Abstract: The interaction mechanism of microperoxidase-8 (MP-8) and Eu³⁺ was determined by using UV-Vis absorption spectroscopic and electrochemical techniques. The study of the UV-Vis absorption spectra indicated that when Eu³⁺ is added into the MP-8 solution, the Sored band shifts to the red direction and its absorbance decreases. The difference between the absorbance of the Soret band of the MP-8 solution without and with Eu³⁺ increases linearly with increasing the molar ratio of Eu³⁺ to MP-8 when the molar ratio is less than 3. When the molar ratio is larger than 3, the difference decreases with increasing the molar ratio. Their extension lines intersect at the molar ratio of 2. The electrochemical study showed that after MP-8 was added into the Eu³⁺ solution with the molar ratio of 10, the area of the anodic or cathodic peak of Eu³⁺ decreases about 20% comparing with that before adding MP-8. All the UV-Vis absorption spectroscopic and elelctrochemical results illustrated that Eu³⁺ ions would coordinate strongly with carboxy oxygen of two propionic acid groups of the heme group in an MP-8 molecule, leading to an increase in the non-planarity of the porphyrin ring, greater exposure degree of the heme group, and better reversibility of the electrochemical reaction of the MP-8 molecule. The excessive Eu³⁺ ions weakly interact with the oxygen-contained groups in the peptide chain, resulting in a slight alteration of the structure of the prophyrin group.

Key words: Microperoxidase-8, Eu³⁺, UV-Vis absorption spectroscopy, Cyclic voltammetry