Threading, Growth, and Aggregation of Pseudopolyrotaxanes

L-asparaginase immobilization in supramolecular nanogels of PEG-grafted poly HPMA and bis(α-cyclodextrin) to enhance pharmacokinetics and lower enzyme antigenicity

L-asparaginase (ASNase) enzyme has limited therapeutic use due to its poor pharmacokinetics and immunogenicity. To overcome these obstacles, we immobilized ASNase in biocompatible poly hydroxypropyl methacrylamide (P(HPMA))-based nanogels simply formed through the host-guest inclusion complex of ASNase-conjugated random copolymer of HPMA and polyethylene glycol (PEG) acrylate (P(HPMA-MPEGA)) and α-cyclodextrin dimer (bisCD) using cystamine as a linker. The effects of bisCD and polymer concentrations on particle size, gelation time, and recovery of enzyme activity were investigated. The ASNase-conjugated bisCD nanogels were discrete, homogeneous, and spherical with a mean projected diameter of 148 ± 41 nm. ASNase immobilized in the bisCD nanogels caused cytotoxicity on HL-60 cell line with IC₅₀ of 3 IU/ml. In-vivo rat study revealed that the immobilized ASNase reduced the enzyme antigenicity and resulted in 8.1 folds longer circulation half-life than the native enzyme. Conclusively, immobilization of ASNase in P(HPMA-MPEGA) and bisCD supramolecular nanogels could enhance the therapeutic value of ASNase in cancer chemotherapy.

Problems with Applying the Ozawa–Avrami Crystallization Model to Non-Isothermal Crosslinking Polymerization

Ozawa has modified the Avrami model to treat non-isothermal crystallization kinetics. The resulting Ozawa–Avrami model yields the Avrami index (n) and heating/cooling function (χ(T)). There has been a number of recent applications of the Ozawa–Avrami model to non-isothermal crosslinking polymerization (curing) kinetics that have determined n and have used χ(T) in place of the rate constant (k(T)) in the Arrhenius equation to evaluate the activation energy (E) and the preexponential factor (A). We analyze this approach mathematically as well as by using simulated and experimental data, highlighting the following problems. First, the approach is limited to the processes that obey the Avrami model. In cases of autocatalytic or decelerating kinetics, commonly encountered in crosslinking polymerizations, n reveals a systematic dependence on temperature. Second, χ(T) has a more complex temperature dependence than k(T) and thus cannot produce exact values of E and A via the Arrhenius equation. The respective deviations can reach tens or even hundreds of percent but are diminished dramatically using the heating/cooling function in the form [χ(T)]1/n. Third, without this transformation, the Arrhenius plots may demonstrate breakpoints that leads to questionable interpretations. Overall, the application of the Ozawa–Avrami model to crosslinking polymerizations appears too problematic to be justified, especially considering the existence of well-known alternative kinetic techniques that are flexible, accurate, and computationally simple.

Inclusion complexes of triblock L35 copolymer and hydroxyl propyl cyclodextrins: a physico-chemical study

Polypseudorotaxanes based on triblock L35 copolymer and hydroxyl propyl-modified cyclodextrins (HP-α-CD and HP-β-CD) have been characterized. Their physico-chemical properties have been correlated to the threading process.

Novel self-assembled nanogels of PEG-grafted poly HPMA with bis(α-cyclodextrin) containing disulfide linkage: synthesis, bio-disintegration, and in vivo biocompatibility

Synthesis of self-assembled nanogels of PEG-grafted poly HPMA with bis(α-cyclodextrin) containing disulfide linkage.

Unexpected Properties of Degassed Solutions

Theories of liquids and their simulation ignore any physical effects of dissolved atmospheric gas. Solubilities appear far too low to matter. Long-standing observations to the contrary, like cavitation, the salt dependence of bubble-bubble interactions, and the stability of degassed emulsions, continue to call that assumption into question, and these questions multiply. We herein explore more unexpected effects of dissolved gas that are inexplicable by classical theory. Electrical conductivities of different salts in water were measured as a function of concentration before and after degassing the liquid. The liquid/liquid phase separation of binary mixtures containing water, n-hexane, or perfluorooctane was significantly retarded after degassing. We anticipate that preliminary attempts at explaining these effect probably lie in self-organization of dissolved gas, like nanobubbles and cooperativity in gas molecular interactions. These are salt- and liquid-dependent.

