Construction of positively-charged layered assemblies assisted by cyclodextrin complexation

A beta-cyclodextrin dimer is found to be effective in preparing a layer-by-layer architecture of positively charged ferrocene-appended poly(allylamine) presumably on the basis of strong beta-cyclodextrin-ferrocene host-guest interaction.     

Development and evaluation of cyclodextrin complexed hydroxyapatite nanoparticles for preferential albumin adsorption

Our study focuses on the incorporation of β-CD into the HA structure, its effects on the phase of HA and the biological responses to proteins and blood cells. Hydroxyapatite (HA) containing levels of β-cyclodextrin (β-CD) of upto 0.9 wt% has been produced by co-precipitation method. The complexes were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, thermal gravimetric analysis (TG) and differential scanning analysis (DSC) methods. The size of the complexes as analyzed using DLS (dynamic light scattering) was between 150 nm and 350 nm. The results show that an increase in concentration of β-CD in the prepared samples that leads to an increase in hydrophobicity seems to promote an affinity for albumin adsorption. The PAGE results were substantiated by Lowry measurements and the results reveal that the H2 (containing 0.7 wt% β-CD) sample shows around 40% increase in albumin adsorption when compared to the H1 (containing 0.5 wt% β-CD) sample. The preferential adsorption of albumin has not been demonstrated in vivo. The ability to design particles that can preferentially interact with particular protein can obtain desired targeting effects. So the results indicate that HA/β-CD complexes have immense potential in targeted delivery of drugs. The in vivo potential of the developed samples was further confirmed in vitro by the results of cell aggregation and haemolytic activity.     

Layered assemblies composed of sulfonated cyclodextrin and poly(allylamine)

Layered assemblies containing cyclodextrin (CD) have been prepared by layer-by-layer deposition of poly(allylamine) and sulfonated α-CD or β-CD on a solid surface. A quartz crystal microbalance study revealed that α-CD and β-CD form multilayers in the films upon each deposition. The sulfonated-α-CD-containing film bound methyl orange (MO) by forming an inclusion complex, resulting in the isolation of MO as a monomer in the film, whereas MO formed aggregates in the sulfonated-β-CD and CD-free films.     

Three-component complexes of cyclodextrins. Exciplex formation in cyclodextrin cavity

The fluorescence quenching of naphthalene, 1-methylnaphthalene, and acenaphthene by trimethylamine (TMA) was studied in aqueous -cyclodextrin (-CD) solutions to know the structural requirements for guest molecules to form three-component complexes. The apparent rates for the fluorescence quenching of the naphthalene derivatives by TMA markedly increased in the presence of -CD. The fluorescence quenching of 1-methylnaphthalene and acenaphthene by TMA was accelerated by -CD more efficently than that of naphthalene. These results suggest the structures of the three-component complexes as the arene-capped -CD including TMA in its cavity.     

Stepwise self-assembly of ordered supramolecular assemblies based on coordination chemistry

Ultrathin multilayers based on transition metal complexes have been prepared by successive deposition and self-assembly. Dendrimer layers were deposited onto SiO2 wafers by alternately immersing the substrate into a solution of terpyridyl (tpy)-pendant poly(amido amine) (PAMAM) dendrimers (dend-n-tpy; n = 8, 16) dissolved in CH2Cl2, followed by the interfacial coordination reaction with cobalt (Co2+) from aqueous solution. The films derived from this simple assembly method have been characterized by electrochemical methods, synchrotron-based X-ray reflectivity (XRR), and X-ray fluorescence (XRF) recorded under grazing incidence. XRR analysis revealed a linear thickness dependence with an increase of 9.9 +/- 0.5 angstroms and 10.8 +/- 0.4 angstroms per growth cycle for both dend-8-tpy and dend-16-tpy, respectively, indicative of a layer-by-layer (LbL) growth of single dendrimer/Co2+ layers. XRF and electrochemical results showed that the amount of Co2+ increases linearly as more layers are deposited, and that the Co2+ concentration (mol/L) in dend-8-tpy/Co2+ films decays slowly as the number of growth cycles (l) increases. Moreover, a preliminary kinetics analysis indicated that the growth of a dendrimer layer in a deposition cycle is a self-limiting process.     

