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Equilibrium and kinetic studies of the mono- and bis- pyridinium-4-aldoxime pentacyanoferrate(II) complexes. Spectrophotometric evaluation of the equilibrium constants.

Pyridinium aldoximes are known as pharmacologically important nucleophilic agents that are effective antidotes against poisoning by organophosphates (pesticides, chemical warfare nerve agents, drugs used in treatment of cholinergic disorders) which cause long-term inhibition of acetylcholinesterase (AChE, E.C.3.1.1.7). Their biological functions and metabolizing mechanisms in living systems are usually related to their structure and coordination ability to the metal ions. Moreover, versatile pharmacological effects of such compounds might be related to the amount of its free or complexed form. A comparative study of the coordination properties of the biologically active mono- and bis- pyridinium- 4-aldoximes toward the aquapentacyanoferrate(II) ion, as a model of important biomolecules, were performed. As a typical representatives of mono-pyridinium-4- aldoxime ligands the 1-methylpyridinium-4- aldoxime iodide (PAM4-I) and 1- benzylpyridinium-4-aldoxime chloride (BPA4-Cl) were used, while the ligands of the bis- pyridinium type were 1, 1’-bis(pyridinium-4- aldoxime)trimethylene dibromide (TMB4-2Br) and 1, 1’-bis(pyridinium-4-aldoxime)oxydimethylene dichloride (TOXO-2Cl, Toxogonin®). Their reactions with the aquapentacyanoferrate(II) ion were studied in aqueous media at pH=6, 25 C and 0.1 M ionic strength by spectrophotometric techniques. Equilibrium constants have been evaluated both by a non-linear fitting of the experimental data to the hyperbolic equation valid for the mole ratio method and with the HYPERQUAD suite of programs. Electronic spectra and the energies of the metal to ligand charge transfer bands of the formed pyridinium-4- aldoxime pentacyanoferrates(II) have been correlated with the sigma-donating and pi- accepting capabilities of the examined ligands. A comparative analysis of the kinetic properties i.e. lability of the formed pentacyanoferrates(II) and the magnitude of pi- back bonding were preformed by kinetic studies of a pyridinium-4-aldoxime substitution from the pentacyanoferrate(II) complex as a function of pH. Consistent with a dissociative mechanism, the rate of ligand dissociation was strongly dependent on the nature of the leaving pyridinium-4-aldoxime. The relative magnitude of the dissociation rate constants of the examined complexes have been interpreted in terms of both electronic and steric effects.

equilibrium constants ; kinetics ; spectrophotometry

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