Abstract
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.
Highlights
Captopril mole ratio can be obtained depending to the experimental conditions
It is applied successfully for treatment of high blood pressure disease in many patients whose hypertension could not be controlled by other available drugs[5,6,7,8,9]. It inhibits the active sites of zinc glycoprotein, the angiotension I converting enzyme (ACE), blocking the conversion of angiotension I to angiotension II, whose levels are elevated in patients
It is interesting to note that the composition nature of the binary mixture has a distinct effect on the protonation constants
Summary
Captopril mole ratio can be obtained depending to the experimental conditions. In acidic pH, the ML complex is more stable, whereas, in the pH range 6.0-8.2, the ML2 complex is the main species[12]. CPL protonation constants and CPL-Cd (II) and CPLPb (II) metal complexes in water and binary methanolwater mixtures were calculated using the program If the dielectric constant and the solvating ability of the solvent are decreased, more energy will be required to separate the cation and anion and the extent of dissociation of the acid will be lowered.
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