Fundamentals and application of ordered molecular assemblies to affinity biosensing

Organization of biomolecules in two/three dimensional assemblies has recently aroused much interest in nanobiotechnology. In this context, the development of techniques for controlling spatial arrangement and orientation of the desired molecules to generate highly-ordered nanostructures in the form of a mono/multi layer is considered highly significant. The studies of monolayer films to date have focused on three distinct methods of preparation: (i) the Langmuir-Blodgett (LB) technique, involving the transfer of a monolayer assembled at the gas-liquid interface; (ii) self-assembly at the liquid-solid interface, based on spontaneous adsorption of desired molecules from a solution directly onto a solid surface; and (iii) Layer-by-layer (LBL) self-assembly at a liquid-solid interface, based on inter-layer electrostatic attractions for fabrication of multilayers. A variety of monolayers have been utilized to fabricate biomolecular electronic devices including biosensors. The composition of a monolayer based matrix has been found to influence the activity(ies) of biomolecule(s). We present comprehensive and critical analysis of ordered molecular assemblies formed by LB and self-assembly with potential applications to affinity biosensing. This critical review on fundamentals and application of ordered molecular assemblies to affinity biosensing is likely to benefit researchers working in this as well as related fields of research (401 references).     

Spectroscopic evidence for the formation of goldfingers

Gold(I) has long been used in the treatment of rheumatoid arthritis, but the therapeutically relevant biological targets of gold(I) are not well understood. Here, we report the results of a spectroscopic investigation into the formation of goldfingers. By exploiting a thiolate to gold charge-transfer band in the UV, we observed that gold(I) interacts with zinc finger peptides with a stoichiometry of one gold ion for each two cysteine residues, forming 1:1, 1.5:1, and 2:1 adducts with zinc finger peptides containing CCHH, CCHC, and CCCC donor sets, respectively. In addition, circular dichroism experiments provided evidence that goldfingers are more ordered than the corresponding metal-free peptides but do not exhibit the canonical zinc finger structure.     

Evidence for vesicle formation during the synthesis of catanionic templated mesoscopically ordered silica as studied by Cryo-TEM

The formation of vesicles during the synthesis of mesostructured surfactant-silicate materials has been evidenced for the first time by applying in situ Cryo-TEM. A mixture of anionic and cationic surfactants was used to fine-tune the packing parameter of the supramolecular surfactant assembly. It is shown that the vesicles are formed during the course of the reaction and suggested to be a consequence of silicate condensation reactions. Furthermore, the study confirms earlier suggestions that the silicate-surfactant micelles are capable of solubilizing hydrophobic additives during the early stages of the reaction.     

A hydrophilic cyclodextrin duplex forming supramolecular assemblies by physical cross-linking of a biopolymer

New beta-cyclodextrin (beta-CD) dimeric species have been synthesised in which the two CD moieties are connected by one or two hydrophilic oligo(ethylene oxide) spacers. Their complexation with sodium adamantylacetate (free adamantane) and adamantane-grafted chitosan (AD-chitosan) was then studied by different complementary techniques and compared with their hydrophobic counterparts that contain an octamethylene spacer. Isothermal titration calorimetry experiments have demonstrated that the use of hydrophilic spacers between the two CDs instead of aliphatic chains makes almost all of the CD cavities available for the inclusion of free adamantane. Investigation of the interaction of the CDs with AD-chitosan by viscosity measurements strongly suggests that the molecular conformation of the CD dimeric species plays a crucial role in their cross-linking with the biopolymer. The derivative doubly linked with hydrophilic arms, also called a duplex, has been shown to be a more efficient cross-linking agent than its singly bridged counterpart, referred to as a dimer. Hence, only 0.5 molar equivalents of the hydrophilic duplex with respect to adamantane was required to obtain the maximum viscosity, whereas in the case of the duplex with aliphatic spacers, the maximum viscosity was achieved with a [duplex]/[AD] ratio of about 1.7 (corresponding to a [CD]/[AD] ratio of 2.5), but with a higher value. To clarify the relationships between the molecular architecture and complexation properties, computational studies were also performed that clearly confirmed the importance of double bridging.     

