Axle Component Research Articles

Formation of [2]- and [3]Rotaxanes through Bridging under Kinetic and Thermodynamic Control.

An efficient synthesis of a doubly stranded [3]rotaxane has been developed through bridging of a pseudo[3]rotaxane featuring two axle components. Reversible azine formation was effective as the bridging reaction. Kinetic and thermodynamic conditions provided the [2]- and [3]rotaxanes, respectively.

Reversible Laser-Induced Bending of Pseudorotaxane Crystals.

This study investigated the dynamic photoresponse of pseudorotaxane crystals with azobenzene and ferrocenyl groups in the axle component. X-ray crystallography showed pseudorotaxanes with a methylazobenzene group and a dibromophenylene ring in the cyclic component to exhibit twisting of the trans-azobenzene groups at torsion angles of 17° and 38°, respectively. Repeated alternating laser irradiation of the crystals at 360 and 445 nm produced bending of 20-30° in opposite directions, with no evidence of decay. Under 445 nm irradiation, bending took place within 0.3 s. A crystal of nonsubstituted pseudorotaxane showed bending of only 2° under 360 nm irradiation due to multiple π-π interactions between the planar trans-azobenzene groups. The pseudorotaxane crystals have two chromophores, bent rapidly and reversibly on irradiation at rates depending on the molecular structure.

Remote electrochemical modulation of pKa in a rotaxane by co-conformational allostery

Allosteric control, one of Nature's most effective ways to regulate functions in biomolecular machinery, involves the transfer of information between distant sites. The mechanistic details of such a transfer are still an object of intensive investigation and debate, and the idea that intramolecular communication could be enabled by dynamic processes is gaining attention as a complement to traditional explanations. Mechanically interlocked molecules, owing to the particular kind of connection between their components and the resulting dynamic behavior, are attractive systems to investigate allosteric mechanisms and exploit them to develop functionalities with artificial species. We show that the pKa of an ammonium site located on the axle component of a [2]rotaxane can be reversibly modulated by changing the affinity of a remote recognition site for the interlocked crown ether ring through electrochemical stimulation. The use of a reversible ternary redox switch enables us to set the pKa to three different values, encompassing more than seven units. Our results demonstrate that in the axle the two sites do not communicate, and that in the rotaxane the transfer of information between them is made possible by the shuttling of the ring, that is, by a dynamic intramolecular process. The investigated coupling of electron- and proton-transfer reactions is reminiscent of the operation of the protein complex I of the respiratory chain.

A Chiral Halogen‐Bonding [3]Rotaxane for the Recognition and Sensing of Biologically Relevant Dicarboxylate Anions

AbstractThe unprecedented application of a chiral halogen‐bonding [3]rotaxane host system for the discrimination of stereo‐ and E/Z geometric isomers of a dicarboxylate anion guest is described. Synthesised by a chloride anion templation strategy, the [3]rotaxane host recognises dicarboxylates through the formation of 1:1 stoichiometric sandwich complexes. This process was analysed by molecular dynamics simulations, which revealed the critical synergy of halogen and hydrogen bonding interactions in anion discrimination. In addition, the centrally located chiral (S)‐BINOL motif of the [3]rotaxane axle component facilitates the complexed dicarboxylate species to be sensed via a fluorescence response.

A Chiral Halogen-Bonding [3]Rotaxane for the Recognition and Sensing of Biologically Relevant Dicarboxylate Anions.

The unprecedented application of a chiral halogen-bonding [3]rotaxane host system for the discrimination of stereo- and E/Z geometric isomers of a dicarboxylate anion guest is described. Synthesised by a chloride anion templation strategy, the [3]rotaxane host recognises dicarboxylates through the formation of 1:1 stoichiometric sandwich complexes. This process was analysed by molecular dynamics simulations, which revealed the critical synergy of halogen and hydrogen bonding interactions in anion discrimination. In addition, the centrally located chiral (S)-BINOL motif of the [3]rotaxane axle component facilitates the complexed dicarboxylate species to be sensed via a fluorescence response.

