Molecular Processing of Polymers with Cyclodextrins

Post-synthesis modification of slide-ring gels for thermal and mechanical reconfiguration

Ring-sliding behavior in polyrotaxanes imbues gels, elastomers, and glasses with remarkable stress-dissipation and actuation properties. Since these properties can be modulated and tuned by structural parameters, many efforts have been devoted to developing synthetic protocols that define the structures and properties of slide-ring materials. We introduce post-synthetic modifications of slide-ring gels derived from unmodified α-cyclodextrin and poly(ethylene glycol) polyrotaxanes that enable (i) actuation and control of the thermo-responsive lower critical solution temperature (LCST) behavior of ring-modified slide-ring hydrogels, and (ii) chemically bonding separate gels into hybrid or shape-reconfigured macro-structures with a slide-ring adhesive solution. The mechanical properties of the post-modified gels have been characterized by shear rheology and uniaxial tensile tests, while the corresponding xerogels were characterized by wide-angle X-ray scattering. These demonstrations show that post-synthetic modification offers a practical solution for re-configuring the properties and shapes of slide-ring gels.

Physical Characterization of Inclusion Complexes of Triphenyl Phosphate and Cyclodextrins in Solution

The goal of this work is to provide physical insights into the formation and stability of inclusion complexes (ICs) in aqueous solution between cyclodextrins (CDs) and a common flame retardant, triphenyl phosphate (TPP). Quantum chemistry calculations reveal the possible energetically favorable geometries of TPP in their 1:1 IC form with α-, β-, and γ-CDs as well as their associated complexation, conformational, and interaction energies. High-resolution mass spectrometry (MS) and tandem MS were used with electrospray ionization to study the soluble ICs formed between TPP and CDs. Successful formation of TPP ICs with both β- and γ-CD in solution was detected in the ratio of 1:1 using high-resolution MS in the positive ion mode. Collision-induced dissociation confirmed the formation of TPP ICs with β- and γ-CDs by generating two product ions, TPP and β- or γ-CD, in both cases. Although quantum chemistry calculations suggest that IC formation with α-CD is energetically possible, an IC with α-CD is not observed in aqueous solution using MS, which aligns with what we also previously observed in the solid state. Since TPP forms stable ICs with β- and γ-CDs both in the solid state and in solution suggests that complexation could be a safer alternative than applying TPP directly to a substrate. In addition, complexation with CDs in solution also opens up new processing methods to create flame-retardant fabrics and foams with TPP.

Reorganizing Polymer Chains with Cyclodextrins

During the past several years, we have been utilizing cyclodextrins (CDs) to nanostructure polymers into bulk samples whose chain organizations, properties, and behaviors are quite distinct from neat bulk samples obtained from their solutions and melts. We first form non-covalently bonded inclusion complexes (ICs) between CD hosts and guest polymers, where the guest chains are highly extended and separately occupy the narrow channels (~0.5⁻1.0 nm in diameter) formed by the columnar arrangement of CDs in the IC crystals. Careful removal of the host crystalline CD lattice from the polymer-CD-IC crystals leads to coalescence of the guest polymer chains into bulk samples, which we have repeatedly observed to behave distinctly from those produced from their solutions or melts. While amorphous polymers coalesced from their CD-ICs evidence significantly higher glass-transition temperatures, Tgs, polymers that crystallize generally show higher melting and crystallization temperatures (Tms, Tcs), and some-times different crystalline polymorphs, when they are coalesced from their CD-ICs. Formation of CD-ICs containing two or more guest homopolymers or with block copolymers can result in coalesced samples which exhibit intimate mixing between their common homopolymer chains or between the blocks of the copolymer. On a more practically relevant level, the distinct organizations and behaviors observed for polymer samples coalesced from their CD-ICs are found to be stable to extended annealing at temperatures above their Tgs and Tms. We believe this is a consequence of the structural organization of the crystalline polymer-CD-ICs, where the guest polymer chains included in host-IC crystals are separated and confined to occupy the narrow channels formed by the host CDs during IC crystallization. Substantial degrees of the extended and un-entangled natures of the IC-included chains are apparently retained upon coalescence, and are resistant to high temperature annealing. Following the careful removal of the host CD lattice from each randomly oriented IC crystal, the guest polymer chains now occupying a much-reduced volume may be somewhat "nematically" oriented, resulting in a collection of randomly oriented "nematic" regions of largely extended and un-entangled coalesced guest chains. The suggested randomly oriented nematic domain organization of guest polymers might explain why even at high temperatures their transformation to randomly-coiling, interpenetrated, and entangled melts might be difficult. In addition, the behaviors and uses of polymers coalesced from their CD-ICs are briefly described and summarized here, and we attempted to draw conclusions from and relationships between their behaviors and the unique chain organizations and conformations achieved upon coalescence.

