Threading Process Research Articles

From Small Cucurbituril Complexes to Large Ordered Networks

AbstractCucurbit[6]uril forms stable complexes with amine compounds in aqueous solutions. Some of the preformed complexes can be used for the synthesis of rotaxanes or polyrotaxanes. Polymers with threaded cucurbit[6]uril are also obtained from these complexes. Even by mixing a solid amino acid with solid cucurbit[6]uril a threading process takes place in the melt during the polycondensation. Structural flexible amino compounds are completely rigid after complex formation with cucurbit[6]uril. As a result, they are used as one component for the formation of polyelectrolyte layers on substrates. Furthermore some of the complexes, together with cations, are able to act as components in the formation of metal‐organic networks. Spectroscopic measurements indicate a well‐ordered formation of these networks.

Theory of capture rate in polymer translocation

The translocation of macromolecules through a nanopore requires the impingement of the molecules at the pore followed by threading through the pore. While most of the discussion on the translocation phenomenon focused so far on the threading process, the phenomenology on the frequency of encounters between the polymer and the pore exhibits diverse features in terms of polymer length, solution conditions, driving force, and pore geometry. We derive a general theory for the capture rate of polyelectrolyte molecules and the probability of successful translocation through a nanopore, under an externally imposed electric field. By considering the roles of entropic barrier at the pore entrance and drift of the polyelectrolyte under the electric field, we delineate two regimes: (a) entropic barrier regime and (b) drift regime. In the first regime dominated by the entropic barrier for the polyelectrolyte, the capture rate is an increasing nonlinear function in the electric field and chain length. In the drift regime, where the electric field dwarfs the role of entropic barriers, the capture rate is independent of chain length and linear in electric field. An analytical formula is derived for the crossover behavior between these regimes, and the general results are consistent with various experimentally observed trends.

Molecular Recognition between the Constituents of a Pseudorotaxane Studied by Scanning Tunneling Microscopy

The formation of intermolecular complexes of two large molecules—a macrocycle and a semiaxle, which have been used in templated syntheses of amide rotaxanes—was studied by scanning tunneling microscopy (STM) and density functional theory (DFT). These experiments mimic the so-called “threading process”, which is based on intermolecular recognition and which is essential for the rotaxane synthesis in solution. First, ordered monolayers of a tetralactam macrocycle (TLM), i.e. the rotaxane wheel, are prepared on a Au(111) surface. Then, semiaxles (SA) are deposited on top of these ordered TLM layers at ca. 140 K. In solution, the SA molecule threads into the TLM cavity by formation of three hydrogen bonds between the amide groups of both molecules. On the Au(111) surface, the scenario is similar, although different in detail due to geometric restrictions given by the underlying Au(111) surface and conformational energy barriers due to the confinement of the TLM geometry in the ordered monolayer structure. Thr...

Efficient Preparation of Separable Pseudo[n]rotaxanes by Selective Threading of Oligoalkylammonium Salts with Cucurbit[7]uril

We describe the efficient preparation of well-defined, homogeneous pseudo[n]rotaxanes (n = 2, 3, 4, 5) by the selective threading of oligoalkylammonium salts with cucurbit[7]uril (CB[7]) and the subsequent simple separation of the pure pseudo[n]rotaxanes from the complicated system. A series of well-defined oligoalkylammonium salts were prepared by stepwise synthesis and their molecular recognition and self-assembly with a solution of CB[7] in CF(3)CO(2)D/D(2)O (1:1 v/v) were investigated. As a result of the high affinity of CB[7] to the p-xylene diammonium units (-NH(2)(+)CH(2)C(6)H(4)CH(2)NH(2)(+)-) in the oligoalkylammonium salts, the CB[7] rings threaded onto these cationic threads to form pseudorotaxane structures. Interestingly, due to the repulsive dipole-dipole electrostatic interactions between the CB[7] units and steric hindrance, the threading process exhibited a high selectivity and a unique self-assembling mode was discovered in which two CB[7] units cannot bind the same ammonium site. The as-formed well-defined pseudo[n]rotaxanes can be easily separated as pure compounds from the mixture by simple counterion exchange with NH(4)PF(6) and the pure pseudo[n]rotaxanes are stable and partially soluble in normal organic solvents. Such facile preparation and separation are mainly ascribed to the high binding constant between CB[7] and the oligoalkylammonium salts and the unique solubility of CB[7] in protonic acid. As most pseudorotaxanes reported in the literature exist as a dynamic mixture and their separation is usually difficult, our research represents one good example in which pure pseudo[n]rotaxanes (n = 2, 3, 4, 5) can be obtained by a simple "threading-followed-by-precipitation" method.

