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Kinetic studies on the formation and dissociation of selected (N-substituted pyridinium-4- aldoxime)pentacyanoferrate(II) complexes

Pyrdinium aldoximes are known as pharmacologically important nucleophilic agents. They are used as effective reactivators of human blood acetylcholinesterase inhibited by organophosphates. Their biological function is related to their chemical structure and coordination ability to metal ions. Aquapentacyanoferrate(II) ion, [Fe(CN)5(H2O)]3-, is used as a model of biologically important macromolecules with labile sixth coordination site. Trying to give more insight into the correlation between chemical structure and biological function of N-substituted pyridinium-4- aldoximes, kinetic studies of the complexes formed in a reaction with aquapentacyanoferrate(II) ion were carried out. N-methylpyridinium-4-aldoxime iodide (PAM4-I), N-benzylpyridinium-4-aldoxime chloride (BPA4-Cl) and N-phenacylpyridinium-4- aldoxime chloride (FEPA4-Cl) were used as N- substituted pyridinium-4-aldoximes. The rates of formation and dissociation of octahedral complexes [Fe(CN)5(PAM4)](3-n)-, [Fe(CN)5(BPA4)](3-n)- and [Fe(CN)5(FEPA4)](3-n)- (n = 0 or 1) at 25 °C in buffered aqueous solutions (pH range: 5 – 10) were determined by spectrophotometric measurements. The ionic strength was maintained at 0.1 M. The ratio of the formation and dissociation rate constants i.e. the stability constants were compared with the stability constants obtained by independent equilibrium technique. The activation parameters ΔH‡ and ΔS‡ for the formation and dissociation of (N-substituted pyridinium-4- aldoxime)pentacyanoferrate(II) complexes were determined from the temperature dependence of the corresponding rate constants. The dissociation rates were in good correlation with σ-donor and π- acceptor capability of examined ligands.

N-substituted pyridinium-4-aldoximes; pentacyanoferrate(II) complexes

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