A Study on Host–Guest Complexation of 5-Amino-2-Mercaptobenzimidazole with β-Cyclodextrin

Complexation of Cyclodextrins with Benzoic Acid in Water-Organic Solvents: A Solvation-Thermodynamic Approach

The aim of this research is to obtain new data about the complexation between β-cyclodextrin (β-CD) and benzoic acid (BA) as a model reaction of the complex formation of hydrophobic molecules with cyclodextrins (CDs) in various media. This research may help developing cyclodextrin-based pharmaceutical formulations through the choice of the appropriate solvent mixture that may be employed in the industrial application aiming to control the reactions/processes in liquid phase. In this paper, NMR results for the molecular complex formation between BA and β-CD ([BA⊂β-CD]) in D₂O-DMSO-d₆ and in D₂O-EtOH have shown that the stability of the complex in the H₂O-DMSO-d₆ varies within the experimental error, while decreases in H₂O-EtOH. Changes in the Gibbs energy of BA resolvation in water and water–dimethylsulfoxide mixtures have been obtained and have been used in the analysis of the reagent solvation contributions into the Gibbs energy changes of the [BA⊂β-CD] molecular complex formation. Quantum chemical calculations of the interaction energy between β-CD and BA as well as the structure of the [BA⊂β-CD] complex and the energy of β-CD and BA interaction in vacuum and in the medium of water, methanol and dimethylsulfoxide solvents are carried out. The stability of [BA⊂β-CD] complex in H₂O-EtOH and H₂O-DMSO solvents, obtained by different methods, are compared. The thermodynamic parameters of the [BA⊂β-CD] molecular complexation as well as the reagent solvation contributions in H₂O-EtOH and H₂O-DMSO mixtures were analyzed by the solvation-thermodynamic approach.

Supramolecular Interactions between β-Cyclodextrin and the Nucleobase Derivatives of Ferrocene

Novel conjugates of ferrocene with uracil, 5-fluorouracil, tegafur, or acyclovir are reported. Their synthesis involved (i) the azide-alkyne 1,3-dipolar cycloaddition or (ii) the formation of the ester linkage. For the first time, we present an in-depth insight into the supramolecular interactions between β-cyclodextrin and ferrocene-nucleobase derivatives. Spectroscopic and voltammetric analyses performed within this work suggested that the ferrocene or adamantane unit of the conjugates interacted with the β-cyclodextrin's inner cavity. The methods applied for the supramolecular studies included ¹H-¹H ROESY NMR, ¹H NMR titration, Fourier-transform infrared spectroscopy, cyclic voltammetry, fluorescence spectra titration, and ¹H DOSY NMR. ¹H DOSY NMR was also employed to evaluate the apparent binding constants for all the complexes. The ferrocene-acyclovir conjugate Fc-5 featured the highest apparent binding constant value among all the complexes tested.

Separately enhanced dual emissions of the amphiphilic derivative of 2-(2'-hydroxylphenyl) benzothiazole by supramolecular complexation

Here, we report separately enhanced dual emissions of the amphiphilic derivative of 2-(2'-hydroxyphenyl)benzothiazole (denoted as HBT-11) by supramolecular complexation with cyclodextrins (CDs). When dispersed in water, HBT-11 shows two relatively weak emission bands, which can be assigned to the emissions of enol- and keto-forms, the two tautomers, owing to excited-state intramolecular proton transfers. Upon the addition of α-CD and β-CD, the keto- and enol-emissions, respectively, are separately enhanced; the enhancement effect is due to the formation of HBT-11/α-CD and HBT-11/β-CD complexes through multiple hydrogen bonding and host-guest interactions, respectively. It is worth to note that the keto-emission caused by the complex of HBT-11/α-CD has a much shorter wavelength compared with that of the aggregates formed by pure HBT-11. To the best of our knowledge, this is the first time that a study on keto-emission of the isolated HBT chromophore has been reported.

Inclusion complexes of organic salts with β-cyclodextrin as organocatalysts for CO2 cycloaddition with epoxides

The inclusion complexes between β-cyclodextrin and 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU)-based phenolates have been developed and employed as heterogeneous catalysts for the cycloaddition of CO₂ to cyclic carbonate.

Photophysical and Photoprototropic Characteristics of 2-Aminobenzothiazole in β-Cyclodextrin Medium

The formation of stable inclusion complexes between neutral and monocationic species of 2-aminobenzothiazole (2ABT) with β-CDx is observed with significant enhancement in absorbance and fluorescence intensity. The stoichiometry of the 2ABT and 2ABTH^⊕ with β-CDx is found to be 1:1 at pH 6.8 and 1.1. Their binding constant values for neutral and monocationic species are found to be 1239.3 and 1259.2 M^-1, respectively. Lifetime analysis, FT-IR spectral and SEM image analysis strongly supports the formation of the inclusion complex between 2ABT and β-CDx. Photoprototropic study shows that there is no remarkable difference in the pK _a and pK _a* in aqueous and β-CDx media, which is rules out the formation of the inclusion complex with -NH₂ group of 2ABT inside the cavity of β-CDx i.e., the -NH₂ group of 2ABT is in aqueous environment. Based on stoichiometry, binding constant and acidity constant values, the structure of the 1:1 complex is proposed.