Adjustable Receptor Based on a [3]Rotaxane Whose Two Threaded Rings Are Rigidly Attached to Two Porphyrinic Plates: Synthesis and Complexation Studies

The design and synthesis of a new type of receptor based on a [3]rotaxane, consisting of one thread and two threaded rings, is reported, as well as some of its complexing properties toward given guests. Two rings rigidly attached to porphyrins are threaded by a stiff rod incorporating two 2,2'-bipyridine-like chelates, the threading process being driven by two Cu(I) atoms acting as templates. A double-stoppering reaction based on click chemistry leads to the copper-complexed [3]rotaxane in which the rings are located close to the central part of the thread and the distance between the two porphyrin plates is short. Removal of the two Cu(I) cations releases the two rings which are now free to move along and around the thread. In these two states of the [3]rotaxane, free and complexed with copper, the two zinc(II) porphyrins attached to the rings can bind different ditopic guests bearing pyridyl groups or amines as terminal functions. UV-visible and NMR DOSY experiments were realized with guests of different sizes, and the association constants were determined. The free [3]rotaxane is both a strong and highly adaptable receptor with high stability constants for the host/guest complexes, log K being in the range of 6.3-7.5 for guests with a length varying between 2.8 and 18 A. The copper-complexed [3]rotaxane is still a good receptor for small guests due to an entropic gain for this preorganized molecule compared to the free [3]rotaxane, but it is a less strong receptor for guests which do not fit the short distance between the two porphyrins.

Towards Controlling the Threading Direction of a Calix[6]arene Wheel by Using Nonsymmetric Axles

Traffic control: By exploiting the interplay of kinetic and thermodynamic effects, the direction of threading/dethreading in a nonsymmetric calixarene wheel can be selected by an appropriate choice of the head group incorporated in the molecular axle (see figure).The possibility of obtaining full control on the direction of axle threading in calix[6]arene wheel 1 either from its upper or lower rim was evaluated in solution. To this aim, we prepared nonsymmetric axles characterised by a 4,4'-bipyridinium recognition unit with two alkyl side chains, one of which terminates with a stopper, and the other with either ammonium (2), hydroxy (3) or methyl (4 and 5) head groups. When the axles were mixed with 1 in apolar solvents at room temperature, the formation of oriented pseudorotaxanes derived from the threading of the axles from the upper rim was observed. The stability constants of such complexes are in the order of 10(7) m(-1) and are almost independent of the type of axle. A detailed thermodynamic and kinetic study revealed that stability constants and activation parameters for complex formation between 1 and axles 2 and 3 are of the same order of magnitude, suggesting a common threading process. However, upon heating a solution of 1 and 2 in benzene at 340 K, the formation of another supramolecular complex was observed, the structure of which is consistent with an oriented pseudorotaxane derived from the threading of axle 2 from the lower rim of the calixarene wheel. By carrying out the threading-stoppering reaction sequence between 1 and 2 in the presence of an excess of diphenylacetyl chloride, the orientational rotaxane isomers R1 and R2, derived from lower- and upper-rim threading, respectively, were collected in about a ratio of 7:3 as the unique chromatographic fraction. Our results suggest that at room temperature the threading process is under kinetic control for all axles. On increasing the temperature only the threading behaviour of axle 2 is substantially modified, most likely because the process becomes thermodynamically controlled owing to the peculiar recognition properties of the ammonium head of this axle.