Size limitations for the formation of ordered striped nanoparticles

A combination of immiscible molecules in the ligand shell of a gold nanoparticle (NP) has been shown to phase separate into a rippled structure; this phase separation can be used to direct the assembly of the NPs into chains. Here we demonstrate that only NPs within a certain size range can form chains, and we conclude that the rippled morphology of the ligand shell also exists only within that given size range. We corroborate this result with simulations of the ligand arrangement on NPs of various sizes.     

Spectroscopic characterizations of molecularly linked gold nanoparticle assemblies upon thermal treatment

The understanding of surface properties of core-shell type nanoparticles is important for exploiting the unique nanostructured catalytic properties. We report herein findings of a spectroscopic investigation of the thermal treatment of such nanoparticle assemblies. We have studied assemblies of gold nanocrystals of approximately 2 nm core sizes that are capped by alkanethiolate shells and are assembled by covalent or hydrogen-bonding linkages on a substrate as a model system. The structural evolution of the nanoparticle assemblies treated at different temperatures was probed by several spectroscopic techniques, including UV-visible, Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS). The results show that the capping/linking shell molecules can be effectively removed to produce controllable surface and optical properties. The data further revealed that the thermally induced evolution of the surface plasmon resonance property of gold nanoparticles is dependent on the chemical nature of the linker molecule. The spectral evolution is discussed in terms of changes in particle size, interparticle distance, and dielectric medium properties, which has important implications for controlled preparation and thermal processing of core-shell nanostructured metal catalysts.     

Spectroscopic investigation of the structures of dialkyl tartrates and their cyclodextrin complexes

Structures of three dialkyl tartrates, namely, dimethyl tartrate, diethyl tartrate, and diisopropyl tartrate, in CCl4, dimethyl sulfoxide (DMSO)/DMSO-d6, and H2O/D2O solvents have been investigated using vibrational absorption (VA), vibrational circular dichroism (VCD), and optical rotatory dispersion (ORD). VA, VCD, and ORD spectra are found to be dependent on the solvent used. Density functional theory (DFT) calculations are used to interpret the experimental data in CCl4 and DMSO. The trans-COOR conformer with hydrogen bonding between the OH group and the C=O group attached to the same chiral carbon is dominant for dialkyl tartrates both in vacuum and in CCl4. The experimental VA, VCD, and ORD data of dialkyl-D-tartrates in CCl4 correlated well with those predicted for dimethyl-(S,S)-tartrate molecule as both isolated and solvated in CCl4. In DMSO solvent, dialkyl tartrate molecules favor formation of intermolecular hydrogen bonding with DMSO molecules. Clusters of dimethyl-(S,S)-tartrate, with one molecule of dimethyl-(S,S)-tartrate hydrogen bonded to two DMSO molecules, are used for the DFT calculations. A trans-COOR cluster and a trans-H cluster are needed to obtain a reasonable agreement between the predicted and experimental data of dimethyl tartrate in DMSO solvent. VA, VCD, and optical rotations are also measured for dialkyl tartrate-cyclodextrin complexes. It is noted that these properties are barely affected by complexation of dialkyl tartrates with cyclodextrins, indicating weak interaction between tartrates and cyclodextrin. Binding constants of alpha-CD and beta-CD with diethyl L-tartrate in both H2O and DMSO have been determined using isothermal titration calorimetry technique. The smaller binding constants (less than 100) confirmed the weak interaction between tartrates and cyclodextrin in the solution state.     

Mesoscopic pattern formation of nanostructured polymer assemblies

Hierarchical mesoscopic structures of the nanoscopic supramolecular assemblies, which consist of polyelectrolytes and bilayer-forming amphiphiles, are prepared by a simple and new solvent-casting method. Submicron scale 2-D structures, e.g. regular dots, stripes, and networks, are formed when highly diluted organic solutions of polymer assemblies are cast on solid surfaces. Dynamic mesoscopic regular structures, the so-called ‘dissipative structures’, formed in the non-equilibrium processes of solvent-casting are fixed as hierarchically structured polymer assemblies.     

