Electrochemical determination of electroinactive guests of β-cyclodextrin at a self-assembled monolayer interface

A novel determination method of electroinactive molecules by means of electrochemical technique is presented. A new self-assembled monolayer containing cyclodextrin (CD) is prepared with mono(6-o-p-tolylsulfonyl)-β-cyclodextrin. Although this derivatization process leads to a β-CD coverage of 10% of a full monolayer, this layer shows an effective host-guest response to ferrocene. The interfacial ferrocene complexation gives a response similar to that expected for a Langmuir adsorption isotherm yielding a stability constant of 4.2 ×104 mol-1· L and a maximum ferrocene coverage of 8.6×10-12 mol/cm2. The redox peak currents of the surface-confined ferrocene decrease upon addition of competing β-CD guest species to the solution, such as m-toluic acid (mTA) and sodium dodecyl sulfonate (SDS). This principle has been used for the determination of the electroinactive molecules, mTA and SDS in the concentration ranges of 0.8-2.7 μmol/L and 5-100 nmol/L, respectively.     

Durable chiral sensor based on quartz crystal microbalance using self-assembled monolayer of permethylated β-cyclodextrin

A novel chiral sensor based on quartz crystal microbalance (QCM) with a self-assembled monolayer of permethylated β-cyclodextrin as the enantioselective coating has been derived which exhibit improved enantioselectivity and excellent long-term environmental stability when used in gas phase sensing.     

Synthesis of a β-cyclodextrin derivative bearing an azobenzene group on the secondary face

An oxidative coupling Sonogashira-type reaction has been used to synthesize a β-cyclodextrin derivative bearing an azobenzene group on the secondary face for the first time starting from a β-cyclodextrin propargylated at one of its C-2 positions. The de-O-propargylation reaction and the formation of an oxidative homocoupling dimer were found to compete with the desired product under several Sonogashira-type reaction conditions. However, the use of a diluted reductive atmosphere of H₂ avoided the former and diminished the latter.     

Molecular recognition on supramolecular systems (XXXV)——Synthesis of novel β-cyclodextrin derivative bearing pyridinio group and its chiral discrimination of amino acids

A novel p-cyclodextrin derivative 4 bearing a pyridinio group on the primary side was synthesized by the reaction of 2-aminopyridine with 6-p-cyclodextrin monoaldehyde 3, and its complexation stability constants with several aliphatic amino acids have been determined in phosphate buffer solution ( pH = 7.2, 0.1 mol ?L~(-1)) at 25℃by using spectrofluorometric titrations. The stoichiometry is 1 : 1 for the inclusion complexation of amino acids with compound 4. Circular dichroism study indicates that the aromatic moiety was embedded shallowly into the cyclodextrin cavity. As a spectral probe, the pyridinio group in the modified cyclodextrin can recognize not only differences of the size and shape of amino acid molecules, but also the L/D-amino acid chiral iso-mer. As compared with mono-[6-(1-pyridinio)-6-deoxy]-p-cyclodextrin 5, compound 4 switched the enantiomer preference for L- to D-isomer, and showed the highest enantioselectivity of 5.4 for D/L-serine. These results are discussed from the viewpoints of ge     

Aversatile β-cyclodextrin functionalized silver nanoparticle monolayer for capture of methyl orange from complex wastewater

The toxic organic dye contaminants in wastewater are extremely harmful to the ecosystem.Surface enhanced Raman scattering(SERS) is a technique with high sensitivity and chemical specificity which fulfills the requirements for monitoring dye contaminants in wastewater.However,as one of the common dye contaminants,methyl orange(MO) has very weak affinity to metallic surfaces and is difficult to be detected by SERS at low concentrations.Therefore,a new type of SERS substrate with Ag nanoparticle monolayer functionalized by mono-6-deoxy-6-thio-β-cyclodextrin(β-CD-SH) was prepared to efficiently capture and detect MO in wastewater with a limit of detection of 5×10^-7 mol/L.The hydrophobic cavity of β-CD is responsible for the efficient trap and enrichment of MO on the Ag NPs surface,achieving a strong SERS signal of MO at low concentrations and at different pH values.This study provides new insight into designing a well-performed adsorbent for the capture and detection of organic contaminants.     