Near-Quantitative Aqueous Synthesis of Rotaxanes via Bioconjugation to Oligopeptides and Proteins

In spite of widespread interest in rotaxane-based molecular machines and materials, rotaxanes have not been attached covalently to proteins. We describe the near-quantitative aqueous synthesis of [2]rotaxanes based on neutral and charged aqueous hosts-cucurbit[7]uril (CB7) and cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺), respectively-using the thiol-ene addition of cysteine and maleimide as a stoppering protocol. After verifying the high efficiency of the reaction using glutathione (GSH) as an oligopeptide stopper, we have employed cytochrome C (CytC) as a protein stopper to produce the first well-characterized protein-rotaxane bioconjugates. We anticipate that this methodology will enable the preparation of novel materials that combine the unique properties of proteins and mechanical bonds.

Structural Reorganization and Fibrinogen Adsorption Behaviors on the Polyrotaxane Surfaces Investigated by Sum Frequency Generation Spectroscopy

Polyrotaxanes, such as supramolecular assemblies with methylated α-cyclodextrins (α-CDs) as host molecules noncovalently threaded on the linear polymer backbone, are promising materials for biomedical applications because they allow adsorbed proteins possessing a high surface flexibility as well as control of the cellular morphology and adhesion. To provide a general design principle for biomedical materials, we examined the surface reorganization behaviors and adsorption conformations of fibrinogen on the polyrotaxane surfaces with comparison to several random copolymers by sum frequency generation (SFG) vibrational spectroscopy. We showed that the polyrotaxane (OMe-PRX-PMB) with methylated α-CDs as the host molecule exhibited unique surface structures in an aqueous environment. The hydrophobic interaction between the methoxy groups of the methylated α-CD molecules and methyl groups of the n-butyl methacrylate (BMA) side chains may dominate the surface restructuring behavior of the OMe-PRX-PMB. The orientation analysis revealed that the orientation of the fibrinogen adsorbed on the OMe-PRX-PMB surface is close to a single distribution, which is different from the adsorption behaviors of fibrinogen on other polyrotaxane or random copolymer surfaces.

One-pot synthesis of water-soluble, β-cyclodextrin-based polyrotaxanes in a homogeneous water system and its use in bio-applications

A small, nano-sized, water-soluble polyrotaxane (PR) was synthesized using a highly efficient one-pot synthesis strategy in a homogeneous water system, formed from β-cyclodextrin-(COOH)₂, poly(propylene glycol)bis(2-aminopropyl ether) (PPG, 2 kDa) and a mono-(6-azido-6-desoxy)-β-cyclodextrin stopper via room temperature click chemistry. β-cyclodextrin-(COOH)₂ and PR were characterized by one- and two-dimensional NMR as well as by high resolution transmission electron microscopy (HR-TEM). The number of carboxyl groups in one PR was determined by ¹H NMR. Two-dimensional diffusion-ordered NMR spectroscopy (2D DOSY) and nuclear Overhauser enhancement spectroscopy (2D NOESY) show that β-cyclodextrin-(COOH)₂ and PPG successfully formed an inclusion complex. HR-TEM revealed the morphology of water-soluble PR as a spherical nanoparticle with a size of approximately 3.5 nm ± 1.5 nm. PR was labeled with rhodamine to assess its biocompatibility and cell membrane penetrability in vitro. The in vivo real-time fluorescent imaging biodistribution experiments indicated that water-soluble PR can actively target tumor sites using an enhanced permeability and retention (EPR) effect, with a significantly prolonged blood circulation time in tumor-bearing mice.