Retention on mixed cyclodextrin/polysiloxane stationary phases in capillary GC—Evidence for the interaction of cyclodextrin and polysiloxane

The dependence of the retention on stationary phases consisting of mixtures of a cyclodextrin (perpentyl-β-cyclodextrin) and a polysiloxane (cyanopropyl-7%-, phenyl-7%-methyl-86%-or OV-1701) was investigated as function of the cyclodextrin concentration. In order to study the effect of mixing, the data on the mixed stationary phases were compared with those obtained on pure OV-1701 and perpentylated β-CD and on coupled columns, individually coated with the pure phases. The validity of the retention model proposed in the literature by Schurig and co-workers was checked. Deviation from linearity was observed for some racemates. A possible explanation of the deviation is presented and a more general retention model on mixed cyclodextrin-polysiloxane phases is proposed.     

Excimer formation of fluoradene. Evidence for aggregation in alcohols

The absorption of fluorescence spectra of fluoradene in several solvents have been measured. In addition to the expected fluorescence band, a broad structureless band at 440 nm is observed in hexane which increases in intensity with increasing fluoradene concentration. In alcoholic solutions and in polycrystalline solid, only the broad, red-shifted band is observed. This broad band is attributed to excimer emission. It is shown that in alcoholic solutions that rate of excimer formation via diffusion is unacceptably large and it is suggested that fluoradene is aggregated in these solvents.     

Highly ordered assemblies of dendritic molecules bearing multi‐hydrophilic head groups

A new class of dendritic amphiphiles with multi‐hydrophilic head groups has been synthesized and characterized. The results from their surface pressure vs. area isotherms and hysteresis curves indicate that they form highly compressible and stable monolayers at the air‐water interface. The topology of dendritic monolayer and Langmuir‐Blodgett films was investigated by atomic force microscopy, which showed a highly ordered aggregation.     

Complex formation of unsaturated cyclodextrin solutions with various polymers

By using methyl orange as a competitive complexant, it was shown that solutions of poly(ethylene glycol), poly(propylene glycol) and poly(N-vinylpyrrolidone) equivalent to a concentration of 1.67 mM monomer are able to form soluble inclusion complexes with 0.20 mM solutions of -, - and -cyclodextrins. The complex formation is independent of polymer chain length. Poly(ethylene glycol) solutions complexed, on average with 40%, 45% and 75% of -, -, and -cyclodextrin, respectively. A poly(propylene glycol) solution complexed 55% -cyclodextrin, 80% -cyclodextrin and 90% -cyclodextrin. Poly(N-vinylpyrrolidone) solutions complexed on average with 90% of -, -, and -cyclodextrin.     

Inhibition of amyloid aggregation by formation of helical assemblies

The formation of amyloid aggregates is responsible for a wide range of diseases, including Alzheimer's and Parkinson's disease. Although the amyloid-forming proteins have different structures and sequences, all undergo a conformational change to form amyloid aggregates that have a characteristic cross-β-structure. The mechanistic details of this process are poorly understood, but different strategies for the development of inhibitors of amyloid formation have been proposed. In most cases, chemically diverse compounds bind to an elongated form of the protein in a β-strand conformation and thereby exert their therapeutic effect. However, this approach could favor the formation of prefibrillar oligomeric species, which are thought to be toxic. Herein, we report an alternative approach in which a helical coiled-coil-based inhibitor peptide has been designed to engage a coiled-coil-based amyloid-forming model peptide in a stable coiled-coil arrangement, thereby preventing rearrangement into a β-sheet conformation and the subsequent formation of amyloid-like fibrils. Moreover, we show that the helix-forming peptide is able to disassemble mature amyloid-like fibrils.     

Organized assemblies of thiol-terpyridine and thiophenol on gold surfaces: preferential composition of mixed species evidenced

Mixed self-assembled monolayers based on thiol-terpyridine and thiophenol molecules on gold surfaces show a tendency to form films whose composition is almost independent of the molar fraction of the components in the starting solution, and this indicates the formation of particularly stable assemblies.