Chelation Study of Captopril with Cd²⁺ and Pb²⁺ Ions

The protonation constants of Captopril, (1-(3-mercapto-2-(S)-methyl-1-oxopropyl)-S(L) proline, (CPL) and stabilities of its two divalent metal ions Cd(II) and Pb(II) were determined potentiometrically in two different metal to ligand ratio 1:1 and 1:2 systems and in water and methanol-water binary mixtures using the computer Best program. Two protonation constants were obtained which were assigned to the carboxylic and thiol groups. All protonation and stability constants increased with decreasing the dielectric constant on going from the pure water to the binary mixtures. An excellent similarity in stabilities of studied metal ions strongly suggests that both metal ions are coordinated by CPL in a same manner through the thiol group.