Functional supramolecular polymers for biomedical applications

As a novel class of dynamic and non-covalent polymers, supramolecular polymers not only display specific structural and physicochemical properties, but also have the ability to undergo reversible changes of structure, shape, and function in response to diverse external stimuli, making them promising candidates for widespread applications ranging from academic research to industrial fields. By an elegant combination of dynamic/reversible structures with exceptional functions, functional supramolecular polymers are attracting increasing attention in various fields. In particular, functional supramolecular polymers offer several unique advantages, including inherent degradable polymer backbones, smart responsiveness to various biological stimuli, and the ease for the incorporation of multiple biofunctionalities (e.g., targeting and bioactivity), thereby showing great potential for a wide range of applications in the biomedical field. In this Review, the trends and representative achievements in the design and synthesis of supramolecular polymers with specific functions are summarized, as well as their wide-ranging biomedical applications such as drug delivery, gene transfection, protein delivery, bio-imaging and diagnosis, tissue engineering, and biomimetic chemistry. These achievements further inspire persistent efforts in an emerging interdisciplin-ary research area of supramolecular chemistry, polymer science, material science, biomedical engineering, and nanotechnology.

Dilution-stable PAMAM G1-grafted polyrotaxane supermolecules deliver gene into cells through a caveolae-dependent pathway

Numerous preclinical studies have demonstrated that polycation mediated gene delivery systems successfully achieved efficient gene transfer into cells and animal models. However, results of their clinical trials to date have been disappointing. That self-assembled gene and polycation systems should be stable undergoing dilution in the body is one of the prerequisites to ensuring efficiency of gene transfer in clinical trials, but it was neglected in most preclinical studies. In this account, we developed the dilution-stable PAMAM G1-grafted polyrotaxane (PPG1) supermolecules in which PAMAM G1-grafted α-cyclodextrins are threaded onto a PEG chain capped with hydrophobic adamantanamine. The PPG1/pDNA polyplex (approximate 100 nm in diameter) was very stable and kept its initial particle size and a uniform size distribution at ultrahigh dilution, whereas DNA/PEI 25K polyplex was above three times bigger at a 16-fold dilution than the initial size and their particle size distribution indicated multiple peaks mainly due to forming loose and noncompacted aggregates. PPG1 supermolecules showed significantly superior transfection efficiencies compared to either PEI 25K or Lipofectamine 2000 in most cell lines tested including normal cells (HEK293A) and cancer cells (Bel7402, HepG2, and HeLa). Furthermore, we found that the PPG1 supermolecules delivered DNA into HEK293A through a caveolae-dependent pathway but not a clathrin-dependent pathway as PEI 25K did. These findings raised the intriguing possibility that the caveolae-dependent pathway of PPG1 supermolecule/pDNA polyplex avoiding lysosomal degradation was attributed to their high transfection efficiency. The dilution-stable PPG1 supermolecule polyplex facilitating caveolae-dependent internalization has potential applications to surmount the challenges of high dilutions in the body and lysosomal degradation faced by most gene therapy clinical trials.

Molecular logistics using cytocleavable polyrotaxanes for the reactivation of enzymes delivered in living cells

The intracellular delivery of enzymes is an essential methodology to extend their therapeutic application. Herein, we have developed dissociable supermolecule-enzyme polyelectrolyte complexes based on reduction-cleavable cationic polyrotaxanes (PRXs) for the reactivation of delivered enzymes. These PRXs are characterized by their supramolecular frameworks of a polymeric chain threading into cyclic molecules, which can form polyelectrolyte complexes with anionic enzymes while retaining their three dimensional structure, although their enzymatic activity is reduced. Upon the addition of a reductant, the PRXs dissociate into their constituent molecules and release the enzymes, resulting in a complete recovery of enzymatic activity. Under the intracellular environment, the PRX-based enzyme complexes showed the highest intracellular enzymatic activity and efficient activation of anticancer prodrugs to induce cytotoxic effects in comparison with the non-dissociable complexes and the commercial cell-penetrating peptide-based reagents. Thus, the intracellularly dissociable supermolecules are an attractive system for delivering therapeutic enzymes into living cells.