Inclusion Complexation between α-Cyclodextrin and Oligo(ethylene glycol) Methyl Ether Methacrylate

The preparation of inclusion complexes based on α-cyclodextrin (α-CD) and oligo(ethylene glycol) methyl ether methacrylate (OEGMA) was investigated aiming to reveal complexation particularities and thermodynamic and kinetic aspects as a function of the oligomer architecture. Small-angle X-ray scattering and isothermal titration calorimetry measurements revealed that oligomer molecular weight controls both the kinetics and thermodynamics of inclusion. Unlike linear ethylene glycol polymers, OEGMA groups possess a methacrylate group, which seems to act as a stopper, affecting their mode of complexation. Nuclear magnetic resonance spectra and relaxation measurements support the fact that methacrylate groups lie outside the α-CD ring and that a full sequential complexation of the oligomer ethylene oxide groups is not observed. These results allied to the temperature sensitivity of these oligomers and enable possible routes for chemical modifications and design of new stimuli-responsive materials.

How Does PHEMA Pass through the Cavity of γ-CDs to Create Mismatched Overfit Polypseudorotaxanes?

A syndiotactic-rich PHEMA oligomer ( rr = 74%, DP = 29, PDI = 1.19) was synthesized and subsequently subjected to self-assembly with a varying amount of γ-CDs in its aqueous solution to create mismatched overfit polypseudorotaxanes (PPRs). The inclusion complexation proceeded in an obvious mismatched manner between the cavity of γ-CDs and the cross-sectional area of an incoming PHEMA chain. The 2D-NOESY NMR analysis provided direct evidence indicating that two adjacent pendant hydroxyethyl groups in PHEMA preferably adopt a curled conformation to pass through the cavity of γ-CDs, giving the PPRs characteristics of a mismatched overfit instead of a matched tight-fit crystal structure. The results suggested that the mutual adaption of pendant side chains of HEMA units with the cavity geometry of γ-CDs would play a dominant role in this unfavorable overfit inclusion complexation besides the size of γ-CDs and the stereoregularity of the PHEMA chain.

Natural-Product-Tailored Polyurethane: Size-Dictated Construction of Polypseudorotaxanes with Cyclodextrin-Triterpenoid Pairs

Cyclodextrin (CD)-based polyrotaxanes (PRs) and polypseudorotaxanes (PPRs) have attracted considerable attention due to their unique topological structures and functions. However, limited by the simple chemical structures and the single functionalization of guest polymer units like poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG), to date the construction of CD-based PRs and PPRs with precisely controllable supramacromolecular structures is fairly rare. In this work, two kinds of molecular necklace-like PPRs with CD-triterpenoid pairs were prepared via the size-dictated construction, where the threaded guest polymer was a natural product-tailored polyurethane (PU-PEG-GA) with the alternating structure of triterpenoid and PEG segments via a simple step-growth polymerization. Taking advantage of the differentiation in host-guest interactions between β/γ-CD and triterpenoid pairs, β-CD simultaneously located on both PEG segments and triterpenoid units in PU-PEG-GA, while γ-CD selectively recognized triterpenoid units. Consequently, the assembly morphology of PU-PEG-GA was adjusted hierarchically from micelles to worms and vesicles upon addition of β-CD, whereas they gradually collapsed to disappear in the presence of γ-CD. Our biocompatible PPRs with precisely controllable supramacromolecular structures may lead to the exploration on understanding and simulating macromolecular recognition using natural products.