Solid Corrosion Products in the Periprosthetic Tissues of a Distal Femoral Endoprosthesis Used to Treat Osteosarcoma

The case of a child is described who was treated for distal femoral osteosarcoma with chemotherapy, removal of the distal femoral bone, and the implantation of a series of distal femoral endoprostheses with hinged tibial components. The design of this modular prosthesis allows for expansion during the years that the bones continue to grow. The tissues from nine surgical procedures were examined, along with one tibial bearing and axle component. Wear debris was noted in variable amounts in nearly all of the tissues but solid corrosion products from the modular junctions were abundant only in the last revision tissues. The amount of wear and corrosion of the explants seemed inconsistent with the size and amount of corrosion products in the tissues. The source of the debris is likely to be in the modular connections that were not revised.

Preparation of Pillar[5]arene-Based [2]Rotaxanes by a Stopper-Exchange Strategy.

A pillar[5]arene-containing rotaxane building block bearing exchangeable stoppers has been prepared in multigram scale quantities with high yields from the reaction of 2,4-dinitrophenol (DNP) with the inclusion complex resulting from the association of dodecanedioyl chloride with 1,4-diethoxypillar[5]arene. Stopper exchange reactions have been achieved by treatment of the resulting DNP diester with various amines through an addition-elimination mechanism preventing the unthreading of the axle component during the reaction and thus preserving the [2]rotaxane structures. The resulting diamide [2]rotaxane derivatives have thus been obtained in good to excellent yields. Importantly, [2]rotaxanes difficult or impossible to prepare by direct introduction of the two stoppers in a single synthetic step are now easily available.

Topology-transformable polymers: linear–branched polymer structural transformation via the mechanical linking of polymer chains

In this review article, we discuss the synthesis and dynamic nature of macromolecular systems that have mechanically linked polymer chains capable of undergoing a topology transformation driven by a rotaxane molecular switch. The rotaxane linking of polymer chains plays a crucial role in these systems. A linear polymer possessing a crown ether/sec-ammonium salt-type [1]rotaxane moiety at the axle chain terminal was prepared via the rotaxane linking of a single polymer chain. A linear–cyclic polymer topology transformation was achieved via the movement of the wheel component from one end to the other end of the axle component using the rotaxane macromolecular switch function. The successful synthesis of a macromolecular [2]rotaxane (M2R) possessing a single polymer axle and one crown ether wheel led to a variety of unique applications, such as the development of topology-transformable polymers and the synthesis of rotaxane crosslinked polymers (RCPs). The introduction of a polymer chain to the wheel component of M2R (rotaxane linking of two polymer chains) produced a rotaxane-linked AB block copolymer that transformed its topology from linear to branched. Furthermore, a rotaxane-linked three-component polymer and an ABC triblock copolymer were synthesized and transformed to the corresponding 3-arm star (co)polymers, and the transformation was confirmed by measuring the hydrodynamic volume change. The structural transformation of 4-arm and 6-arm star polymers was also accomplished using the dynamic mobility of a similar rotaxane. As a useful application, M2R-based vinylic crosslinkers (RCs) were prepared and applied to the synthesis of RCPs, whereby the addition of RCs into radical polymerization systems of vinyl monomers afforded polymers with excellent toughness by enhancing both of tradeoff properties that cannot be achieved using typical covalent crosslinkers. Synthesis and dynamic nature of macromolecular systems having mechanically linked polymer chains, which can undertake the topology transformation, are described. The rotaxane linking of polymer chains plays a crucial role in these systems. Successful syntheses of macromolecular [2]rotaxane (M2R) possessing single polymer axle and one crown ether wheel made possible the development of topology-transformable polymers from star to linear. Furthermore, rotaxane-linked ABC triblock copolymer and 4-arm and 6-arm star polymers were also synthesized and transformed to other topological polymers along with the property change.

PH-Switched fluorescent pseudorotaxane assembly of cucurbit[7]uril with bispyridinium ethylene derivatives

The host-guest properties of cucurbit[7]uril (Q[7]) and bispyridinium ethylene derivatives have been studied by 1H NMR spectroscopy, UV–vis absorption spectra, and fluorescence emission analysis. The proton shifts associated with the guest encapsulated by the host suggested that the Q[7]-based [2]pseudorotaxane behaves like a fast molecular shuttle along the bispyridinium ethylene axle of the guest upon protonation and deprotonation of the terminal carboxylates. In particular, the distinct fluorescent response signals indicated that the bispyridinium ethylene moiety not only behaves as the axle component for the pseudorotaxane system, but also acts as an optical reporting unit during the host–guest complexation.

