In this paper, the interaction mechanism between La3+ and microperoxidase-11 (MP-11) in the imitated physiological solution was investigated with the electrochemical and spectroscopic methods. It was found that when the molar ratio of La3+, and MP-11 is low, such as 2, La3+ can coordinate with oxygen in the propionic acid group of the heme group in the MP-11 molecule, forming the La-MP-11 complexes and leading to the increase in the non-planarity of the porphyrin cycle in the heme group and then the increase in the extent of exposure of the electrochemically active center, Fe(I I I) in the porphyrin cycle of the heme group. The increase in the extent of exposure of the electrochemically active center, Fe(III) in the porphyrin cycle of the heme group would increase the reversibility of the electrochemical reaction of the La-MP-11 complexes and its electrocatalytic activity for the reduction of H2O2. The results of the chromatographic analysis demonstrated that the average molar ratio of La3+ and MP-11 in the La-MP-11 complexes is 1.62. When the molar ratio of La3+ and MP-11 is high, such as 3, La3+ would shear some amino acid residues of the peptide of MP-11. Therefore, many La3+ ions can bind to the oxygen- and/or nitrogen-containing groups in the sheared amino acid residues except coordinating with the sheared and non-sheared MP-11 molecules. The results of the chromatographic analysis demonstrated that the complexes of La3+ with different microperoxiadases were formed and the average molar ratio of La3+ and microperoxidase in the La microperoxidase complexes is 1.16. Because the microperoxidase molecule with the short peptide is easy to form the dimmer due to the pi-pi interaction between the heme groups and the dimmer formed due to the pi-pi interaction between the heme groups would increase the planarity, the planarity of the La microperoxidase complexes with the molar ratio of 1.16 is better than and the extent of exposure of Fe(III) in the porphyrin cycle of the heme group is less than that of the La-MP-11 complexes with the molar ratio of 1.62. Therefore, the reversibility of the electrochemical reaction of the La-microperoxidase complexes with the molar ratio of 1.16 and its electrocatalytic activity for the reduction of H2O2 is lower than that of the La-MP-11 complexes with the molar ratio of 1.62. (c) 2006 Elsevier B.V. All rights reserved.