Supramolecular Hybrid Structures and Gels from Host-Guest Interactions between α-Cyclodextrin and PEGylated Organosilica Nanoparticles

Polypseudorotaxanes are polymer chains threaded by molecular rings that are free to unthread; these "pearl-necklace" can self-assemble further, leading to higher-order supramolecular structures with interesting functionalities. In this work, the complexation between α-cyclodextrin (α-CD), a cyclic oligosaccharide of glucopyranose units, and poly(ethylene glycol) (PEG) grafted to silica nanoparticles was studied. The threading of α-CD onto the polymeric chains leads to their aggregation into bundles, followed by either the precipitation of the inclusion complex or the formation of a gel phase, in which silica nanoparticles are incorporated. The kinetics of threading, followed by turbidimetry, revealed a dependence of the rate of complexation on the following parameters: the concentration of α-CD, temperature, PEG length (750, 4000, and 5000 g mol^-1), whether the polymer is grafted or free in solution, and the density of grafting. Complexation is slower, and temperature has a higher impact on PEG grafted on silica nanoparticles compared to PEG free in solution. Thermodynamic parameters extracted from the transition-state theory showed that inclusion complex formation is favored with grafted PEG compared to free PEG and establishes a ratio of complexation of five to six ethylene oxide units per cyclodextrin. The complexation yields, determined by gravimetry, revealed that much higher yields are obtained with longer chains and higher grafting density. Thermogravimetric analysis and Fourier transform infrared spectroscopy on the inclusion complex corroborate the number of macrocycles threaded on the chains. A sol-gel transition was observed with the longer PEG chain (5k) at specific mixing ratios; oscillatory shear rheology measurements confirmed a highly solid-like behavior, with an elastic modulus G' of up to 25 kPa, higher than that in the absence of silica. These results thus provide the key parameters dictating inclusion complex formation between cyclodextrin and PEG covalently attached to colloidal silica and demonstrate a facile route toward soft nanoparticle gels based on host-guest interactions.

Supramolecular Control over the Interparticle Distance in Gold Nanoparticle Arrays by Cyclodextrin Polyrotaxanes

Amphiphilic nonionic ligands, synthesized with a fixed hydrophobic moiety formed by a thiolated alkyl chain and an aromatic ring, and with a hydrophilic tail composed of a variable number of oxyethylene units, were used to functionalize spherical gold nanoparticles (AuNPs) in water. Steady-state and time-resolved fluorescence measurements of the AuNPs in the presence of α-cyclodextrin (α-CD) revealed the formation of supramolecular complexes between the ligand and macrocycle at the surface of the nanocrystals. The addition of α-CD induced the formation of inclusion complexes with a high apparent binding constant that decreased with the increasing oxyethylene chain length. The formation of polyrotaxanes at the surface of AuNPs, in which many α-CDs are trapped as hosts on the long and linear ligands, was demonstrated by the formation of large and homogeneous arrays of self-assembled AuNPs with hexagonal close packing, where the interparticle distance increased with the length of the oxyethylene chain. The estimated number of α-CDs per polyrotaxane suggests a high rigidization of the ligand upon complexation, allowing for nearly perfect control of the interparticle distance in the arrays. This degree of supramolecular control was extended to arrays formed by AuNPs stabilized with polyethylene glycol and even to binary arrays. Electromagnetic simulations showed that the enhancement and distribution of the electric field can be finely controlled in these plasmonic arrays.

Two sides of the coin. Part 2. Colloid and surface science meets real biointerfaces

Part 1 revisited developments in lipid and surfactant self assembly over the past 40 years [1]. New concepts emerged. Here we explore how these developments can be used to make sense of and bring order to a range of complex biological phenomena. Together with Part 1, this contribution is a fundamental revision of intuition at the boundaries of Colloid Science and Biological interfaces from a perspective of nearly 50 years. We offer new insights on a unified treatment of self assembly of lipids, surfactants and proteins in the light of developments presented in Part 1. These were in the enabling disciplines in molecular forces, hydration, oil and electrolyte specificity; and in the role of non Euclidean geometries-across the whole gammut of physical, colloid and surface chemistry, biophysics and membrane biology and medicine. It is where the early founders of the cell theory of biology and the physiologists expected advances to occur as D'Arcy Thompson predicted us 100 years ago.

