Ascorbic acid is one of the essential compounds for living organisms. The reductive properties of ascorbate depend significantly on the dissociation of ascorbic acid, which, in turn, is influenced by the ionic strength value. The protonation of ascorbate dianion was studied in an aqueous solution at T = 298.2 K; p = 0.1 MPa; and I = 0.10, 0.25, 0.5, 1.0, 2.0, 3.0, and 5.0 mol L–1 (NaCl) using the spectropotentiometric technique. Protonation constants were calculated from the spectral data. The analysis of ionic strength influence on the ascorbic acid dissociation constants was performed using the modified Davies equation (the so-called Vasil’ev equation) and specific ion interaction theory.

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.

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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.

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