Kinetics of the Self-Assembly of α-Cyclodextrin [2]Pseudorotaxanes with 1,12-Bis(4-(α-alkyl-α-methylmethanol)pyridinium)dodecane Dications in Aqueous Solution

Pseudorotaxane formation affected by stereo-electronic effects. A theoretical and experimental study

We report a theoretical and experimental study on different complexes of pseudorotaxanes possessing pyridine axles. In order to evaluate the stereo-electronic effects of the methyl substituents in the pyridine ring, complexes with different substitution patterns were synthesized. In this way, it was possible to analyze the different behaviors of these complexes according to the positions of their methyl substituents. Combined techniques of molecular dynamics and quantum mechanical calculations with the help of molecular electrostatic potentials for a simpler visualization of the electronic effects were employed. We have sought experimental support of NMR spectroscopy analysis to corroborate the conclusions obtained from the molecular simulations. Our results not only clearly demonstrate that both electronic and steric effects play key roles in the feasibility of the formation of such complexes, but also the simulations reported here might predict the degree of difficulty of their formation. The combination of computational techniques employed here seems to be an excellent approach to be able to predict whether or not a complex can be formed and with what degree of difficulty. In addition, our experimental and theoretical results have allowed us to visualize the formation of external complexes in the rotaxanes reported here. In this case, the use of bolaforms with trimethylammonium groups at both ends was very useful to evaluate in detail the formation of the so-called external complexes in these systems.

Complexation Kinetics of Cyclodextrins with Bile Salt Anions: Energy Barriers for the Threading of Ionic Groups

Binding constants for thousands of cyclodextrin complexes have been reported in the literature, but much less is known about the kinetics of these host-guest complexes. In the present study, inclusion complexes of bile salts with β-cyclodextrin, γ-cyclodextrin, and a methylated β-cyclodextrin were studied by nuclear magnetic resonance (NMR) lineshape analysis to explore the structural factors that govern the complexation kinetics. For complexes with β-cyclodextrin, the association rate constants ranged from 2 × 10⁶ to 2 × 10⁷ M^-1 s^-1 while the dissociation rate constants ranged from 12 to 6000 s^-1 at 25 °C. The kinetics were thus significantly slower than for any other β-cyclodextrin complex reported in the literature, due to the large energy barrier for threading the ionic sidechains of the bile salt anions. Bile salts with taurine and glycine sidechains had identical binding affinities, but the kinetics differed by a factor of 10. Introduction of a single hydroxyl group at the binding site of the bile salts reduced the lifetimes and binding constants of the complexes by more than 50 times. The strong temperature dependence of the rate constants revealed that the large activation energies were mainly enthalpic with a small contribution from entropy. The larger γ-cyclodextrin was threaded by the nonionic end of the bile salts, and the kinetics were too fast to be accurately determined. The study demonstrates that ionic groups on guest molecules constitute significant energy barriers for the threading and dethreading of β-cyclodextrin hosts.

Association of liquid-assisted grinding with aging accelerates the inherently slow slipping-on of a dibenzo-24-crown-8 over the N-hydroxysuccinimide ester of an ammonium-containing thread

Very efficient slipping-on of the dibenzo-24-crown-8 over the NHS end of an ammonium-containing molecular axle was carried out through a solvent-less procedure.

The relationship between the conformational degree of freedom of template-containing threads and slippage in the formation of [2]rotaxane building blocks

Slippage is correlated with the degree of freedom of the molecular thread to be encircled. The straighter the thread, the faster is the slippage.

Kinetic Study of [2]Pseudorotaxane Formation with an Asymmetrical Thread

Kinetic and thermodynamic studies on cyclodextrin (CD)-based [2]pseudorotaxane formation have been carried out by a combination of NMR and calorimetric techniques using bolaform surfactants as axles. Experimental evidence of the formation of an external complex between the trimethylammonium head groups of the axle and the external hydrogen atoms of α-cyclodextrin (α-CD) is reported. Inclusion of this external complex in the reaction pathway allows us to explain the kinetic behavior as well as the nonlinear dependence of the observed rate constant on CD concentrations. The equilibrium constant for [2]pseudorotaxane formation is strongly affected by the spacer length of the axle. This effect is a consequence of increasing rotaxane stability because the threading rate constant is almost independent of the spacer length, but dethreading strongly decreases on increasing the axle size. Using a nonsymmetrical axle with tripropyl and trimethylammonium cations precludes CD threading by the large head side. CDs will thread this asymmetrical bolaform by both their wide and narrow sides, yielding two isomeric [2]pseudorotaxanes. Threading by the wide side of the CD is 60% more favorable than that by the narrow one, but dethreading rate constants are the same for both isomers.