Molecular recognition of naphthalenediamine by polyamine modified β-cyclodextrin:yttrium metal complexes

The 6-OH group of β-cyclodextrin was modified by diethylene triamine and triethylene tetramine, respectively, mono[6-diethylenetriamino]-6-deoxy-β-cyclodextrin (DTCD) and mono[6-triethylenetetraamino]-6-deoxy-β-cyclodextrin (TTCD) were synthesized, which included 1,5-naphthalenediamine and 1,8-naphthalenediamine, respectively, in the presence of rare earth metal yttrium chloride. As a result, four ternary inclusion complexes (host–guest-metal) formed, which were characterized via ¹HNMR spectroscopy. The chemical shift variations of host and guest molecules were studied. The stoichiometric proportion of host and guest molecules is 2:1 for all the complexes. Signal degeneration still exists for the guest molecules after the inclusion process, which verifies the symmetrical conformation of guest molecules inside the cavities of two host molecules. All the four complexes exhibit “sandwich”-typed structure.     

Solubilization of cholesterol in aqueous solution by two β-cyclodextrin dimers and a negatively charged β-cyclodextrin derivative

Two βCD dimers (linked by succinic acid, 2, or ethylenediaminetetraacetic acid, EDTA, 3, bridges) and a negatively charged monomer derivative of βCD, 1, have been synthesized and their ability to solubilize cholesterol in aqueous solution was studied. The three compounds exhibit a great capacity in solubilizing cholesterol as, for instance, concentrations up to 6 mM of cholesterol were measured in the presence of 25 mM of 3. The phase-solubility diagrams of the two dimers exhibit A _L type profiles while the monomer 1 follows an A _P isotherm. The cholesterol/dimer complexes have 1:1 stoicheiometries while monomer 1 forms two complexes with molar ratios of 1:1 and 1:2 (cholesterol/1). The equilibrium constants are K _1:1 = (5.9 ± 0.3) × 10⁴ M^?1 and K _1:1 = (8.8 ± 0.2) × 10⁴ M^?1 for 2 and 3, respectively, and K _1:1 = 73 ± 19 M^?1 and K _1:2 = 204 ± 65 M^?1 for 1. The comparison of K _1:1(3) with the product K _1:1 × K _1:2 (1) reveals that a chelate effect in binding the cholesterol by 3 exists. The structure of the cholesterol/3 complex was studied by ROESY experiments and by molecular dynamics simulations.     

Photophysics of self-assembled monolayers of a π-conjugated quinquethiophene derivative

The photophysics of fully and partially covered self-assembled monolayers (SAMs) of a quinquethiophene (5T) derivative have been investigated. The monolayers behave as H-aggregates. The fluorescence of fully covered SAMs is weak and red-shifted, and the extinction is blue-shifted as compared to that of single molecules. The fluorescence of partially covered SAMs is dominated by that of single molecules on the surface. The extinction spectra are similar for fully and partially covered monolayers, which show that even the smallest islands are H-aggregates. The extinction spectra furthermore closely resemble those for 5T single crystals, which demonstrates that in oligothiophene crystals the intermolecular interactions within one layer molecules are stronger than the interlayer electronic coupling.     

