The formation constants of the species formed in the systems H+ + Mo(VI) + aspartic and glutamic acids and H+ + aspartic and glutamic acids have been determined in different aqueous solutions of methanol [(0 to 40) % v/v] at 25 °C and constant ionic strength (0.1 mol·dm−3 sodium perchlorate), using a combination of spectrophotometric and potentiometric techniques. The composition of the complexes was determined by the continuous variations method. It was shown that molybdenum(VI) forms a mononuclear 1:1 complex with the amino acids of the type MoO3L2− at pH 5.8. The protonation and formation constants in various media were analyzed in terms of Kamlet and Taft’s parameters. Single-parameter correlations of the formation constant, K S , versus α (hydrogen-bond donor acidity), β (hydrogen-bond accepter basicity), and π* (dipolarity/polarizability) are poor in all solutions, but multiparameter correlations represent significant improvements with regard to the single-parameter models. Linear correlation is observed when the experimental log K S values are plotted versus the calculated ones, while all the Kamlet and Taft’s parameters are considered. Finally, the results are discussed in terms of the effect of solvent on protonation and complexation.

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The interaction of guanosine 5 -monophosphate, GMP, with the thallium(I) ion was studied by UV-vis and potentiometric titration methods and 31 P NMR spectroscopy. Both NMR spectra and UV-vis titration data have shown that GMP coordinates via guanine to the thallium(I) ion in the pH range 1.5-10. Our study of the system Tl(I) + GMP was performed in water-methanol mixtures with different volume ratios of methanol. The complexation equilibrium in the pH range of study led to the following mononuclear species: TlH 2 (GMP) + , TlH(GMP) and Tl(GMP) − , where (GMP) 2− represents the fully dissociated ligand. The formation constants of the species were calculated in the various media at constant temperature (25 • C) and constant ionic strength of sodium perchlorate (0.1 mol dm −3 ) using a suitable computer program. The formation constants were analyzed in terms of Kamlet and Taft's parameters. A single-parameter correlation of the formation constants, β 121 , β 111 and β 101 vs α (hydrogen-bond donor acidity), β (hydrogen-bond acceptor basicity) and for π * (dipolarity/polarizability) are relatively poor in all solutions, but multi-parameter correlations represent significant improvements with regard to the single-parameter model. In this work, we have also used the normalized polarity parameter, E T N , alone and in combination with some of the Kamlet-Taft parameters to find a better correlation of the formation constants in different methanol-water mixtures.

The formation constants of species formed in the systems H + + Mo(VI) + aspartic acid and H + + aspartic acid have been determined in aqueous solution for 4 e pH e 9 at 25 °C and different ionic strengths ranging from (0.1 to 0.8) mol dm -3 (NaClO 4 ), using polarimetric, spectrophotometric, and potentiometric techniques. The composition of the complex was determined by the continuous variations method. It was shown that molybdenum(VI) forms a mononuclear 1:1 complex with aspartic acid of the type MoO 3 L -2 at pH ) 5.8. The dependence of the protonation of aspartic acid and the stability constant of the complex on ionic strength is described by a Debye-Huckel-type equation, and finally the results have been compared with data previously reported.

The formation constants of the species formed in the systems H+ + dioxovanadium(V) + penicillamine and H+ + penicillamine have been determined in aqueous solutions of methanol at 25 °C and constant ionic strength, using spectrophotometric and potentiometric techniques. It was shown that dioxovanadium(V) forms two mononuclear 1:1 and 1:2 complexes with penicillamine. The formation constants in various media were analyzed in terms of Kamlet and Taft's parameters. Single-parameter correlation of the formation constants, β121 and β142, versus α (hydrogen-bond donor acidity) and β (hydrogen-bond acceptor basicity) is poor in all solutions, except for π* (dipolarity/polarizability), but multiparameter correlation represents significant improvement with regard to the single-parameter models. Linear relationships are observed when log β121 and log β142 are plotted versus π*. Finally, the results are discussed in terms of the effect of the solvent on complexation.

The protonation constants of tryptophan (K 1 and K 2) were determined in binary mixtures of water with methanol, ethanol, and 1-propanol containing (0, 10, 20, 30, 40, 50, 60, 70, and 80) % (v/v) using a potentiometric method at 25 °C and constant ionic strength (0.1 mol·dm−3 sodium perchlorate). The autoprotolysis constant values (K ap) of the media were also determined in the same binary mixtures. The protonation constants of tryptophan and the autoprotolysis constant of the medium in different binary mixtures were analyzed in terms of Kamlet, Abboud, and Taft (KAT) parameters. Single-parameter correlations of the constants versus α (hydrogen-bond donor acidity), β (hydrogen-bond acceptor basicity), and π* (dipolarity/polarizability) are poor in all solutions. Multiparameter correlations show better results, but dual-parameter correlations represent significant improvements with regard to the single- and multiparameter models. Linear correlation is observed when the experimental log K ap, log K 1, and log K 2 values are plotted versus the calculated ones, while the KAT parameters are considered. Finally, the results are discussed in terms of the effect of the solvent on the protonation and autoprotolysis constants.

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The protonation constants of phenylalanine and the stability constants of the complexes between dioxovanadium(V) ion and phenylalanine have been determined spectrophotometrically, in the temperature range 15–35°C and ionic strength ranging from 0.1 to 1.5 mol dm−3 sodium perchlorate as a background salt, in the pH range 1.5–10.5, with high ligand to metal ratios. The values of enthalpy and entropy changes based on these formation constants were calculated. The dependence of protonation and the stability constants on ionic strength are described by a Debye-Huckel type equation.