On the formation of inclusion complexes at the solid/liquid interface of anchored temperature-responsive PNIPAAM diblock copolymers with γ-cyclodextrin

The thermal responsive behavior of adsorbed layers of diblock copolymers of poly(N-isopropylacrylamide) (PNIPAAM) and poly((3-acrylamidopropyl)trimethylammonium chloride) (PAMPTMA(+)) with γ-cyclodextrin (γ-CD) at the solid/liquid interface has been investigated using three in situ techniques: null ellipsometry, quartz-crystal microbalance with dissipation monitoring, and neutron reflectometry. The measurements provided information about the adsorbed amounts, the layer thickness, hydration and viscoelastic properties, and the interfacial structure and composition. The copolymers adsorb to silica with the cationic PAMPTMA(+) blocks sitting as anchors in a flat conformation and the PNIPAAM chains extending into the solution. The copolymer system alone exhibits reversible collapse above the lower critical solution temperature of PNIPAAM. The addition of γ-CD to pre-adsorbed copolymer layers results in a highly extended conformation as well as some loss of copolymer from the surface, which we discuss in terms of the formation of surface-invoked lateral steric repulsion of formed inclusion complexes.

Hydroxypropyl-β-CD vs. its α-homologue for a 3D modified polyrotaxane network formation and properties: the relationship between modified CD and polymer revealed through comparison

The threading mechanism of the hydroxypropyl-cyclodextrin (Hy-CD)/tetrahedron-like poly(ethylene glycol) (tetra-PEG) based host-guest complex and the relationship between Hy-CD and poly(ethylene oxide) (PEO) in the three-dimensional modified polyrotaxane (PR) formed by the complex were revealed through the comparison between Hy-β-CD/tetra-PEG and Hy-α-CD/tetra-PEG based systems from the macroscopic material view to the microscopic molecular view. The complexation between Hy-CD and tetra-PEG in water experiences a threading-dethreading-rethreading process which is controlled by the intermolecular interaction intensity or molecular hindrance depending on the feed ratio of Hy-CD to tetra-PEG. In the 3D modified PR, the methyl group of the Hy part on one Hy-CD can insert into the cavity of the adjacent Hy-CD and interacts with both the interior surface of the cavity and the PEO segment within the cavity if the cavity of Hy-CD is large enough. The threaded Hy-CD in the PR straightens the chain of PEO and suppresses the segment motion of the PEO. With the decrease of the cavity size of Hy-CD, the degree of suppression on the segment motion of PEO increases. Hy-CD threaded on the PEO chain can also deform when the 3D modified PR is compressed, and the degree of deformation increases with the increase of the cavity size of Hy-CD. These results of the modified CD/PEG based complex system set it apart from the unmodified CD/PEG based one, and reveal the structure-property relationship of this new type of Hy-CD/tetra-PEG based 3D modified PR material.

Structure and rheology of gel nanostructures from a vitamin C-based surfactant

The structure and rheology behaviour of gels produced by water dispersions of a vitamin C-derived surfactant (ascorbyl-6-O-dodecanoate) were investigated by means of SAXS and rheology experiments for the first time. The gel state is formed upon heating and is due to an anisotropic expansion of the tightly compact lamellar structure. The phase transition involves primarily the melting of the alkyl chains and a significant increment in the interlamellar water layer. In particular, our results show that in the gel the hydrophobic chains are in a liquid-like state, as in the core of a micelle, while the head groups release their acidic proton, become negatively charged and determine the onset of strong electrostatic interactions between facing lamellae. The full hydration of the anionic head groups and the uptake of a significant amount of water increase the interlamellar thickness and stabilise the gel structure. Rheology and SAXS measurements together provide an updated picture for the gel state. Moreover, for the first time we show the presence of a concentration threshold, above which the self-assembled aggregates interact more strongly and deplete some of the water that is retained in the interlamellar region.