Enantioselective Anion Recognition by Chiral Halogen-Bonding [2]Rotaxanes.

The application of chiral interlocked host molecules for discrimination of guest enantiomers has been largely overlooked, which is surprising given their unique three-dimensional binding cavities capable of guest encapsulation. Herein, we combined the stringent linear geometric interaction constraints of halogen bonding (XB), the noncovalent interaction between an electrophilic halogen atom and a Lewis base, with highly preorganized and conformationally restricted chiral cavities of [2]rotaxanes to achieve enantioselective anion recognition. Representing the first detailed investigation of the use of chiral XB rotaxanes for this purpose, extensive 1H NMR binding studies and molecular dynamics (MD) simulation experiments revealed that the chiral rotaxane cavity significantly enhances enantiodiscrimination compared to the non-interlocked free axle and macrocycle components. Furthermore, by examining the enantioselectivities of a family of structurally similar XB [2]rotaxanes containing different combinations of chiral and achiral macrocycle and axle components, the dominant influence of the chiral macrocycle in our rotaxane design for determining the effectiveness of chiral discrimination is demonstrated. MD simulations reveal the crucial geometric roles played by the XB interactions in orientating the bound enantiomeric anion guests for chiral selectivity, as well as the critical importance of the anions' hydration shells in governing binding affinity and enantiodiscrimination.

Synthesis of Mechanically Planar Chiral rac-[2]Rotaxanes by Partitioning of an Achiral [2]Rotaxane: Stereoinversion Induced by Shuttling.

Mechanically planar chiral [2]rotaxanes were synthesized by the introduction of bulky pyrrole moieties into the axle component of an achiral [2]rotaxane. The enantiomers were separated by chiral HPLC. The shuttling of the ring component between the two compartments at high temperature induced the stereoinversion of the mechanically planar chiral [2]rotaxane. The rate of the stereoinversion was studied quantitatively, and the kinetic parameters were determined.

Four-State Molecular Shuttling of [2]Rotaxanes in Response to Acid/Base and Alkali-Metal Cation Stimuli.

In this study, we synthesized [2]rotaxanes possessing three recognition sites-a dialkylammonium, an alkylarylamine, and a tetra(ethylene glycol) stations-in their dumbbell-like axle component and dibenzo[24]crown-8 (DB24C8) as their macrocyclic component. These [2]rotaxanes behaved as four-state molecular shuttles: i) under acidic conditions, the DB24C8 unit encircled both the dialkylammonium and alkylarylammonium stations; ii) under neutral conditions, the dialkylammonium unit was the predominant station for the DB24C8 component; iii) under basic conditions, when both ammonium centers were deprotonated, the alkylarylamine unit became a suitable station for the DB24C8 component; and iv) under basic conditions in the presence of an alkali-metal cation, the tetra(ethylene glycol) unit recognized the DB24C8 component through cooperative binding of the alkali-metal ion. In addition, we observed that the [2]rotaxanes exhibited selective recognition for metal cations. These shuttling motions of the macrocyclic component proceeded reversibly.

Vinylic Rotaxane Cross‐Linker Comprising Different Axle Length for the Characterization of Rotaxane Cross‐linked Polymers

SummaryThe [2]rotaxane cross‐linkers (RCs) comprising vinyl group‐terminated axle component having different axle lengths and vinyl group‐containing crown ether were synthesized, and applied to prepare the corresponding rotaxane cross‐linked polymers (RCPs). The preparations of RCP were performed by the radical polymerizations of n‐butyl acrylate in the presence of 0.5 mol% of RCs. Properties of the RCPs were evaluated by swelling ratio, clearly demonstrating that axle length of RC affects swelling ratio of RCPs.

Strong and Selective Halide Anion Binding by Neutral Halogen-Bonding [2]Rotaxanes in Wet Organic Solvents.

The design and construction of neutral interlocked host molecules for anion recognition are rare. Using an active-metal template approach, the preparation of a family of neutral halogen bonding (XB) rotaxanes containing two, three and four iodotriazole groups integrated into the macrocycle and axle components is achieved. In spite of the interlocked hosts' neutrality, such rotaxane systems are capable of binding halide anions strongly and selectively in wet organic solvent mixtures. Importantly, halide-binding strength and selectivity can be modulated by varying the number and position of the halogen bond donor iodotriazole groups within the interlocked cavity; the rotaxane containing the largest number of halogen bond donor groups exhibits the highest halide anion-binding affinities. By varying the percentage of water content in the solvent, neutral XB donor-mediated anion-binding strength is also demonstrated to be highly sensitive to solvent polarity.

