Stretching and Rupturing Individual Supramolecular Polymer Chains by AFM

Single-molecule mechanics of synthetic aromatic amide helices: Ultrafast and robust non-dissipative winding

A series of poly(lactide) (PLA)‐based supramolecular polymers based on linear PLLA‐b‐PCL‐b‐PLLA triblock copolymers (PLLA, poly(l‐lactide); PCL, poly(ε‐caprolactone)) or/and three‐arm star (PCL‐b‐PDLA)3 block copolymers (PDLA, poly(d‐lactide)) were synthesized. The effects of the structure and composition on crystal structure, crystallization behavior, spherulite morphology and mechanical properties of the synthesized supramolecular polymers were investigated. The results of DSC and polarized optical microscopy indicated that the supramolecular polymer exhibited poor crystallization ability with respect to PCL/PLA block copolymer, and the crystallinity of the supramolecular polymer with alternating PCL/PLA multiblock structure was stronger than that with similar crosslinked network structure. The presence of molten PCL blocks disturbed the orientation of lamellae, forming spherulites with feather‐like dendrites, and ring‐banded spherulites were observed as the molecular weight of the PLA blocks increased. The results of tensile tests demonstrated that supramolecular polymers with larger molecular weight of PLA blocks showed the pronounced ductile fracture. On this basis, stereocomplexed supramolecular polymers were also synthesized, and it was found that the stereocomplex crystals had a significant impact on the crystallization and mechanical properties of the supramolecular polymers. Therefore, in this work a novel technique for manufacturing toughened PLA‐based material and tuning its performances is proposed, which may promote the application of PLA‐based materials in more fields. © 2022 Society of Industrial Chemistry.

A new method of supramolecular emulsion interfacial polymerization is developed, which can be used to fabricate supramolecular polymeric nanospheres. We designed a water-soluble monomer containing two maleimide end groups, acting as both building block and surfactant, and an oil-soluble supramonomer bearing two thiol groups connected by quadruple hydrogen bonds. With the assist of ultrasonication, hollow nanospheres can be controllably prepared by thiol-maleimide reaction of two monomers at the emulsion interface, which exhibit good stability and dynamic property. In addition, the encapsulated guest molecules could be controllably released from the supramolecular polymeric nanospheres, owing to their stimuli-responsiveness. It is anticipated that this approach will enrich the methodology of supramolecular polymerization and can be applied to constructing supramolecular materials with controllable structures and functions.

Small-sized albumin gel microspheres, MSs, containing 5-fluorouracil (5FU) with targeting moieties on their surfaces (average diameter: 1.5 μm) were prepared by the glutaraldehyde crosslinking method and suspension technique. Since galactose is known to interact specifically with the asialoglycoprotein receptor on hepato-cyte, the galactose residues were introduced on the surface of MSs as the targeting moieties for hepatoma through polyethylene glycol (PEG) spacers. PEG spacers were employed to depress the immu-nogenicity of albumin, to keep the mobility of the galactose residues, and to heighten the distributive stability of the MSs in aqueous solution. It was confirmed by ESC A analysis that the PEG chains were introduced onto the surfaces of MSs. The amount of galactose residues introduced to MS were estimated to be 0.013 wt%. The intra-MSs aggregation was observed by the addition of Ricinus Communis Agglutinin I (RCA120) into the MS suspension, and then the aggregation of MSs was dissociated...

A well-defined quadruple hydrogen bonding strategy involving dimerization of 2-ureido-4[1H]-pyrimidone (UPy) units is innovatively designed to prepare polyureas with high overall mechanical properties. Three polyureas containing different amounts of UPy units were synthesized by replacing a portion of isophorone diisocyanate (IPDI) with a UPy-derived diisocyanate. The formation of quadruple hydrogen bonds in hard segments via UPy dimers was confirmed by nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The mechanical properties of the polyureas were evaluated by uniaxial tensile testing. Compared to the polyurea without UPy units, remarkable improvements in Young’s modulus, tensile strength, and toughness were simultaneously achieved when UPy units were incorporated. The mechanism behind the strong strengthening effect rooted in the stronger intermolecular forces among hard segments brought by the quadruple hydrogen bonds, which were stronger than the inherent bidentate and monodentate hydrogen bonds among urea groups, and the slower soft segmental dynamics reaveled by both increased Tg and relaxation time of the soft segments. The mechanism behind the strong toughening effect was ascribed to more effective energy dissipation brought by the quadruple hydrogen bonds that served as stronger sacrificial bonds upon deformation. This work may offer new insight into the design of polyurea elastomers with comprehensively improved mechanical properties.