Facile and efficient formation and dissociation of a pseudo[2]rotaxane by a slippage approach using pillar[5]arene-based cyclic host liquid and solvent

A pillar[5]arene-based [2]rotaxane was synthesized by heating an axle with valine ends in a cyclic host liquid.

Zn(II)-cyclam based chromogenic sensors for recognition of ATP in aqueous solution under physiological conditions and their application as viable staining agents for microorganism

Two chromogenic complexes, L.Zn (where L is (E)-4-((4-(1,4,8,11-tetraazacyclotetradecan-1-ylsulfonyl)phenyl)diazenyl)-N,N-dimethylaniline) and its [2]pseudorotaxane form (α-CD.L.Zn), were found to bind preferentially to adenosine triphosphate (ATP), among all other common anions and biologically important phosphate (AMP, ADP, pyrophosphate, and phosphate) ions in aqueous HEPES buffer medium of pH 7.2. Studies with live cell cultures of prokaryotic microbes revealed that binding of these two reagents to intercellular ATP, produced in situ, could be used in delineating the gram-positive and the gram-negative bacteria. More importantly, these dyes were found to be nontoxic to living microbes (eukaryotes and prokaryotes) and could be used for studying the cell growth dynamics. Binding to these two viable staining agents to intercellular ATP was also confirmed by spectroscopic studies on cell growth in the presence of different respiratory inhibitors that influence the intercellular ATP generation.

Face-Selective [2]- and [3]Rotaxanes: Kinetic Control of the Threading Direction of Cyclodextrins

New [2]- and [3]pseudorotaxanes containing alpha-cyclodextrin (alpha-CDs) molecules as rotors and alkyl pyridinium derivatives as axles were prepared by a slipping process. The inclusion behavior of these rotaxanes was investigated by using one- and two-dimensional NMR spectroscopy. The methyl group at the 2-position of the pyridinium moiety at the end of each axle molecule was found to control the rates of threading of the alpha-CD onto the axle molecules. alpha-CD can approach axle molecules from a particular direction to form inclusion complexes. Axle molecules that contain a 2-methylpyridinium moiety at one end and a bulky stopper at the other end can regulate the direction of approach to give a [2]pseudorotaxane such as 2 b-alpha-CD. A [3]pseudorotaxane in which two alpha-CD molecules are arranged facing in the same direction at two stations of the tetracationic axle molecule was also obtained. These face-selective behaviors are dominated by kinetic processes rather than thermodynamic processes.

Kinetics and Mechanism of Cyclodextrin Inclusion Complexation Incorporating Bidirectional Inclusion and Formation of Orientational Isomers

The kinetics of cyclodextrin (CD) inclusion complexation has been usually analyzed in terms of a one-step reaction or a consecutive two-step reaction involving intracomplex structural transformation as a second step. These schemes presume the inclusion of guest molecules through only one side of the CD cavity and the formation of unidirectional CD complexes. However, there has been increasing experimental evidence for the inclusion of guests through both sides of the CD cavity and the formation of orientational isomers for noncentrosymmetric guest molecules. This article presents a novel parallel reaction scheme for CD inclusion complexation, incorporating bidirectional inclusion and the formation of orientational isomers into the scheme. It is shown that the parallel reaction scheme gives the same concentration versus reaction time relationship as the consecutive two-step reaction scheme. The experimental methods for determining the microscopic directional rate constants are presented. The kinetic parameters of the two-step reaction scheme are expressed as functions of the directional rate constants. The ratios of orientational isomers of alpha-CD-based [2]-pseudorotaxanes and the microscopic directional rate constants of the threading and dethreading reactions are estimated from the reported thermodynamic and kinetics data obtained by using either the one-step or two-step reaction scheme. It is shown that the thermodynamic preference of an isomer over the other is mainly due to the slow dethreading rate of the isomer.

