Scientific publications

The coordination of zinc(II)tetraphenylporphine with pyridine N-oxides in methylene chloride was studied by electron spectroscopy. Linear correlations are observed between the logarithms of the stability constants of complexes, the shifts of the maximum absorption bands of Zn-TPP in electronic spectra during complexation, the basicity of the ligand, and the σ-constants of the substituent Gammet. In the case of pyridines in CHCl3, CCl4, and 1,2-dichloroethane, complexation is isoequilibrium compensation, but with N-oxides of pyridines, the replacement of chloroform (isoenthalpic process) with methylene chloride (isoequilibrium anticompensation process) leads to drastic changes in the thermodynamics of coordination. The change of the isoentalpic behavior of pyridine N-oxides in chloroform to the anticompensation behavior in methylene chloride in coordination with Zn-TPP may indicate a change in the mechanism of complexation. It is suggested that this may be due to a change in the type of hybridization of the oxygen atom of the N→O group of the ligand depending on the polarity of the solvent. In favor of this hypothesis is the fact that coordination anilines with Zn-TPP in chloroform (as in the case of N-oxides is possible rehybridization of nitrogen atom of the amino group) is also isoenthalpic process, but in the methylene chloride becomes isoequilibrium unlike the N-oxides of pyridines with positive value β. We found good linear correlations between the rate constants of horseradish peroxidase reactions in aqueous solutions with anilines and the stability constants of Zn-TPP complexes with the same ligands in chloroform. We hope that this work will help clarify important patterns in this area of biochemistry, in particular thermodynamic features of peroxidase functioning.