Dielectric relaxation of β-cyclodextrin complex with 4-t-butylbenzyl alcohol

The frequency and temperature dependence of the real (ε′) and imaginary (ε′) parts of the dielectric constant of polycrystalline complex β-cyclodextrin-4-t-butylbenzyl alcohol [β-CD · TERB · 11.2H2O] and β-cyclodextrin [β-CD · 9.8H2O] and of the corresponding dried forms (β-CD · TERB · 3.8H2O and β-CD · 2.4H2O, respectively) has been investigated, in the frequency range 0-100 kHz and temperature range 130-350 K. The dielectric behaviour is described well by Debye-type relaxation (α dispersion). All systems except for the β-CD · TERB · 3.8H2O, exhibit an additional Ω dispersion at low frequencies, which usually is attributed to proton transport. In the non-dried samples the temperature dependence of ε′ and ε″max exhibits two steps, whereas in the dried samples it exhibits only the low temperature step. The low temperature step is due to the tightly bound water molecules, whereas that at higher temperatures is due to easily removable water. The temperature dependence of ε″ shows a peak which has been attributed to a transition between ordered and disordered hydroxyl β-CD groups, and water molecules. The relaxation time varies exponentially with temperature (in the range 8-12 μsec), in a reverse V like curve, with maximum values located at the corresponding order-disorder transition temperatures. Activation energies of the order of ∼2.5 kJ mol-1 are calculated for the transition in every sample. The disorder in the hydrogen bonding is equivalent to a system of two dipoles with opposite directions, and the model of Fröhlich can be applied to explain the order-disorder transition and the temperature dependence of the relaxation time. An apparent negative activation energy before the transition temperature can be attributed to reorientation of the hydrogen bonding around the cyclodextrin molecules, and it is related to endothermic drifts observed by calorimetric studies of β-CD. The order-disorder transition can be probed also from the phase shift component of the current passing through the sample relative to the applied signal. © 1999 Taylor & Francis Ltd

Organisation of long aliphatic monocarboxylic acids in β-cyclodextrin channels: Crystal structures of the inclusion complexes of tridecanoic acid and (Z)-tetradec-7-enoic acid in β-cyclodextrin

In the crystalline state, infinite channels of β-cyclodextrin dimers host infinite arrays of self associated linear aliphatic monocarboxylic acids, thus enclosing the hydrophilic carboxy ends inside the hydrophobic channels

Photochromism and thermochromism of solid trans-N,N'-bis-(salicylidene)-1,2-cyclohexanediamines and trans-N,N'-bis-(2-hydroxy-naphylidene)-1,2-cyclohexanediamine

The photochromic and thermochromic properties of trans-N,N′-bis(salicylidene)-1,2-cyclohexanediamine (1), trans-N,N′-bis(3,5-dichloro-salicylidene)-1,2-cyclohexanediamine (2), trans-N,N ′-bis(3,5-di-t-butyl-salicylidene)-1,2- cyclohexanediamine (3) and trans-N,N ′-bis(2-hydroxy-naphylidene)-1,2-cyclohexanediamine (4) were investigated by UV and fluorescence spectroscopies in the crystalline state at various temperatures and the molecular structures of 2 and 4 were determined by single-crystal X-ray diffraction. The existence of the two Schiff base groups on a single molecule does not seem to differentiate the chromobehavior of the present compounds (except possibly for 3) with respect to the usual Schiff bases of salicylaldehyde. It is suggested that for this class of compounds also, what determines the thermochromic behavior is the enhanced basicity of the nitrogen atom, due the absence of π,π- and n,π-conjugation with an aryl ring bound to it. The role of the crystal structure in this case is important only in so far as it affects the electron density on the nitrogen atom. Photochromism, however, is structure dependent and requires space for the generation of the photoproduct that involves cis to trans isomerization in the excited state. © 2004 Elsevier B.V. All rights reserved

The structure of the 2[4Fe-4S] ferredoxin from Pseudomonas aeruginosa at 1.32-Å resolution: Comparison with other high-resolution structures of ferredoxins and contributing structural features to reduction potential values