Cationic all-halogen bonding rotaxanes for halide anion recognition.

A family of cationic halogen bonding [2]rotaxanes have been synthesised via an active-metal template synthetic strategy. 1H NMR spectroscopic anion titration investigations reveal these interlocked host systems recognize halides selectively over oxoanions in aqueous-organic solvent media. Furthermore, systematically modulating the rigidity and size of the rotaxanes' anion binding cavities via metal complexation, as well as by varying the number of halogen bond-donor groups in the axle component, was found to dramatically influence halide anion selectivity.

Halogen bonding rotaxanes for nitrate recognition in aqueous media.

Targeting the biologically and environmentally important nitrate anion, halogen bonding (XB) has been incorporated into three novel [2]rotaxane structural frameworks via an axle component containing covalently linked 3,5-bis-iodotriazole pyridine-pyridinium motifs. This has enabled the recognition of nitrate in aqueous media containing up to 90% water with equivalent binding affinity to chloride, illustrating the potency of XB for anion recognition in highly competitive aqueous solvent mixtures.

Porphyrinoid rotaxanes: building a mechanical picket fence.

Building on recent progress in the synthesis of functional porphyrins for a range of applications using the Cu-mediated azide-alkyne cycloaddition (CuAAC) reaction, we describe the active template CuAAC synthesis of interlocked triazole functionalised porphyrinoids in excellent yield. By synthesising interlocked analogues of previously studied porphyrin-corrole conjugates, we demonstrate that this approach gives access to rotaxanes in which the detailed electronic properties of the axle component are unchanged but whose steric properties are transformed by the mechanical "picket fence" provided by the threaded rings. Our results suggest that interlocked functionalised porphyrins, readily available using the AT-CuAAC approach, are sterically hindered scaffolds for the development of new catalysts and materials.

The response of a high-speed train wheel to a harmonic wheel-rail force

The maximum speed of China's high-speed trains currently is 300km/h and expected to increase to 350-400km/h. As a wheel travels along the rail at such a high speed, it is subject to a force rotating at the same speed along its periphery. This fast moving force contains not only the axle load component, but also many components of high frequencies generated from wheel-rail interactions. Rotation of the wheel also introduces centrifugal and gyroscopic effects. How the wheel responds is fundamental to many issues, including wheel-rail contact, traction, wear and noise. In this paper, by making use of its axial symmetry, a special finite element scheme is developed for responses of a train wheel subject to a vertical and harmonic wheel-rail force. This FE scheme only requires a 2D mesh over a cross-section containing the wheel axis but includes all the effects induced by wheel rotation. Nodal displacements, as a periodic function of the cross-section angle 6, can be decomposed, using Fourier series, into a number of components at different circumferential orders. The derived FE equation is solved for each circumferential order. The sum of responses at all circumferential orders gives the actual response of the wheel.

Diastereoselective synthesis of optically active rotaxane amine N-oxides via through-space chirality transfer

Selective synthesis of optically active rotaxane amine N-oxides was achieved with high diastereoselectivity via the effective through-space chirality transfer. Oxidation of tert-amine moiety on axle component of rotaxane with an optically active wheel component having (R)-binaphthyl group was carried out. The oxidation of several rotaxanes with dimethyldioxirane was conducted in dichloromethane at −78°C to give the corresponding amine N-oxides with high diastereoselectivity up to 95%, indicating the conversion via the effective through-space chirality transfer. Higher diastereoselectivity was observed with the rotaxane possessing the rigid skeleton and the N-benzyl substituent. The optimized structures suggested the stereochemistry of the nitrogen center of (R)-configuration.

Potential of an improved energy efficiency in the chassis

In this paper, an approach is presented to determine the losses of the different components in the chassis system. An enhanced two-track model is extended by sub-models to simulate losses and efficiencies. These sub-models are describing the loss behavior, which is compared to the state of art. The energy saving potential of each component is analyzed by optimizing the package of axle components and regarding technology improvements. The study is based on measured user behavior, which includes lateral and longitudinal dynamics as well as road unevenness.