Molecular and quantum mechanical studies on the monomer recognition of a highly-regular β-helical antifreeze protein

The possible interaction models for an antifreeze protein from Tenebrio molitar (TmAFP) have been systematically studied using the methods of molecular mechanics, molecular dynamics and quantum chemistry. It is hoped that these approaches would provide insights into the nature of interaction between protein monomers through sampling a number of interaction possibilities and evaluating their interaction energies between two monomers in the course of recognition. The results derived from the molecular mechanics indicate that monomer's β-sheets would be involved in interaction area and the side chains on two β-faces can match each other at the two-dimensional level. The results from molecular mechanics and ONIOM methods show that the strongest interaction energy could be gained through the formation of H-bonds when the two β-sheets are involved in the interaction model. Furthermore, the calculation of DFT and analysis of van der Waals bond charge density confirm further that recognition between the two     

Molecular and quantum mechanical studies on the monomer recognition of a highly-regular β-helical antifreeze protein

The possible interaction models for an antifreeze protein from Tenebrio molitar (TmAFP) have been systematically studied using the methods of molecular mechanics, molecular dynamics and quantum chemistry. It is hoped that these approaches would provide insights into the nature of interaction between protein monomers through sampling a number of interaction possibilities and evaluating their interaction energies between two monomers in the course of recognition. The results derived from the molecular mechanics indicate that monomerś β-sheets would be involved in interaction area and the side chains on two p-faces can match each other at the two-dimensional level. The results from molecular mechanics and ONIOM methods show that the strongest interaction energy could be gained through the formation of H-bonds when the twoβ-sheets are involved in the interaction model. Furthermore, the calculation of DFT and analysis of van der Waals bond charge density confirm further that recognition between the two TCTs mainly depends on inter-molecular hydroxyls. Therefore, our results demonstrate that during the course of interaction the most favorable association of TmAFPs is via their β-sheets.     

Potentiometric discrimination of organic amines by liquid membrane electrodes based on a long alkyl chain derivative of β-cyclodextrin

A long alkyl chain derivative of -cyclodextrin (6^A,6^B,6^C,6^D,6^E,6^F,6^G-hepta-S-dodecyl-6^A, 6^B,6^C,6^D,6^E,6^F,6^G-heptathio--cyclodextrin tetradecaacetate) was examined as a new type of sensory element of an ion-selective electrode for organic amines. Poly(vinyl chloride) (PVC) matrix liquid membrane electrodes based on this host were prepared by using several membrane solvents, among which 2-fluoro-2-nitrodiphenyl ether (FNDPE) was found to be by far most effective for potentiometric responses by the cyclodextrin host. Guest-induced potential changes were examined at pH 5.0 for several groups of protonated organic amine guests having different types of nonpolar moieties. The magnitude of the guest-induced potential change was in the order of 2-phenylethylamine > 1-adamantanamine > benzylamine > cyclohexanamine > dopamine. Potentiometric discrimination was also observed for the positional isomers of di- and trimethoxybenzylamines, though the discrimination between the isomers of monomethoxybenzylamines was negligible. These potentiometric selectivities were quite different from those for a liquid membrane electrode containing no particular sensory element. No appreciable effect was observed by addition of potassium tetrakis(p-chlorophenyl)borate as a fixed anionic site. Such a characteristic feature of this cyclodextrin-based liquid membrane electrode was explained in terms of the availability of an inclusion complex between the cyclodextrin host and each guest in a geometry that would be stable at the membrane surface.Presented in part at the IUPAC 2nd International Symposium on Bioorganic Chemistry, Fukuoka, Japan, June 6–10, 1993. Abstracts, p. 239     

The application of β-cyclodextrin derivative functionalized aligned carbon nanotubes for electrochemically DNA sensing via host-guest recognition

