inflammatory Drug as Supramolecular Gelators: Synthesis,Characterization, and Biological Studies

Indomethacin ( IND ), which is a well‐known nonsteroidal anti‐inflammatory drug (NSAID), was conjugated with various naturally occurring amino acids. Most of these bioconjugates were capable of gelling pure water, a solution of NaCl (0.9 wt %), and phosphate‐buffered saline (pH 7.4), as well as a few organic solvents. The gels were characterized by table‐top and dynamic rheology, and electron microscopy. Variable‐temperature ¹H NMR spectroscopy studies on a selected gel were performed to gain insights into the self‐assembly process during gel formation. Both 1D and 2D hydrogen‐bonded networks were observed in the single‐crystal structures of two of the gelators. Plausible biological applications of the hydrogelators were evaluated with the ultimate aim of drug delivery in a self‐delivery fashion. All hydrogelators were stable in phosphate‐buffered saline at pH 7.4 at 37 °C, and biocompatible in mouse macrophage RAW 264.7 cell line (3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide assay). Two of the most biocompatible hydrogelators displayed an anti‐inflammatory response comparable to that of the parent drug IND in prostaglandin E₂ assay. Release of the bioconjugates into the bulk solvent interfaced with the corresponding hydrogels indicated their plausible future application in drug delivery.     

Self‐Assembly of a Highly Organized,Hexameric Supramolecular Architecture: Formation,Structure and Properties

Two derivatives, ³ L and ⁹ L , of a ditopic, multiply hydrogen‐bonding molecule, known for more than a decade, have been found, in the solid state as well as in solvents of low polarity at room temperature, to exist not as monomers, but to undergo a remarkable self‐assembly into a complex supramolecular species. The solid‐state molecular structure of ³ L , determined by single‐crystal X‐ray crystallography, revealed that it forms a highly organized hexameric entity ³ L ₆ with a capsular shape, resulting from the interlocking of two sets of three monomolecular components, linked through hydrogen‐bonding interactions. The complicated ¹H NMR spectra observed in o‐dichlorobenzene (o‐DCB) for ³ L and ⁹ L are consistent with the presence of a hexamer of D₃ symmetry in both cases. DOSY measurements confirm the hexameric constitution in solution. In contrast, in a hydrogen‐bond‐disrupting solvent, such as DMSO, the ¹H NMR spectra are very simple and consistent with the presence of isolated monomers only. Extensive temperature‐dependent ¹H NMR studies in o‐DCB showed that the L ₆ species dissociated progressively into the monomeric unit on increasing th temperature, up to complete dissociation at about 90 °C. The coexistence of the hexamer and the monomer indicated that exchange was slow on the NMR timescale. Remarkably, no species other than hexamer and monomer were detected in the equilibrating mixtures. The relative amounts of each entity showed a reversible sigmoidal variation with temperature, indicating that the assembly proceeded with positive cooperativity. A full thermodynamic analysis has been applied to the data.     

Non‐Covalent Polyvalent Ligands by Self‐Assembly of Small Glycodendrimers: A Novel Concept for the Inhibition of Polyvalent Carbohydrate–Protein Interactions In Vitro and In Vivo

Polyvalent carbohydrate–protein interactions occur frequently in biology, particularly in recognition events on cellular membranes. Collectively, they can be much stronger than corresponding monovalent interactions, rendering it difficult to control them with individual small molecules. Artificial macromolecules have been used as polyvalent ligands to inhibit polyvalent processes; however, both reproducible synthesis and appropriate characterization of such complex entities is demanding. Herein, we present an alternative concept avoiding conventional macromolecules. Small glycodendrimers which fulfill single molecule entity criteria self‐assemble to form non‐covalent nanoparticles. These particles—not the individual molecules—function as polyvalent ligands, efficiently inhibiting polyvalent processes both in vitro and in vivo. The synthesis and characterization of these glycodendrimers is described in detail. Furthermore, we report on the characterization of the non‐covalent nanoparticles formed and on their biological evaluation.