The structure of the 2[4Fe-4S] ferredoxin (PaFd) from Pseudomonas aeruginosa, which belongs to the Allochromatium vinosum (Alvin) subfamily, has been determined by X-ray crystallography at 1.32-Å resolution, which is the highest up to now for a member of this subfamily of Fds. The main structural features of PaFd are similar to those of AlvinFd. However, the significantly higher resolution of the PaFd structure makes possible a reliable comparison with available high-resolution structures of [4Fe-4S]-containing Fds, in an effort to rationalize the unusual electrochemical properties of Alvin-like Fds. Three major factors contributing to the reduction potential values of [4Fe-4S]2+/+ clusters of Fds, namely, the surface accessibility of the clusters, the N-H·S hydrogen-bonding network, and the volume of the cavities hosting the clusters, are extensively discussed. The volume of the cavities is introduced in the present work for the first time, and can in part explain the very negative potential of cluster I of Alvin-like Fds. © SBIC 2006

Insight into the protein and solvent contributions to the reduction potentials of [4Fe-4S]2+/+ clusters: Crystal structures of the Allochromatium vinosum ferredoxin variants C57A and V13G and the homologous Escherichia coli ferredoxin

The crystal structures of the C57A and V13G molecular variants of Allochromatium vinosum 2[4Fe-4S] ferredoxin (AlvinFd) and that of the homologous ferredoxin from Escherichia coli (EcFd) have been determined at 1.05-, 1.48-, and 1.65-Å resolution, respectively. The present structures combined with cyclic voltammetry studies establish clear effects of the degree of exposure of the cluster with the lowest reduction potential (cluster I) towards less negative reduction potentials (E°). This is better illustrated by V13G AlvinFd (high exposure, E° = -594 mV) and EcFd (low exposure, E° = -675 mV). In C57A AlvinFd, the movement of the protein backbone, as a result of replacing the noncoordinating Cys57 by Ala, leads to a +50-mV upshift of the potential of the nearby cluster I, by removal of polar interactions involving the thiolate group and adjustment of the hydrogen-bond network involving the cluster atoms. In addition, the present structures and other previously reported accurate structures of this family of ferredoxins indicate that polar interactions of side chains and water molecules with cluster II sulfur atoms, which are absent in the environment of cluster I, are correlated to the approximately 180-250 mV difference between the reduction potentials of clusters I and II. These findings provide insight into the significant effects of subtle structural differences of the protein and solvent environment around the clusters of [4Fe-4S] ferredoxins on their electrochemical properties. © 2009 SBIC

Cooperative Self-Assembly Enables Two-Dimensional H-type Aggregation of a Sterically Crowded Perylene-Bisimide Dimer

Identifying the role of multiple cooperative supramolecular interactions and the working mechanism underlying the formation of sophisticated, well-defined self-assembled architectures is definitely a challenging and formidable task in understanding the complexity in chemical systems and engineering the properties of advanced materials. The topological design of multifunctional tectons, capable of self-organizing into patterned supramolecular assemblies comprising stacked aromatic molecules, is of particular importance because it can lead to the predictable emergence of controlled functions with tailored electronic properties. Herein, we provide spectroscopic, structural, and mechanistic insights on metal-ion-mediated self-assembly of a charged, amphiphilic perylene-bisimide (PBI) dimer S into two-dimensional (2D) arrays consisting of parallel columnar PBI stacks with a precise spatial arrangement and pattern behavior, using a readily accessible design strategy. The building block (S), a centrosymmetric PBI homodimer bearing a disulfonated trans-stilbene core, was designed to concurrently feature high complexation directionality with a strong binding affinity through multiple supramolecular interactions. In solvents that efficiently solvate PBI, e.g., chloroform, the zinc ion interacts strongly through electrostatic interactions with the negatively charged core of S, and with the πcloud of the stilbene moiety (cation-πinteractions) forming simple 1:1 adducts. In methanol, the findings manifest the efficient formation of well-defined aggregates with H-type excitonic coupling. A single-crystal X-ray structure reveals, despite the sterically crowded bay area of PBIs constituting S, an unprecedented pattern of 2D arrays comprising face-to-face, slipped π-stacked PBI interdimers that pack in parallel columns. This molecular arrangement explains the quenched fluorescence in solution, as well as the appearance of weak excimer-like fluorescence both in solution and crystals. The spectroscopic and structural findings converge to the conclusion that the development of aggregates in solution proceeds by a cooperative growth process driven by a collection of different supramolecular interactions, i.e., electrostatic (core of S), π-πstacking (terminal PBIs), and multiple C-H···π(bay substituents). A corresponding aggregation model fits satisfactorily the experimental data in solution and allows extracting the association constants and spectra of the equilibrated species. © Copyright © 2019 American Chemical Society