Modulating the self-assembly of amphiphilic X-shaped block copolymers with cyclodextrins: structure and mechanisms

Inclusion complexes between cyclodextrins and polymers-so-called pseudopolyrotaxanes (PPR)-are at the origin of fascinating supramolecular structures, which are finding increasing uses in biomedical and technological fields. Here we explore the impact of both native and a range of modified cyclodextrins (CD) on the self-assembly of X-shaped poly(ethylene oxide)-poly(propylene oxide) block copolymers, so-called Tetronics or poloxamines, by focusing on Tetronic 904 (T904, Mw 6700). The effects are markedly dependent on the type and arrangement of the substituents on the macrocycle. While native CDs drive the formation of a solid PPR, most substituted CDs induce micellar breakup, with dimethylated β-CD (DIMEB) having the strongest impact and randomly substituted CDs a much weaker disruptive effect. Using native α-CD as a "molecular trap", we perform competitive binding experiments-where two types of CDs thread together onto the polymer chains-to establish that DIMEB indeed has the highest propensity to form an inclusion complex with the polymer, while hydroxypropylated CDs do not thread. 1D (1)H NMR and ROESY experiments confirm the formation of a soluble PPR with DIMEB in which the CD binds preferentially to the PO units, thus providing the drive for the observed demicellization. A combination of dynamic light scattering (DLS) and small-angle neutron scattering (SANS) is used to extract detailed structural parameters on the micelles. A binding model is proposed, which exploits the chemical shifts of selected protons from the CD in conjunction with the Hill equation, to prove that the formation of the PPR is a negatively cooperative process, in which threaded DIMEBs hamper the entrance of subsequent macrocycles.

Inclusion complexes synthesized from an ABA triblock polymer and β-cyclodextrins: amplification of hydrophobic interaction along a hydrophilic polymer chain

β-CD can accommodate PEG segments in aqueous solution through a hydrophobic stabilizing and hydrogen-bond inducing effect.

More on polypseudorotaxanes formed between poly(ethylene glycol) and α-cyclodextrin

A new interpretation for the mechanism associated with the spontaneous threading of α-CD, onto a PEG chain followed by the supramolecular hydrogel formation, is described. Beyond a specific stoichiometry, the complexation of α-CD and PEG results in the formation of a two-phase system. Besides the phase separation, for PEG with a molecular weight higher than 6000 Da, part of the polymer chains are unthreaded by the α-CD, leading to the formation of a supramolecular hydrogel. The kinetics for the complexation and the determination of the yield for equilibrated systems consisting of PEG (linear and star) are used to investigate the number of α-CD threaded before and after the phase separation. The results are compared with the prediction obtained from the application of the Poisson distribution and reveal the ratio between α-CD and PEG in each step of the process. Additionally, the kinetics for the hydrogel formation and its inner structure are investigated by using the proton NMR spin-spin relaxation of water.

Effect of composition of PDMAEMA-b-PAA block copolymers on their pH- and temperature-responsive behaviors

A series of poly(2-(dimethylamino) ethyl methacrylate) (PDMAEMA) homopolymers and poly(2-(dimethylamino)ethyl methacrylate)-b-poly(acrylic acid) (PDMAEMA-b-PAA) diblock copolymers were synthesized by atomic transfer radical polymerization. Thanks to a fine-tuning of the hydrophobic-hydrophilic balance by varying the molecular weight of the polymers and the pH of the aqueous solutions, as well as the composition for the block copolymers, the lower critical solution temperature (LCST) and the aggregation-dissolution kinetics of PDMAEMA homopolymers and PDMAEMA-b-PAA block copolymers can be adjusted. For the block copolymers, the results show that larger relative size of the PDMAEMA blocks leads to an increasing tendency to form micellar aggregates and a decrease of the LCST of the aqueous solution, which is consistent with the increasing copolymer hydrophobicity. A significant difference of the stimuli-responsive behavior between PDMAEMA-rich and PAA-rich copolymers is observed, because the former exhibit thermo-responsive behavior in a broad temperature range of 40-60 °C in basic media, while the pH-responsive behavior is dominant, and only a weak thermo-responsive behavior is exhibited around the specific isoelectric point (IEP) in the latter case. The aggregation rate is strongly influenced by temperature, molecular weight, structure, and composition of the polymer. Specifically, temperature has a stronger effect than the molar ratio of the PDMAEMA segment in the copolymer (related to its hydrophobicity) and the chain molecular weight, although the PDMAEMA-b-PAA copolymers show faster aggregation rate than do the PDMAEMA homopolymers.