We functionalized aligned carbon nanotubes (ACNTs) electrode with a new kind of β-cyclodextrin (β-CD) derivative through diazotization reaction. The resulting β-CD/ACNTs electrode was used to detect DNA hybridization in homogeneous solution based on host–guest recognition technology. In the sensing protocol, one special DNA probe was designed with a stem-loop structure and both ends modified, which we called dually labeled DNA probe (DLP). One end of the DLP was labeled with dabcyl as guest molecule for β-CD/ACNTs electrode capture, and the other end was labeled with a CdS nanoparticle as an electrochemical tag to indicate the occurrence of DNA hybridization. In the absence of the target DNA sequence, the DLP maintains its hairpin structure in solution phase and would not be captured and detected by the β-CD/ACNTs electrode. In the presence of the complementary target sequence, the conformational structure of the DLP was altered and a double-stranded DNA (dsDNA) molecule was formed by the hybridization of DLP and complementary DNA sequence. Consequently, the dsDNA was captured by the β-CD/ACNTs electrode owing to guest–host recognition between β-CD and dabcyl. The electrochemical signal from the CdS nanoparticle–dsDNA/β-CD/ACNTs was then measured. Under optimized detection conditions, the proposed method showed high sensitivity and selectivity with a detection limit of 5.0 × 10⁻¹³ M for complementary DNA sequence.     

Molecular recognition and biological application of modified β-cyclodextrins

The molecular recognition based on cyclodextrins(CDs) has become a focus of interest in modern supramolecular chemistry.CDs are known to encapsulate various ions and organic/inorganic molecules in their hydrophobic cavities and form stable inclusion complexes through cooperative noncovalent interactions. During the past few decades, a large variety of modified CDs have been elaborately designed and synthesized, which significantly promotes our molecular-level understanding of the structure–function relationship in many supramolecular systems. Through the accurate analysis on the molecular binding behaviors, one can create a library of CD-based nanoassemblies with controlled physicochemical properties. In this review, we will focus on the stability constant-directed molecular recognition and the biological activities of β-CDs toward some representative bioactive substrates, including metal ions, steroids, porphyrins, amino acids and oligopeptides, as well as drug molecules, with the final goal of promoting their practical applications in the biomedical field.     

Synthesis of a β-cyclodextrin derivate and its molecular recognition behavior on modified glassy carbon electrode by diazotization

A novel β-cyclodextrin (β-CD) derivative containing mono-phenylamino (MPA-β-CD) was newly synthesized by classical Mitsunobu reaction in good yield, and its structure has been confirmed by ¹H NMR, ¹³C NMR and electrospray ionization mass spectra. The compound MPA-β-CD was immobilized onto glassy carbon electrode (GCE) by diazotization, and with this modified electrode the binding behavior of MPA-β-CD for ferrocene (Fc) was investigated qualitatively, and the comparison of differential pulse voltammetry before and after immersion in ferrocene solution indicated that the MPA-β-CD immobilized GCE exhibited the molecular recognition behavior between β-CD and ferrocene.     

Antifungal activity of silver nanoparticle-encapsulated β-cyclodextrin against human opportunistic pathogens

Silver nanoparticles were synthesised by reducing silver acetate with a long-chain aliphatic amine. β-Cyclodextrin (CD)-stabilised silver nanoparticles were successfully synthesised and characterised by the UV–vis spectroscopy and scanning electron microscopy analysis. This system was examined for their antifungal activity against opportunistic human pathogens such as Aspergillus fumigatus, Mucor ramosissimus and Chrysosporium species. This study clearly demonstrates that the present system is a powerful antifungal agent against human opportunistic pathogenic fungi.     

Molecular dynamics simulations of low-ordered alzheimer β-amyloid oligomers from dimer to hexamer on self-assembled monolayers