A set of fluorene oligomers has been synthesized by stepwise palladium-catalyzed (Suzuki) couplings of fluorene monomers. Ureidopyrimidinones (UPy), functional groups that can dimerize via quadruple hydrogen bonds, were attached to both ends of the oligofluorenes. The resulting bis-UPy-terminated oligomers self-assemble into supramolecular chain polymers. For comparison, oligofluorenes of the same oligomer lengths but without terminal hydrogen-bonding groups were synthesized. Chains of hydrogen-bonded fluorenes can be simply endcapped by a variety of chain stoppers, molecules that have one UPy group. In this manner, we have endcapped the hydrogen-bonded fluorene chains with either oligo(p-phenylenevinylene) or perylene bisimide. Energy-transfer experiments in solution and the solid state demonstrate that oligofluorenes can donate energy to a variety of energy acceptors, but that this energy transfer occurs most effectively when the donor fluorene is hydrogen-bonded to the acceptor.

A facile method for preparing supramolecular waterborne polyurethane (WPU) based on quadruple hydrogen bonds (4H‐WPU) is reported. Herein, 4H‐WPU with quadruple hydrogen bonds in main chain were synthesized by poly (1,4‐buthylene‐neopentylene adipate glycol) as soft segment, 2‐ureido‐4[1H]‐pyrimidinone (UPy) functionalized monomer, isophorone diisocyanate, 2,2‐Bis(hydroxymethyl)propionic acid (hydrophilic monomer), isophorondiamine, triethylamine (neutralizer), and monoethanolamine (blocking agent) as hard segment. The molecular weight of 4H‐WPU was controlled around 18,000 consistently. The properties of 4H‐WPU with different content of hydrophilic group, hard segment, and UPy units were characterized and the results could provide the reference for preparing supramolecular WPU with high mechanical and self‐healing performance while maintaining dispersion stability. The tensile strength of 4H‐WPU was 10 times of the blank sample. The healing time of scratched 4H‐WPU film was inversely proportional to the content of UPy‐ functionalized monomer and the shortest healing time is 2.5 hr at 80°C. Mechanical performance of healed films can be restored to more than 90%.

It is still challenging and attractive to prepare polyurethane (PU) materials with excellent self-healing ability while improving their mechanical properties and high ductility. Here, a multifunctional linear PU supramolecular elastomer was successfully prepared by introducing a cross-linking network of quadruple hydrogen bonds and thermo-reversible Diels–Alder bonds and rigid ring structure to the linear backbone. The results exhibited that the obtained PU elastomer displayed a high tensile strength (6.30 MPa), elongation (1957.84%), toughness (84.48 MJ/m3), and excellent repair efficiency (93.33%). The quadruple hydrogen bonds from 5-(2-hydroxyethyl)-6-methyl-2-aminouracil and thermo-reversible Diels–Alder bonds from the conjugated reaction of 4,4′-bismaleimide diphenylmethane with furfuryl alcohol, due to its synergetic dual reversible bonds, formed the PU elastomer that possessed excellent mechanical, self-healing, shape recovery, and reprocessing properties. The prepared multifunctional PU can be used as a substrate for flexible conductive materials or as conductive composite material with conductive materials, which can self-repair many times when the surface is damaged, can be recycled, and greatly improve the service life of the material. Therefore, the prepared multifunctional high-performance self-healing PU materials have potential applications in several fields.

Force and function: probing proteins with AFM-based force spectroscopy

Heterodimerization of human epidermal growth factor receptor 2 (HER2) with HER3 initiates aberrant downstream growth-signaling pathways in tumors. Our objective was to construct bispecific radioimmunoconjugates (bsRICs) that recognize HER2 and HER3 and evaluate their ability to image tumors in athymic mice that express one or both receptors using small-animal SPECT/CT. bsRICs were constructed by reacting the maleimide-derivatized trastuzumab Fab fragments that bind HER2 with a thiolated form of the HER3-binding peptide of heregulin-β1 (HRG) with or without a 12- or 24 mer polyethylene glycol (PEG) spacer. bsRICs were derivatized with diethylenetriaminepentaacetic acid for labeling with (111)In. The ability of (111)In-bsRICs to bind HER2 or HER3 was determined in competition assays with unlabeled Fab or HRG on cells expressing one or both receptors. Tumor and normal-tissue uptake were examined in CD1 athymic mice bearing subcutaneous tumor xenografts that expressed HER2, HER3, or both receptors, with or without the preadministration of unlabeled Fab or HRG to determine the specificity of uptake. Conjugation of Fab to HRG was confirmed by sodium dodecyl sulphate polyacrylamide gel electrophoresis-Western blot and size-exclusion high-performance liquid chromatography. Improved HER2 and HER3 binding and greater displacement of binding by competitors was found for (111)In-bsRICs that incorporated a PEG spacer, with the PEG(24) spacer being optimal. The highest uptake of (111)In-bsRICs (7.8% ± 2.1% injected dose per gram [%ID/g]) in BT-474 human breast cancer xenografts (HER2-positive/HER3-positive) occurred at 48 h after injection. The preadministration of trastuzumab Fab decreased uptake in SK-OV-3 (HER2-positive/HER3-negative) human ovarian cancer xenografts from 7.0 ± 1.2 to 2.6 ± 1.5 %ID/g (P < 0.001). The preadministration of an excess of HRG decreased uptake in MDA-MB-468 (HER2-negative/HER3-positive) human breast cancer xenografts from 4.4 ± 0.9 to 2.6 ± 0.5 %ID/g (P < 0.05). All tumors were imaged by small-animal SPECT/CT. (111)In-bsRICs composed of trastuzumab Fab and HRG exhibited specific binding in vitro to tumor cells displaying HER2 or HER3 and were taken up specifically in vivo in tumors expressing one or both receptors, permitting tumor visualization by small-animal SPECT/CT. These agents could be useful for imaging heterodimerized HER2 and HER3 receptors because their bivalent properties may result in preferential binding to the heterodimerized forms. The approach may also be extended to constructing bsRICs for visualizing other peptide growth factor receptors.

