Schiff Bases Derived from Pyridoxal 5'-Phosphate and 2-X-Phenylamine (X = H, OH, SH): Substituent Effects on UV-Vis Spectra and Hydrolysis Kinetics.

Schiff bases are compounds that are widely distributed in nature and have practical value for industry and biomedicine. Another important use of Schiff bases is identifying metal ions and different molecules, including proteins. Their proneness to hydrolysis limits the utilization of Schiff bases to mainly non-aqueous solutions. However, by introducing -OH and -SH substituents to aromatic amine-bearing rings, it is possible to increase the resilience of the Schiff base to destruction in water. The present paper discusses how the hydroxyl or thiol group influences the spectral properties and kinetics of the hydrolysis and formation of Schiff bases derived from pyridoxal 5'-phosphate and aniline, 2-hydroxyaniline, and 2-mercaptoaniline using quantum chemical data. The spectral variation between different imines can be explained by taking into account the geometry and frontier molecular orbital alteration induced by the substituents. The changes in the hydrolysis rate are analyzed using the computed values of local reactivity indices.

Complexes of Gold(III) with Hydrazones Derived from Pyridoxal: Stability, Structure, and Nature of UV-Vis Spectra.

Pyridoxal and pyridoxal 5'-phosphate are aldehyde forms of B6 vitamin that can easily be transformed into each other in the living organism. The presence of a phosphate group, however, provides the related compounds (e.g., hydrazones) with better solubility in water. In addition, the phosphate group may sometimes act as a binding center for metal ions. In particular, a phosphate group can be a strong ligand for a gold(III) ion, which is of interest for researchers for the anti-tumor and antimicrobial potential of gold(III). This paper aims to answer whether the phosphate group is involved in the complex formation between gold(III) and hydrazones derived from pyridoxal 5'-phosphate. The answer is negative, since the comparison of the stability constants determined for the gold(III) complexes with pyridoxal- and pyridoxal 5'-phosphate-derived hydrazones showed a negligible difference. In addition, quantum chemical calculations confirmed that the preferential coordination of two series of phosphorylated and non-phosphorylated hydrazones to gold(III) ion is similar. The preferential protonation modes for the gold(III) complexes were also determined using experimental and calculated data.

Shedding Light on Heavy Metal Contamination: Fluorescein-Based Chemosensor for Selective Detection of Hg2+ in Water.

This article discusses the design and analysis of a new chemical chemosensor for detecting mercury(II) ions. The chemosensor is a hydrazone made from 4-methylthiazole-5-carbaldehyde and fluorescein hydrazide. The structure of the chemosensor was confirmed using various methods, including nuclear magnetic resonance spectroscopy, infrared spectroscopy with Fourier transformation, mass spectroscopy, and quantum chemical calculations. The sensor's ability in the highly selective and sensitive discovery of Hg2+ ions in water was demonstrated. The detection limit for mercury(II) ions was determined to be 0.23 µM. The new chemosensor was also used to detect Hg2+ ions in real samples and living cells using fluorescence spectroscopy. Chemosensor 1 and its complex with Hg2+ demonstrate a significant tendency to enter and accumulate in cells even at very low concentrations.

Geometry and UV-Vis Spectra of Au3+ Complexes with Hydrazones Derived from Pyridoxal 5'-Phosphate: A DFT Study.

Gold(III) complexes with different ligands can provide researchers with a measure against pathogenic microorganisms with antibiotic resistance. We reported in our previous paper that the UV-Vis spectra of different protonated species of complexes formed by gold(III) and five hydrazones derived from pyridoxal 5'-phosphate are similar to each other and to the spectra of free protonated hydrazones. The present paper focuses on the reasons of the noted similarity in electron absorption spectra. The geometry of different protonated species of complexes of gold(III) and hydrazones (15 structures in total) was optimized using the density functional theory (DFT). The coordination polyhedron of gold(III) bond critical points were further studied to identify the symmetry of the gold coordination sphere and the type of interactions that hold the complex together. The UV-Vis spectra were calculated using TD DFT methods. The molecular orbitals were analyzed to interpret the calculated spectra.

Complexation of Gold(III) with Pyridoxal 5'-Phosphate-Derived Hydrazones in Aqueous Solution.

Today, complexes of gold(I) and gold(III) are recognized as promising drugs for the treatment of bacterial infectious diseases and oncological diseases, respectively. It is of interest to broaden the area of potential use of gold(III) compounds to the pathogenic microorganism as well. The first step towards the development of new antibacterial drugs based on Au3+ complexes is the study of their stability in an aqueous solution. The present contribution reports on the investigation of gold(III) complexation with five hydrazones derived from a well-known biologically active compound, pyridoxal 5'-phosphate (one of the aldehyde forms of the B6 vitamin). The complex formation in aqueous solutions was confirmed by mass spectrometry and fluorescent spectroscopy. The stoichiometric composition of the complexes formed and their stability constants were determined using a UV-Vis titration method. The complexes are quite stable at physiological values of pH, as the speciation diagrams show. The results of the paper are helpful for further studies of gold(III) complexes interaction with biomacromolecules.

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 ß-cyclodextrin (ß-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 ß-CD ([BA⊂ß-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⊂ß-CD] molecular complex formation. Quantum chemical calculations of the interaction energy between ß-CD and BA as well as the structure of the [BA⊂ß-CD] complex and the energy of ß-CD and BA interaction in vacuum and in the medium of water, methanol and dimethylsulfoxide solvents are carried out. The stability of [BA⊂ß-CD] complex in H2O-EtOH and H2O-DMSO solvents, obtained by different methods, are compared. The thermodynamic parameters of the [BA⊂ß-CD] molecular complexation as well as the reagent solvation contributions in H2O-EtOH and H2O-DMSO mixtures were analyzed by the solvation-thermodynamic approach.

La(III), Ce(III), Gd(III), and Eu(III) Complexation with Tris(hydroxymethyl)aminomethane in Aqueous Solution.

Lanthanides such as cerium(III), europium(III), and gadolinium(III) are widely used for designing fluorescent probes or magnetic resonance imaging contrasting agents for biological systems. The synthesis and study of lanthanide complexes in buffer solutions imitating biological fluids are often complicated because of a lack of data on the lanthanide interactions with buffer solution components. Therefore, Ln(III) [where Ln(III) = La(III), Ce(III), Gd(III), Eu(III)] complexation with a widely used buffer agent, tris(hydroxymethyl)aminomethane (Tris), in aqueous solution is studied using potentiometry, spectrofluorimetry, and 139La NMR spectroscopy. The stoichiometric composition of complexes is determined using mass spectrometry. The thermodynamic stability constants of Ln(III)-Tris complexes are calculated from potentiometric and spectral data; the difficulties in the study of these systems, reliability, and accuracy of the obtained constants are discussed. The possible structures of free Tris and its complexes with lanthanides(III) are optimized on the density functional theory/PBE0 level; the peculiarities of metal-ligand bonds were studied by Quantum Theory Atoms in Molecules analysis.