Anion templated assembly of an indolocarbazole containing pseudorotaxane on beads and silica nanoparticles

The surface covalent attachment of fluorescent axles of indolocarbazole enables anion templation to be exploited in the formation of pseudorotaxane assembliesvia the threading of neutral isophthalamide macrocycles. In utilising the surface of polystyrene beads this threading process can be followed by both magnetic resonance methods and changes in the axle fluorescent emission spectrum. The analogous surface assembly and anion templated threading can be achieved, with fluoride and sulfate, on silica nanoparticles where anion recognition and macrocycle threading are associated with equivalent and specific optical change.

Molecular Puzzle Ring: pseudo[1]Rotaxane from a Flexible Cyclodextrin Derivative

A pseudo[1]rotaxane formed by a flexible cyclodextrin (CD) derivative (1-R) with a bulky end group has been investigated on kinetic quantitation. 1-Rs have the cinnamamide moiety as a guest and a bulky end group (R) as a rate-determining moiety of the threading process. The R groups play an important role for the formation of pseudo[1]rotaxane, and kinetics of the self-inclusion process was found to be controlled by the size and shapes of the R groups. 1-Ad and 1-Me derivatives, which have an adamantyl and methyl end group, respectively, formed self-inclusion complexes by threading of the arm moiety with a conformational conversion of altrose from (1)C(4) form to (4)C(1) form. Flexibility of the altro-alpha-CD cavity resulted in an induced fit (from (1)C(4) to (4)C(1)) to the arm moiety, and introducing a bulky end group allowed the stability of this pseudo[1]rotaxane to be enhanced.

DNA Polymorphism as an Origin of Adenine-Thymine Tract Length-Dependent Threading Intercalation Rate

Binuclear ruthenium complexes that bind DNA by threading intercalation have recently been found to exhibit an exceptional kinetic selectivity for long polymeric adenine-thymine (AT) DNA. A series of oligonucleotide hairpin duplexes containing a central tract of 6-44 alternating AT base pairs have here been used to investigate the nature of the recognition mechanism. We find that, above a threshold AT tract length corresponding to one helix turn of B-DNA, a dramatic increase in threading intercalation rate occurs. In contrast, such length dependence is not observed for rates of unthreading. Intercalation by any mechanism that depends on the open end of the hairpin was found not to be important in the series of oligonucleotides used, as verified by including in the study a hairpin duplex cyclized by a copper-catalyzed "click" reaction. Our observations are interpreted in terms of a conformational pre-equilibrium, determined by the length of the AT tract. We finally find that mismatches or loops in the oligonucleotide facilitate the threading process, of interest for the development of mismatch-recognizing probes.

Iron(II)-Templated Synthesis of [3]Rotaxanes by Passing Two Threads through the Same Ring

The three-dimensional template effect of an octahedral iron(II) center has been exploited for threading two coordinating molecular units, each one is a bidentate 3,3‘-biisoquinoline ligand bearing p-anisyl groups, through a coordinating 41-membered macrocycle containing the same endocyclic but nonsterically hindering chelate. The double threading process is driven by coordination of three bidentate ligands to the iron(II) center. By replacing the threads decorated with p-anisyl substituents by analogous long-chain ligands bearing appropriate end-functions (azide), the double threading process is as efficient as with the shorter (p-anisyl) chelates. An efficient quadruple end-functionalization reaction based on the copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition (“click” chemistry) affords the desired iron(II)-complexed two-string [3]rotaxane in excellent yield.