Complexation between α-cyclodextrin and PEGylated-PAMAM dendrimers at low and high pH values

Poly(ethylene glycol)-grafted-poly(amido amine) (PEGylated-PAMAM) dendrimers have attracted increasing amounts of attention because of their improved stability, toxicity, and better particle drug leakage property. The complexation of α-cyclodextrin (α-CD) with grafted PEG segments on the surface of PAMAM dendrimers was elucidated by light scattering and titration calorimetry. At pH 10, complexation between α-CD and PEGylated-PAMAM occurred once α-CD was titrated into the PAMAM solution. We observed for the first time a unique phenomenon at pH 2, where no binding took place until a critical α-CD concentration (C*) of ∼8.0 mM was reached. The size of the nanostructures increased from 6.7 to 57.6 nm when the α-CD concentration was increased from 0.5 to 15 mM at pH 2. The zeta potential of PEGylated-PAMAM at pH 2 was +6.7 mV. Thus, the dendrimers possessed positive charges attributed to the protonation of primary amine groups on PAMAM chains that impart electrostatic repulsive forces to the system. The morphology of the complex is expected to be different at two different pH values (2 and 10) because the former produces a clear solution and the latter forms a turbid solution with white precipitates.

Formation and self-organization kinetics of alpha-CD/PEO-based pseudo-polyrotaxanes in water. A specific behavior at 30 degrees C

alpha-Cyclodextrins (alpha-CDs) have the ability to form inclusion complexes with poly(ethylene oxide) (PEO) polymer chains. These pseudo-polyrotaxanes (PPRs) can be obtained by quenching an alpha-CD/PEO mixture in water from 70 degrees C down to a lower temperature (typically in the range from 5 to 30 degrees C) thanks to favorable interactions between alpha-CD cavities and PEO chains. Moreover, starting from a liquid alpha-CD/PEO mixture at a total mass fraction of 15% w/w at 70 degrees C, the formation of PPRs with time at a lower temperature induces a white physical gel with time, and phase separation is observed. We established that PPR molecules are exclusively found in the precipitated phase although unthreaded alpha-CD molecules and unthreaded PEO chains are in the liquid phase. At 30 degrees C, the physical gel formation is much slower than at 5 degrees C. At 30 degrees C, we established that, in a first step, alpha-CDs thread onto PEO chains, forming PPR molecules which are not in good solvent conditions in water. At a higher length scale, rapid aggregation of the PPR molecules occurs, and threaded alpha-CD-based nanocylinders form (cylinder length L = 5.7 nm and cylinder radius R = 4.7 nm). At a higher length scale, alpha-CD-based nanocylinders associate in a Gaussian way, engendering the formation of precipitated domains which are responsible for the high turbidity of the studied system. At the end of this first step (i.e., after 20 min), the system still remains liquid and the PPRs are totally formed. Then, in a second step (i.e., after 150 min), the system undergoes its reorganization characterized by a compacity increase of the precipitated domains and forms a physical gel. We found that PPRs are totally formed after 20 min at 30 degrees C and that the system stays in a nongel state up to 150 min. This opens new perspectives regarding the PPR chemical modification: between these two characteristic times, we can easily envisage an efficient chemical modification of the PPR molecules in water, as for instance an end-capping reaction leading to the synthesis of polyrotaxanes.