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Stability and Reactivity of the Biologically Active Benzoylethylpyridinium−4−aldoxime cation

The aralkyl derivatives of pyridinium chloride, such as 1-benzoylethylpyridinium chloride (BEP) and 1-benzoylethylpyridinium-4-aldoxime chloride (BEPA-4), are reversible inhibitors of human blood cholinesterase [1]. Thus, they represent possible protectors of this enzyme upon phosphorylation by organophosphorus poisons. In order to recognize the reactive forms of the BEPA-4 molecule, the previously reported UV-Vis characterization of this compound [2] was extended by an additional study of electronic spectra of its ethanol and aqueous solutions. Absorption spectra were analyzed in comparison with the linear summation of absorptions of its individual chromophores: acethophenone and pyridinium-4-aldoxime chloride. The enol form of the BEPA-4 cation was not detected in the examined solutions. An observed fluctuation of spectra in time was tentatively attributed to the compound degradation and will be further investigated by Raman spectroscopy. The reactivity of BEPA-4 in aqueous solution was determined by a kinetic study of its reactions with the aquapentacyanoferrate(II) ion, [Fe(CN)5OH2]3-, as a model of biologically important macromolecules with labile sixth coordination site. The electronic spectrum of the BEPA-4 substituted pentacyanoferrate (II) complex revealed a strong MLCT band at 440 nm. Although BEPA-4 possesses two potential donor groups, the keto and the oxime one, the carbonyl group was found to participate in coordination to the iron centre. Such observation is in accordance with the pre established reactivity of the 1- benzoylethylpyridinium cation itself [3]. The complex formation and dissociation rate constants, along with activation parameters, will be presented and discussed in terms of dissociative mechanism. The equilibrium constant for the complex formation was deduced from kinetic measurements. References 1.M. Škrinjarić-Špoljar, N. Burger and J. Lovrić, J. Enz. Inhib. 14 (1999) 331 2. J. Lovrić, N. Burger, V. Deljac and Z. Mihalić, Croat. Chem. Acta (1999) 123 3. J. Lovrić, B. Foretić and N. Burger, Z. Phys. Chem. 208 (2004) 1289

Kinetics ; Aquapentacyanoferrate(II) ; 1-Benzoylethylpyridinium-4-aldoxime chloride ; Electronic spectra

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