A novel direct fibre generation technique for preparing functionalized and compound scaffolds and membranes for applications within the life sciences

Fibres, scaffolds and membranes have shown in the past decade their tremendous applicability to the life sciences. Essentially, these constructs have recently spearheaded focused applications in and within approaches to assist in the development of biologically active tissues to effective mechanisms for targeted and controlled cellular/drug delivery. There are several routes for forming continuous fibres from which functionalized scaffolds to membranes are prepared. One such technique that has demonstrated its wide versatility is none other than electrospinning. This is an electrified threading process capable of generating micro- and nano-sized (<50 nm) threads, which have been explored with a wide range of material compositions in their many manifestations. Although this threading process is economical and flexible, the process has its downsides, namely the associated high voltage to the preclusion of threading highly conducting viscoelastic media. In the current work we unveil a three-needle pressure-assisted spinning (PAS) approach comparable to coaxial- or co-electrospinning, without the hazardous high voltage and possessing the ability to thread highly conducting viscoelastic media from which pre-designed to functionalized compound structural units could be generated. Previously in our hands we demonstrated this technique with a two-needle system, which was only able to process a single- or multi-phase suspension at any given time. In its present form, two single/multi-phase miscible or immiscible media could be processed simultaneously. Therefore, PAS would be most useful to the biomedical world because a majority of biological media containing biological matter such as living cells have within them unprecedented concentrations of ions, which are needed by the living organisms for maintaining the cells/organisms' intricate metabolisms. In our hands, pressure-assisted spinning will compete directly with electrospinning and have a revolutionary effect on the fibre, scaffold and membrane preparation arena as applied to the plethora of applications within the life sciences.

PRESSURE-ASSISTED SPINNING: A UNIQUE AND VERSATILE APPROACH FOR DIRECTLY FABRICATING MEMBRANES WITH MICRO- AND NANOFIBERS

Fibers are structurally interesting components most useful in a range of applications spanning the physical and life science areas of research. These membrane (scaffold) forming fibers have been explored in applications ranging from microfilteration to advanced biological investigations in tissue engineering to controlled and targeted drug delivery. One such robust fiber generation approach investigated for over a century, which has recently been exploited, is the well-established threading process referred to as electrospinning. In this technique, single- or multi-phase media are charged within a conducting needle and later exposed to an electric field which promotes the formation of a continuous micro- to nanosized fiber(s) which over a period of collection time has been reported for forming scaffolds and membranes. This process has been explored for a wide range of polymer composite-based materials and the technique has now reached the point where it has moved into industrial production. We report here as a first example a comparable fiber to membrane fabrication approach completely driven by the coupling of a coaxial needle system with a pressure. We refer to this novel methodology as pressure-assisted spinning (PAS) where the hazardous element of high voltage (as in the case of electrospinning) is nonexistent. Hence, our discovery introduces both a directly competing fiber, scaffold to membrane fabrication approach, which is versatile and has no associated hazards as those in electrospinning. Furthermore as our technique is nonelectric field driven, the media spun into fibers could have a high electrical conductivity, which in this case has no effect on the stability in processing near-uniform fibers/scaffolds to membranes. The fabricated fibers and membranes generated by means of this approach could directly be used for a plethora of applications spanning the engineering and biological areas of research.

Processive Rotaxane Systems. Studies on the Mechanism and Control of the Threading Process

The threading behavior of a zinc analogue of a previously reported processive manganese porphyrin catalyst onto a series of polymers of different lengths is reported. It is demonstrated that the speed of the threading process is determined by the opening of the cavity of the toroidal porphyrin host, which can be tuned with the help of axial ligands that coordinate to the metal center in the porphyrin.

Kinetics of Threading α-Cyclodextrin onto Cationic and Zwitterionic Poly(bola-amphiphiles)

The amphiphilic polymers, poly(iminoundecamethylene), poly((N-methylimino)undecamethylene), poly((N,N-dimethylammonio)undecamethylene), and poly((N-methylimino)undecamethylene-N-oxide) were synthesized from nylon-11 by BH3 reduction of amide groups to amino groups and subsequent methylations. The poly(N-oxide) was obtained by H2O2 oxidation of the polymeric tertiary amine. Since the polymers and their inclusion compounds were water-soluble, threading kinetics of α-cyclodextrin rings onto these polymers could be investigated by 1H NMR. Kinetics were fitted by an empirical root exponential association function Y = Y∞(1 − exp(−sqrt(5.3t/t90))). The time t90, necessary to reach 90% completion of the threading process, was taken as a measure of the steric hindrance exerted by the hydrophilic groups along the polymer chain. The vaIues of t90 decreased by more than 3 orders of magnitude as the diameters of the hydrophilic groups decreased from 5.5 to 4.3 Å.