Experimental and density functional theory study of Raman and SERS spectra of 5-amino-2-mercaptobenzimidazole

Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS) and density functional theory (DFT) simulations were employed to study 5-amino-2-mercaptobenzimidazole (5-A-2MBI) molecules. Ag colloids were used as SERS substrates which were prepared by using hydroxylamine hydrochloride as reducing agent. Raman vibration modes and SERS characteristic peaks of 5-A-2MBI were assigned with the aid of DFT calculations. The molecular electrostatic potential (MEP) of 5-A-2MBI was used to discuss the possible adsorption behavior of 5-A-2MBI on Ag colloids. The spectral analysis showed that 5-A-2MBI molecules were slightly titled via the sulfur atoms adhering to the surfaces of Ag substrates. The obtained SERS spectral intensity decreased when lowering the 5-A-2MBI concentrations. A final detection limit on the concentration of 5×10(-7) mol · L(-1) was gained. SERS proved to be a simple, fast and reliable method for the detection and characterization of 5-A-2MBI molecules.

Preparation and characterization of host-guest system between inosine and β-cyclodextrin through inclusion mode

Inosine is a nucleoside that is formed when hypoxanthine is attached to a ribose ring (also known as a ribofuranose) via a β-N₉-glycosidic bond. Inosine is commonly found in tRNAs. Inosine (INS) has been used widely as an antiviral drug. The inclusion complex of INS with β-CDx in solution phase is studied by ground and excited state with UV-visible and fluorescence spectroscopy, respectively. A binding constant and stoichiometric ratio between INS and β-CDx are calculated by BH equation. The lifetime and relative amplitude of INS is increases with increasing the concentrations of β-CDx, confirms the formation of inclusion complex in liquid state. The solid complexes are prepared by kneading method (KM) and co-precipitation method (CP). The solid complex is characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), powder X-ray diffraction (XRD) and differential scanning colorimetry (DSC). CP method gives the solid product with good yield than that of physical mixture and KM method. The structure of complex is proposed based on the study of Patch - Dock server.

A spectroscopic study on the coordination and solution structures of the interaction systems between biperoxidovanadate complexes and the pyrazolylpyridine-like ligands

In order to understand the substitution effects of pyrazolylpyridine (pzpy) on the coordination reaction equilibria, the interactions between a series of pzpy-like ligands and biperoxidovanadate ([OV(O2)2(D2O)](-)/[OV(O2)2(HOD)](-), abbrv. bpV) have been explored using a combination of multinuclear ((1)H, (13)C, and (51)V) magnetic resonance, heteronuclear single quantum coherence (HSQC), and variable temperature NMR in a 0.15 mol L(-1) NaCl D2O solution that mimics the physiological conditions. Both the direct NMR data and the equilibrium constants are reported for the first time. A series of new hepta-coordinated peroxidovanadate species [OV(O2)2L](-) (L = pzpy-like chelating ligands) are formed due to several competitive coordination interactions. According to the equilibrium constants for products between bpV and the pzpy-like ligands, the relative affinity of the ligands is found to be pzpy > 2-Ester-pzpy ≈ 2-Me-pzpy ≈ 2-Amide-pzpy > 2-Et-pzpy. In the interaction system between bpV and pzpy, a pair of isomers (Isomers A and B) are observed in aqueous solution, which are attributed to different types of coordination modes between the metal center and the ligands, while the crystal structure of NH4[OV(O2)2(pzpy)]·6H2O (CCDC 898554) has the same coordination structure as Isomer A (the main product for pzpy). For the N-substituted ligands, however, Isomer A or B type complexes can also be observed in solution but the molar ratios of the isomer are reversed (i.e., Isomer B type is the main product). These results demonstrate that when the N atom in the pyrazole ring has a substitution group, hydrogen bonding (from the H atom in the pyrazole ring), the steric effect (from alkyl) and the solvation effect (from the ester or amide group) can jointly affect the coordination reaction equilibrium.

The formation of host-guest complexes between surfactants and cyclodextrins

Cyclodextrins are able to act as host molecules in supramolecular chemistry with applications ranging from pharmaceutics to detergency. Among guest molecules surfactants play an important role with both fundamental and practical applications. The formation of cyclodextrin/surfactant host-guest compounds leads to an increase in the critical micelle concentration and in the solubility of surfactants. The possibility of changing the balance between several intermolecular forces, and thus allowing the study of, e.g., dehydration and steric hindrance effects upon association, makes surfactants ideal guest molecules for fundamental studies. Therefore, these systems allow for obtaining a deep insight into the host-guest association mechanism. In this paper, we review the influence on the thermodynamic properties of CD-surfactant association by highlighting the effect of different surfactant architectures (single tail, double-tailed, gemini and bolaform), with special emphasis on cationic surfactants. This is complemented with an assessment of the most common analytical techniques used to follow the association process. The applied methods for computation of the association stoichiometry and stability constants are also reviewed and discussed; this is an important point since there are significant discrepancies and scattered data for similar systems in the literature. In general, the surfactant-cyclodextrin association is treated without reference to the kinetics of the process. However, there are several examples where the kinetics of the process can be investigated, in particular those where volumes of the CD cavity and surfactant (either the tail or in special cases the head group) are similar in magnitude. This will also be critically reviewed.