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In this study, pK a values of glycine, valine, phenylalanine, glycylvaline, and glycylphenylalanine were determined in aqueous solution by an ab initio method. To explain the acidic dissociation constants obtained, we investigated the molecular conformations and solute−solvent interactions of the peptides and amino acid anions, using the density functional theory (DFT) method. Several ionization reactions and equilibria in protic solvents, which possess a high hydrogen-band-donor capability, are shown. The mentioned reactions and equilibria constitute the indispensable theoretical basis to calculate the acidity constants of glycine, valine, phenylalanine, glycylvaline, and glycylphenylalanine. Basis sets at the B3LYP/6-31+G(d) level of theory were used for calculations. Tomasi’s method was used to analyze the formation of intermolecular hydrogen bonds between the existent species and water molecules. In this way, it was determined that in alkaline aqueous solutions the cation, anion, and neutral species of glycine, valine, phenylalanine, glycylvaline, and glycylphenylalanine are solvated with one, two, three, and four molecules of water, respectively. In this study, there is comparable agreement between the experimentally determined pK a values for the acid−base reactions selected by potentiometric and those reported in the literature demonstrating the theoretically calculated pK a values.

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As constantes de estabilidade dos complexos de íons tálio(I) e cádmio(II) com dipeptídeos de glicil-L-fenilalanina e L-fenilalanilglicina foram determinadas em solução aquosa a 25 °C e meio iônico 0,1 mol dm -3 usando uma combinação de técnicas potenciométricas e espectrofotométricas. Perclorato de sódio foi usado para manter a força iônica. A composição dos complexos formados foi determinada e observou-se que tálio (I) The stability constants of the complexes of thallium(I) and cadmium(II) ions with dipeptides of glycyl-L-phenylalanine and L-phenylalanylglycine were determined in aqueous solution at 25 °C and 0.1 mol dm -3 ionic medium using a combination of potentiometric and spectrophotometric techniques. Sodium perchlorate was used to maintain the ionic strength. The composition of the formed complexes was determined and it was shown that thallium(I) and cadmium (

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The stability constant on complexation of VO2 + with glycine has been determined by a combination of spectrophotometric and potentiometric methods at (25 ± 0.1) °C and ionic strengths ranging from 0.2 to 0.8 mol dm-3 (sodium chloride) in the pH range 1.3−2.5, with high ligand-to-metal ratios. Also, the results are discussed in terms of the effect of ionic strengths on protonation and complexation.

The protonation equilibria of glycyl-glycine and glycyl-valine and their complex formation with thorium(IV) ions in aqueous solution were studied over a wide range of pH values (2 to 12) using a combination of potentiometric, cyclic voltametric, and spectrophotometric methods at constant temperature (25 °C) and constant ionic strength (0.1 mol • dm -3 sodium perchlorate). The redox properties of the species obtained by the cyclic voltametry technique have confirmed the spectrophotometric results. Least-squares regression calculations are consistent with the formation of the ThL 3+ , ThHL 2 3+ , and ThL 2 2+ species for the studied systems, where L represents each peptide. Finally, the species concentrations were plotted at different pH values and discussed.

Equilibria of the reaction of molybdenum (VI) with L-glutamic acid have been studied in aqueous solution in the pH range 2-5 to 9'5, using spectrophotometric and optical rotation methods at constant ionic strength (0"15 mol dm -3 sodium perchlorate) and temperature 25 + 0.t°C. Our studies have shown that glutamic acid forms a mononuclear complex with Mo(VI) of the type MoO3 L2 -at pH 5"5. The stability constant of this complexation and the dissociation constants of L-glutamic acid have been determined.