Polyester-Containing α-Cyclodextrin-Based Polyrotaxane: Synthesis by Living Ring-Opening Polymerization, Polypseudorotaxanation, and End Capping Using Nitrile N-Oxide

The first synthesis of polyrotaxanes consisting of polyester axles and α-cyclodextrin (α-CD) wheels was achieved by the catalyst-free click end-capping reaction of polypseudorotaxanes using nitrile N-oxide. The polypseudorotaxanes contain acrylate-functionalized polyesters that are obtained by the living ring-opening polymerization of lactones. The yield and coverage ratio of polyrotaxanes are highly dependent on the reaction time, molecular weight of the polyester, polyester structure, and solvent used. From the thermal properties of the resulting polyrotaxanes, it was found that coverage with α-CDs efficiently suppresses the crystallization of the polyester main chain.

Synthesis of pseudopolyrotaxanes-coated Superparamagnetic Iron Oxide Nanoparticles as new MRI contrast agent

Superparamagnetic Iron Oxide Nanoparticles (SPIONs) were synthesized and coated with pseudopolyrotaxanes (PPRs) and proposed as a novel hybrid nanostructure for medical imaging and drug delivery. PPRs were prepared by addition of α-cyclodextrin rings to functionalized polyethylene glycol (PEG) chain with hydrophobic triazine end-groups. Non-covalent interactions between SPIONs and PPRs led to the assembly of SPIONs@PRs hybrid nanomaterials. Measurements of the (1)H Nuclear Magnetic Resonance (NMR) relaxation times T(1) and T(2) allowed us to determine the NMR dispersion profiles. Comparison between our SPIONs@PRs hybrid nano-compound and the commercial SPION compound, Endorem, showed a higher transverse relaxivity for SPIONs@PRs. In vitro MRI experiments showed that our SPIONs@PRs produces better negative contrast compared to Endorem and can be considered as a novel MRI contrast agent.

Restructuring polymers via nanoconfinement and subsequent release

During the past several years my students and I have been utilizing certain small-molecule hosts to create nanostructured polymers. This is accomplished by first forming noncovalently bonded inclusion complexes (ICs) between these small-molecule hosts and guest polymers, followed by the careful removal of the host crystalline lattice to obtain a coalesced bulk polymer. We have repeatedly observed that such coalesced polymer samples behave distinctly from those produced from their solutions or melts. Coalesced amorphous homopolymers exhibit higher glass-transition temperatures, while crystallizable homopolymers coalesced from their ICs display higher melting and crystallization temperatures, and sometimes different crystalline polymorphs. When ICs are formed with block copolymers or with two or more different homopolymers, the resulting coalesced samples can exhibit intimate mixing between the copolymer blocks, or between entire homopolymer chains. Each of the distinct behaviors observed for polymers coalesced from their ICs is a consequence of the structural organization of the polymer-host-ICs. Polymer chains in host-IC crystals are confined to occupy narrow channels (diameter ~0.5-1.0 nm) formed by the small-molecule hosts around the included guest polymers during IC crystallization. This results in the separation and high extension of the included guest polymer chains, which leads, following the careful removal of the host molecule lattice, to unique behaviors for the bulk coalesced polymer samples. Apparently, substantial degrees of the extended and unentangled natures of the IC-included chains are retained upon coalescence. In this review we summarize the behaviors and uses of coalesced polymers, and attempt to draw conclusions on the relationship between their behavior and the organization/structures/conformations of the constituent polymer chains achieved upon coalescence from their ICs.

New structural insights into mechanically interlocked polymers revealed by ion mobility mass spectrometry

Mechanically interlocked polymers can possess significant additional physical properties, in comparison to those associated with their constituent parts. Their unique properties make them attractive for a range of potential applications, such as as biomaterials and molecular machines. Their efficient and reproducible synthesis is therefore of much interest. Both their synthesis and subsequent characterization are intriguing yet demanding. The properties of mechanically interlocked polymeric systems depend not only on the properties of their individual components but also on the topology of the subsequent product. Here traveling wave ion mobility mass spectrometry has been used to investigate the structural properties of a polyrotaxane system. Ion mobility studies reveal that this system remains linear in form with increase in size. Both ion mobility studies and tandem mass spectrometry studies indicate that the macrocycle preferentially remains associated with the ammonium moiety of the polymeric repeat unit and is impeded from moving freely along the axle. This is consistent with NMR observations of the average structure. Analysis of mechanically interlocked polymers by ion mobility mass spectrometry provides additional structural insights into these systems relating to dynamics, heterogeneity, and topology. This molecule-specific information is vital in order to understand the origin of a system's functional properties.