06/02112 Production of carbon fibers from Castilla's crude-oil deasphalted bottoms: Leal, G. and Peñaloza, J.-I. Ciencia Tecnologia y Futuro, 2002, 2, (3), 41–48.

Obtaining raw materials for the manufacturing of carbon fibers from Castilla's crude-oil deasphalted bottoms was achieved through thermal treatments carried out in inert atmosphere, at temperatures ranging from 573 K to 673 K. The deasphalted bottoms displayed isotropic characteristics. Deasphalted bottoms and thermally-treated fractions were characterized using elemental and thermo-gravimetric analyses; the softening point was also defined. The thermal treatment increases the C/H ratio, which is reflected in the softening temperatures. Thermally-treated fractions showed excellent behavior in the threading process and the obtained fibers were stabilized through air-driven oxidation. Once the fibers were stabilized, they were carbonized in inert atmosphere, at 1073 K and 1273 K. Results showed that when this raw material is processed, it has a performance rating of over 65%, and produces compact, infusible structures that may be classified as general-use carbon fibers.

Towards the stereoselective synthesis of inherently chiral pseudorotaxanes

Herein is reported an investigation towards the stereoselective synthesis of inherently chiral pseudorotaxanes. Chiral ammonium threads were readily prepared in five steps from racemic or enantiopure (M or P) salts of di-n-propyl-1,13-dimethoxyquinacridinium cation. Their self-assembly with DB24C8 or disymmetrically oriented DB24C8F6 rings formed pseudorotaxanes as shown by 1H and 19F NMR spectroscopy as well as MS measurements. A determination of the association constants (Ka) was afforded. The crucial role played by the ammonium counter-ion in the threading process was further demonstrated as salts of TRISPHAT (tris(tetrachlorobenzenediolato)phosphate(V)) anion were quite more effective than their PF6- analogues (x 7.3). A general lack of diastereoselectivity (de <or= 8%) was unfortunately observed.

Monitoring the DNA Binding Kinetics of a Binuclear Ruthenium Complex by Energy Transfer: Evidence for Slow Shuffling

The semirigid binuclear ruthenium complex Delta,Delta-[mu-(11,11'-bidppz)(phen)(4)Ru(2)](4+) has been shown to rearrange slowly from an initial groove-bound nonluminescent state to a final intercalated emissive state by threading one of its bulky Ru(phen)(2) moieties through the DNA base stack. When this complex binds to poly[d(A-T)(2)], a further increase in emission from the complex is observed after completion of the intercalation, assigned to reorganization of the intercalated complex. We here report a study of the threading process in poly[d(A-T)(2)], in which the minor groove binding dye DAPI is used as an energy transfer probe molecule to assess the distribution of ruthenium complex during and also after the actual threading phase. The emission from DAPI is found to change with the same rate as the emission from the ruthenium complex, and furthermore, DAPI does not disturb the binding kinetics of the latter, justifying it as a good probe of both the threading and the reorganization processes. We conclude from the change in the emission from both DAPI and the ruthenium complex with time that DAPI-ruthenium interactions are most pronounced during the process of threading of the complex, suggesting that the complexes are initially threaded slightly anticooperatively and thereafter redistribute along the DNA to reach their thermodynamically most favorable distribution. The final distribution is characterized by a small but significant binding cooperativity, probably as a result of hydrophobic interactions between the complex ions despite their tetravalent positive charges. The mechanism of "shuffling" the complex along the DNA chain is discussed, i.e., whether the ruthenium complex remains threaded (requiring sequential base-pair openings) or if unthreading followed by lateral diffusion within the ionic atmosphere of the DNA and rethreading occurs.