Accumulation of small soluble oligomers of amyloid-β (Aβ) in the human brain is thought to play an important pathological role in Alzheimer's disease. The interaction of these Aβ oligomers with cell membrane and other artificial surfaces is important for the understanding of Aβ aggregation and toxicity mechanisms. Here, we present a series of exploratory molecular dynamics (MD) simulations to study the early adsorption and conformational change of Aβ oligomers from dimer to hexamer on three different self-assembled monolayers (SAMs) terminated with CH(3), OH, and COOH groups. Within the time scale of MD simulations, the conformation, orientation, and adsorption of Aβ oligomers on the SAMs is determined by complex interplay among the size of Aβ oligomers, the surface chemistry of the SAMs, and the structure and dynamics of interfacial waters. Energetic analysis of Aβ adsorption on the SAMs reveals that Aβ adsorption on the SAMs is a net outcome of different competitions between dominant hydrophobic Aβ-CH(3)-SAM interactions and weak CH(3)-SAM-water interactions, between dominant electrostatic Aβ-COOH-SAM interactions and strong COOH-SAM-water interactions, and between comparable hydrophobic and electrostatic Aβ-OH-SAM interactions and strong OH-SAM-water interactions. Atomic force microscopy images also confirm that all of three SAMs can induce the adsorption and polymerization of Aβ oligomers. Structural analysis of Aβ oligomers on the SAMs shows a dramatic increase in structural stability and β-sheet content from dimer to trimer, suggesting that Aβ trimer could act as seeds for Aβ polymerization on the SAMs. This work provides atomic-level understanding of Aβ peptides at interface.     

Polypyrrole on self-assembled monolayers of a pyrrolyl lipoic acid derivative—electrosynthesis and polymer film characterization

The electropolymerization of pyrrole on gold modified by a self-assembled monolayer (SAM) of a pyrrolyl lipoic acid derivative was investigated in detail and the results compared to those obtained on bare substrates. Both under potentiostatic and potentiodynamic control, a slight blocking action of the underlying SAM could be observed for the initial stages of polymer growth but thereafter the electrochemical features were similar to those collected for polypyrrole (PPy) deposition on bare gold. The morphology and structure of PPy films formed on the SAMs were characterized by atomic force microscopy and X-ray diffraction, which revealed that those polymer properties are much more influenced by the electrochemical mode of preparation, than by the underlying SAMs. While, when compared to PPy on bare gold, no effect has been detected on thin layers deposited at constant potential, surface areas with rather irregular morphology, as well as a small but beneficial influence in inducing order on the first few layers of the polymer film, have been observed on similar films formed by cyclic voltammetry. The typical globular morphology of PPy has always been observed for relatively thick layers in which the redox behavior, analyzed by in situ AFM, showed an increase in volume of the polymer nodules upon reduction, largely due to the SAM reorganization induced by the applied potential.     

Ultra-long-range electron transfer through a self-assembled monolayer on gold composed of 120-Å-long α-helices

Electron transfer through α-helices has attracted much attention from the viewpoints of their contributions to efficient long-range electron transfer occurring in biological systems and their utility as molecular-electronics elements. In this study, we synthesized a long 80mer helical peptide carrying a redox-active ferrocene unit at the terminal and immobilized the helical peptide on a gold surface. The molecular length is calculated to be 134 ?, in which the helix accounts for 120 ?. The preparation conditions of the self-assembled monolayers were intentionally changed to obtain monolayers with different physical states to study the correlation between molecular motions and electron transfer. Ellipsometry and infrared spectroscopy showed that the helical peptide forms a self-assembled monolayer with vertical orientation. Electrochemical measurements revealed that an electron is transferred from the ferrocene unit to gold through the monolayer composed of this long helical peptide, and the experimental data are well explained by theoretical results calculated under the assumption that electron transfer occurs by a unique hopping mechanism with the amide groups as hopping sites. Furthermore, we have observed a unique dependence of electron transfer on the monolayer packing, suggesting the importance of structural fluctuations of peptides on the electron transfer controlled by the hopping mechanism.     

Host-guest sensory system of sodium anthranilate-modifiedβ-cyclodextrin: Molecular recognition properties

Sodium anthranilate-modified-cyclodextrin (1) has been prepared as a sensor for detecting organic compounds including terpenoids and steroids.1 shows a pure monomer fluorescence whose intensity is increased or decreased upon addition of the guest species examined. In this system, the sodium anthranilate moiety acts either as a spacer, which enables the cyclodextrin to form a 11 guest complex by narrowing the-cyclodextrin cavity or acts as a hydrophobic cap.1 shows a higher sensitivity for terpenoids than for steroids, but has a higher selective molecular recognition ablity for steroids than for terpenoids.