α- and β-Cyclodextrin [2]rotaxanes with (diethylenetriamine)platinum(II) stoppers

A series of dinuclear platinum(II) complexes, [(dien)Pt(NH2(CH2)nNH2)Pt(dien)]Cl4 (dien = diethylenetriamine, n = 8, 9, 10, and 12) and their corresponding [2]rotaxanes with α-cyclodextrin (α-CD), [(dien)Pt{NH2(CH2)nNH2·α-CD}Pt(dien)]Cl4, have been synthesized and characterized by 1H, 13C, and 195Pt NMR spectroscopy and electrospray mass spectrometry. The rotaxanes were prepared by reacting the {NH2(CH2)nNH2·α-CD} pseudorotaxanes with [Pt(dien)]Cl, to stopper the included linear α,ω-diaminoalkane chains with the inert Pt(II) end groups. The kinetics of the self-assembly and dissociation of the β-CD rotaxane, [(dien)Pt{NH2(CH2)10NH2·β-CD}Pt(dien)]4+, were investigated by using 1H NMR and are indicative of a slippage mechanism, owing to the comparable sizes of the β-CD cavity and the [Pt(dien)]+ end group. A relatively weak inclusion of the end group in the β-CD cavity precedes a thermally promoted passage of the β-CD over the [Pt(dien)]+ end group onto the hydrophobic polymethylene chain of the bridging ligand of the thread. Key words: rotaxanes, pseudorotaxanes, cyclodextrin, platinum complexes, slippage mechanism.

Orientational isomers of α-cyclodextrin [2]semi-rotaxanes with asymmetric dicationic threads

Two series of novel dicationic threading molecules [Quin(CH2)10R]2+ and [3,5-Lut(CH2)10R]2+, where Quin+ = quinuclidinium, 3,5-Lut+ = 3,5-lutidinium, and R+ = N(CH3)3+ and N(CH3)2CH2CH3+, form [2]semi-rotaxanes with [small alpha]-cyclodextrin (alpha-CD) in aqueous solution. The quinuclidinium and 3,5-lutidinium are sufficiently bulky to prevent threading while the R+ groups allow for slow threading by alpha-CD at 25 degrees C. The resulting [2]semi-rotaxanes exist in two orientational isomers owing to the asymmetry of both the alpha-CD cavity and the threading molecules. Two-dimensional 1H NMR spectroscopy and kinetics experiments reveal that the isomer in which the narrower rim (primary OHs) is positioned near the R+ group is the kinetically preferred isomer, while the other isomer is the thermodynamically preferred product. The kinetics and mechanism of the formation, dissociation, and interconversion of the two isomers have been determined at 25 degrees C.

alpha- and beta-cyclodextrin rotaxanes of mu-bis(4-pyridyl)bis[pentacyanoferrate(II)] complexes

The kinetics and mechanism of the self-assemblies of alpha- and beta-cyclodextrin (CD) [2]rotaxanes, [(NC)5Fe-[pyRpy.CD]Fe(CN)5]6-, containing pentacyanoferrate(II)-stoppered 4,4'-bis(pyridyl) threads pyRpy (R = -CH = CH-, -N=N-, -CH=N-N=CH-, and -C(CH3)=N-N=C(CH3)-) have been investigated in aqueous solution by using visible and 1H NMR spectroscopy. The rotaxanes may be formed rapidly by the addition of the [Fe(CN)5OH2]3- ion to the CD-included pyRpy thread or slowly by the addition of an excess of CD to the dimeric [(NC)5Fe(pyRpy)Fe(CN)5]6- complex. In the latter method, the mechanism involves a rate-determining dissociation of a [Fe(CN)5]3- center to form the monomeric complex, which subsequently includes the coordinated pyRpy in the CD cavity to yield the semirotaxane, which is rapidly recomplexed by the [Fe(CN)5OH2]3- ion, generating the [2]rotaxane. Rate and activation parameters and CD inclusion stability constants have been determined for the ligand substitution reactions involving the formations and dissociations of the semirotaxanes and rotaxanes. The extents of the decreases in the formation (kf) and dissociation (kd) rate constants upon CD inclusions of the free and coordinated ligands, respectively, are related to the natures of the CD hosts and the R linkage on the pyRpy guests. The semirotaxanes and rotaxanes exhibit significant bathochromic shifts in their visible MLCT transitions compared with the corresponding monomeric and dimeric iron complexes. A correlation between the extent of the decrease in kd and the change in the MLCT energy upon alpha-CD inclusions of [Fe(CN)5L]3-, where L is an aromatic N-heterocyclic 4-Rpy or pyRpy ligand, has been observed.