Investigation of several PVD coatings for blind hole tapping in austenitic stainless steel

During the last 10 years, the usage of stainless steel materials increased continuously in various industrial applications. However the machineability of these materials is difficult, especially austenitic stainless steels. Blind hole tapping in such materials is problematic due to difficult chip formation, chip transport, the tendency of cold-welding and build-up edges on the tool. HSS-tools are commonly used for tapping. The surface is either untreated, annealed in steam or coated with PVD. In most cases TiN, TiCN or TiAlN coatings are used. In this study, blind hole tests were accomplished with HSS M8-taps in 1.4571 austenitic stainless steel. The tools were coated with hard coatings such as CrN, CrC, TiN, TiAlN, CrAlN; lubricant coatings such as WC/C and DLC, or double layers such as TiCN + WC/C. To investigate the behavior of the different coatings in this cutting process, the cutting torque maxima, the torque slope and different quality criteria for the finished thread were analyzed. The used tools were analyzed by SEM regarding tool wear and cold welding. In addition mechanical properties such as hardness, abrasive wear resistance and friction coefficient were measured. To conclude which coating is suited for this application the combined analysis of the machining data, the SEM-investigations and the mechanical properties were conducted. Coatings with low friction coefficient have the lowest fluctuation of the cutting torque and are the only one having the ability to reduce the reverse torque by a half. Overcoating e.g. TiCN with DLC and WC/C this property will be preserved. In this case the excellent abrasive and adhesive wear resistance of TiCN was combined with excellent friction behavior and abrasive wear resistance of DLC. Out of the accomplished tests no precise conclusion about the lifetime can be made, but to obtain a stable blind hole threading process in austenitic stainless steel a coating system which combines excellent adhesive and abrasive wear resistance as well as excellent friction properties is required.

Viologen-calix[6]arene pseudorotaxanes. Ion-pair recognition and threading/dethreading molecular motions.

A calix[6]arene wheel, whose cavity has been extended and rigidified by N-phenylureido groups on the upper rim, forms pseudorotaxane species with molecular axles containing the viologen (4,4'-bipyridinium) unit in CH(2)Cl(2) solution. In these conditions, the self-assembly process is very efficient, with associated DeltaG degrees values of around -8 kcal mol(-1). The counteranions of the bipyridinium-based threads play indeed an important role in the formation of the complex. The use of either tosylate or hexafluorophosphate salts of the guests affects both the stability of the complexes and the rate of the threading process. Such effects have been interpreted in terms of ion-pair recognition, suggesting that coordination of the counteranions of the viologen thread by the ureido groups of the calixarene wheel is crucial for the breaking of tight ion pairs prior to threading. The rate constants of the threading/dethreading reactions coupled with the redox processes of the viologen unit of the axle have been obtained by means of cyclic voltammetry. The pseudorotaxane species undergo fast dethreading (submicrosecond time scale) on electrochemical reduction of the guest. The heterogeneous electron-transfer kinetics for the reduction of the viologen unit is slowed upon encapsulation into the calixarene cavity.

Bis(polypseudorotaxane)s Formed by Multiple Metallo-Bridged β-Cyclodextrins and the Thermodynamic Origin of Their Molecular Aggregation

Bis(polypseudorotaxane)s possessing the bridged bis(β-cyclodextrin)s with multicoordinated metal centers are prepared, in which β-cyclodextrin (β-CD) acts as the host structure, amino-terminated polypropylene glycol (PPG; MW = 2000) as the guest, and nickel(II) complexes as templating moieties. The assembly behavior is comprehensively studied by NMR spectroscopy, gel permeation chromatography (GPC), end group assay, static light scattering, thermogravimetric (TG), differential thermal analysis (DTA), and scanning tunneling microscopy (STM). The obtained results indicate that not only could the two PPG molecules be respectively threaded into metallo-bridged bis(β-CD)s forming the double-strand structure, but they also could play a stabilizing role in the molecular aggregation. The fluorescence titrations are performed in a mixture of acetonitrile/water to calculate the effective binding constants and thermodynamic parameters for the threading process of 2 onto PPG. The results reveal that the assembly proc...