Modular assembly of bio-inspired supramolecular polymers is a powerful technique to develop new soft nanomaterials, and protein folding is a versatile basis for preparing such materials. Previous work demonstrated a significant difference in the physical properties of closely related supramolecular polymers composed of building blocks in which identical coiled-coil-forming peptides are cross-linked by one of two subtly different organic linkers (one flexible and the other rigid). Herein, we investigate the molecular basis for this observation by isolating a single subunit of the supramolecular polymer chain and probing its structure and conformational flexibility by double electron–electron resonance (DEER) spectroscopy. Experimental spin–spin distance distributions for two different labeling sites coupled with molecular dynamics simulations provide insights into how the linker structure impacts chain dynamics in the coiled-coil supramolecular polymer.

Single-stranded DNA binding proteins (SSB) interact with single-stranded DNA (ssDNA) specifically. Taking advantage of this character, we have employed Bacillus subtilis SSB protein to investigate the nature of force-induced conformation transition of double-stranded DNA (dsDNA) by using AFM-based single molecule force spectroscopy (SMFS) technique. Our results show that, when a dsDNA is stretched beyond its contour length, the dsDNA is partially melted, producing some ssDNA segments which can be captured by SSB proteins. We have also systematically investigated the effects of stretching length, waiting time, and salt concentration on the conformation transition of dsDNA and SSB-ssDNA interactions, respectively. Furthermore, the effect of proflavine, a DNA intercalator, on the SSB-DNA interactions has been investigated, and the results indicate that the proflavine-saturated dsDNA can be stabilized to the extent that the dsDNA will no longer melt into ssDNA under the mechanical force even up to 150 pN, and no SSB-DNA interactions are detectable.

The study on the nanoparticle-polymer interactions is very important for the design/preparation of high performance polymer nanocomposite. Here we present a method to quantify the polymer-particle interaction at single molecule level by using AFM-based single molecule force spectroscopy (SMFS). As a proof-of-concept study, we choose poly-l-lysine (PLL) as the polymer and several different types of polyoxometalates (POM) as the model particles to construct several different polymer nanocomposites and to reveal the binding mode and quantify the binding strength in these systems. Our results reveal that the shape of the nanoparticle and the binding geometry in the composite have significantly influenced the binding strength of the PLL/POM complexes. Our dynamic force spectroscopy studies indicate that the disk-like geometry facilitate the unbinding of PLL/AlMo6 complexes in shearing mode, while the unzipping mode becomes dominate in spherical PLL-P8W48 system. We have also systematically investigated the effects of charge numbers, particle size, and ionic strength on the binding strength and binding mode of PLL/POM, respectively. Our results show that electrostatic interactions dominate the stability of PLL/POM complexes. These findings provide a way for tuning the mechanical properties of polyelectrolyte-nanoparticle composites.

Probing Single Individual Proteins Unfold and Refold with 1-μs Resolution: Improved AFM-Based Single Molecule Force Spectroscopy

Atomic force microscopy (AFM) enables the pick up of a single protein molecule to apply a mechanical force. This technique, called “force spectroscopy,” provides unique information about the intermediates and free energy landscape of the mechanical unfolding of proteins. In this review, we introduce the AFM-based single molecule force spectroscopy of proteins and describe recent studies that answer some fundamental questions such as “is the mechanical resistance of proteins isotropic?”, “what is the structure of the transition state in mechanical unfolding?”, and “is mechanical unfolding related to biological functions?”