Development of a low toxicity, effective pDNA vector based on noncovalent assembly of bioresponsive amino-β-cyclodextrin:adamantane-poly(vinyl alcohol)-poly(ethylene glycol) transfection complexes

A host:guest-derived gene delivery vector has been developed, based on the self-assembly of cationic β-CD derivatives with a poly(vinyl alcohol) (MW 27 kDa) (PVA) main chain polymer bearing poly(ethylene glycol) (MW 750) (PEG) or MW 2000 PEG and acid-labile adamantane-modified (Ad) grafts through an acid-sensitive benzylidene acetal linkage. These components were investigated for their ability to promote supramolecular complex formation with pDNA using two different assembly schemes, involving either precomplexation of the pendent Ad-PVA-PEG polymer with the cationic β-CD derivatives before pDNA condensation (method A) or pDNA condensation with the cationic β-CD derivatives prior to addition of Ad-PVA-PEG to engage host:guest complexation (method B). The pendent polymers were observed to degrade under acidic conditions while remaining intact for more than 5 days at pH 7. HeLa cell culture data show that these materials have 10(3)-fold lower cytotoxicities than 25 kDa bPEI while maintaining transfection efficiencies that are superior to those observed for this benchmark cationic polymer transfection reagent when the method A assembly scheme is employed. These findings suggest that degradable cationic polymer constructs employing multivalent host:guest interactions may be an effective and low-toxicity vehicle for delivering nucleic acid cargo to target cells.

Pendant polymer:amino-β-cyclodextrin:siRNA guest:host nanoparticles as efficient vectors for gene silencing

A novel siRNA delivery vector has been developed, based on the self-assembly of monosubstituted cationic β-CD derivatives with a poly(vinyl alcohol)MW27kD (PVA) main-chain polymer bearing poly(ethylene glycol)MW2000 (PEG) and acid-labile cholesterol-modified (Chol) grafts through an acid-sensitive benzylidene acetal linkage. These components were investigated for their ability to form nanoparticles with siRNA using two different assembly schemes, involving either precomplexation of the pendant Chol-PVA-PEG polymer with the cationic β-CD derivatives before siRNA condensation or siRNA condensation with the cationic β-CD derivatives prior to addition of Chol-PVA-PEG to engage host:guest complexation. The pendant polymer:amino-β-CD:siRNA complexes were shown to form nanoparticles in the size range of 120-170 nm, with a slightly negative zeta potential. Cell viability studies in CHO-GFP cells shows that these materials have 10(3)-fold lower cytotoxicities than 25 kD bPEI, while maintaining gene-silencing efficiencies that are comparable to those of benchmark transfection reagents such as bPEI and Lipofectamine 2000. These results suggest that the degradable Chol-PVA-PEG polymer is able to self-assemble in the presence of siRNA and cationic-β-CD to form nanoparticles that are an effective and low-toxicity vehicle for delivering siRNA cargo to target cells.

Preparation and surface properties of polyrotaxane-containing tri-block copolymers as a design for dynamic biomaterials surfaces

A tri-block copolymer series containing hydrophilic polyrotaxane and hydrophobic poly(iso-butylmethacrylate) (PiBMA) segments was prepared by atom transfer radical polymerization (ATRP), starting from a pseudopolyrotaxane consisting of 2-bromoisobutyryl end-capped poly(ethylene glycol) (PEG) and α-cyclodextrin (α-CD) and followed by methylation. The dynamic wettability and molecular mobility of the copolymer surfaces were evaluated by dynamic contact angle (DCA) and quartz crystal microbalance with dissipation (QCM-D) measurements, respectively. The polyrotaxane tri-block copolymer surfaces were found to show pronounced dynamic wettability and molecular mobility compared to the control surfaces-a tri-block polymer consisting of PEG and PiBMA, and a PiBMA homopolymer-suggesting that a polyrotaxane loop-like structure exists at the outermost surface in an aqueous environment and exhibits dynamic properties attributable to the possible mobile nature of hydrated α-CD molecules along the PEG backbone. Finally, excellent protein adsorption repellency was achieved on the polyrotaxane tri-block copolymer surface, presumably due to the mobile nature of the supramolecular architecture on the surface.

