Solvent Effects on Protonation and Complexation of Cysteine and Thallium(I) in Different Aqueous Solutions of Methanol

The protonation equilibria of cysteine and its complex formation with the Tl(I) ion were studied over a wide range of pH (1 to 11), using a combination of spectrophotometric and potentiometric methods at constant temperature, 25 °C, different methanol−water mixtures, (0 to 40) % (v/v), and constant ionic strength (0.1 mol·dm−3 sodium perchlorate). Least-squares regression calculations are consistent with the formation of TlH2L+, TlHL, and TlL− species, where L2− represents the fully dissociated ligand. The protonation of cysteine and the formation constants of the formed complexes in different media were analyzed in terms of Kamlet, Abboud, and Taft (KAT) parameters. Single-parameter correlations of the formation constants versus α (hydrogen-bond donor acidity), β (hydrogen-bond acceptor basicity), and for π* (dipolarity/polarizability) are relatively poor in all solutions, but multiparameter correlations represent significant improvements with regard to the single-parameter model. Linear correlation is o...