Synthesis of a Series of Zinc Porphyrins and Spectroscopic Changes upon Coordination Reaction with Imidazole Derivatives

Three metal-free porphyrins modified with Boc-L-threonine and their zinc analogs were synthesized and characterized by elemental analysis, H-1 NMR, UV/vis, and fluorescence spectroscopies. The binding of imidazole derivatives to these zinc porphyrins was studied, with emphasis on the binding mechanism in CH2Cl2 solution, by means of UV/vis spectroscopy and quantum chemical methods. Both experimental results and theoretical calculations showed that a coordination reaction occurred between the zinc porphyrins and imidazole derivatives. The association constants between the zinc porphyrins and imidazole derivatives decreased in the order N-MeIm>Im. Increasing the temperature disfavored the interaction. Thermodynamic parameters calculated by the van't Hoff equation showed that the driving force for the reaction was the enthalpy change. The fluorescence changes associated with the interaction between the zinc porphyrins and imidazole derivatives were also studied by fluorescence spectroscopy. The experimental results showed significant quenching between the various zinc porphyrins and imidazole derivatives.