Polyrotaxanes for applications in life science and biotechnology

Due to their low cytotoxicity, controllable size, and unique architecture, cyclodextrin (CD)-based polyrotaxanes and polypseudorotaxanes have inspired interesting exploitation as novel biomaterials. This review will update the recent progress in the studies on the structures of polyrotaxanes and polypseudorotaxanes based on different CDs and polymers, followed by summarizing their potential applications in life science and biotechnology, such as drug delivery, gene delivery, and tissue engineering. CD-based biodegradable polypseudorotaxane hydrogels could be used as promising injectable drug delivery systems for sustained and controlled drug release. Polyrotaxanes with drug or ligand-conjugated CDs threaded on polymer chain with biodegradable end group could be useful for controlled and multivalent targeting delivery. Cationic polyrotaxanes consisting of multiple oligoethylenimine-grafted CDs threaded on a block copolymer chain were attractive non-viral gene carries due to the strong DNA-binding ability, low cytotoxicity, and high gene transfection efficiency. Cytocleavable end caps were also introduced in the polyrotaxane systems in order to ensure efficient endosomal escape for intracellular trafficking of DNA. Finally, hydrolyzable polyrotaxane hydrogels with cross-linked α-CDs could be a desirable scaffold for cartilage and bone tissue engineering.

A supramolecular approach to the preparation of charge-tunable dendritic polycations for efficient gene delivery

A facile supramolecular approach for the preparation of charge-tunable dendritic polycations, by a combination of the multi-functionality of dendritic polymers with the dynamic-tunable ability of supramolecular polymers, has been developed.

Cyclodextrin-based gene delivery systems

Cyclodextrin (CD) history has been largely dominated by their unique ability to form inclusion complexes with guests fitting in their hydrophobic cavity. Chemical funcionalization was soon recognized as a powerful mean for improving CD applications in a wide range of fields, including drug delivery, sensing or enzyme mimicking. However, 100 years after their discovery, CDs are still perceived as novel nanoobjects of undeveloped potential. This critical review provides an overview of different strategies to promote interactions between CD conjugates and genetic material by fully exploiting the inside-outside/upper-lower face anisotropy of the CD nanometric platform. Covalent modification, self-assembling and supramolecular ligation can be put forward with the ultimate goal to build artificial viruses for programmed and efficient gene therapy (222 references).

pH-sensitive supramolecular polypeptide-based micelles and reverse micelles mediated by hydrogen-bonding interactions or host-guest chemistry: characterization and in vitro controlled drug release

A versatile strategy is provided for the fabrication of pH-sensitive polypeptide-based normal micelles and reverse micelles from the same polypeptide-based copolymers via hydrogen-bonding interactions or host-guest chemistry. The pH-sensitive self-assembly of both linear and dendron-like/linear poly(L-glutamic acid)-b-poly(ethylene oxide) (Dm-PLG-b-PEO) block copolymers was investigated in detail by means of UV-vis, dynamic light scattering, NMR, fluorescence spectroscopy, and transmission electron microscopy. It was demonstrated that both the copolymer topology and the composition controlled the morphology of the polypeptide-cored normal micelles. Importantly, a novel class of polypeptide-shelled reverse micelles was for the first time generated by host-guest-chemistry-mediated self-assembly of these copolymers and alpha-cyclodextrin (alpha-CD) in alkaline solution. The supramolecular inclusion complexation between PEO and alpha-CD was confirmed by wide-angle X-ray diffraction, differential scanning calorimetry, and NMR. Moreover, the zeta potential of the reverse micelles ranged from -20.2 to -24.2 mV, convincingly demonstrating that the reverse micelles had an anionic PLG shell. Furthermore, the anticancer doxorubicin (DOX)-loaded micelles fabricated from the dendron-like/linear copolymer showed a higher DOX loading efficiency (38%) and capacity (24%) and sustained a longer drug-release period (approximately 70 days) than the linear counterpart. Consequently, this will provide a platform for the fabrication of supramolecular polypeptide-cored and polypeptide-shelled micelles for the anticancer drug delivery systems.