| The application of rare-earth fertilizers in the agriculture was firstly suggested with Chinese scientists. A lot of investigations about the biological mechanism showed that the rare-earth fertilizers in the certain concentration range could increase the biological activities of some enzymes, such as peroxidase (POD). However, its chemical mechanism is still not clear and the investigation in this area is very few. Because of the high chemical activity, the structural complexity and the conformational flexibility of enzymes, so that it is difficult to study of the interaction of rare earth ions with enzymes. Therefore, in this thesis, as the model of POD, microperoxidase-8 (MP-8), whose active center and bioactivity are similar to POD, was selected and La3+ and Eu3+ were used as the typical rare earth ions to investigate the interaction of MP-8 with La3+ and Eu3+ using electrochemical method and UV-vis absorption spectroscopy. It would lay a foundation of the further investigation of the interaction of rare earth ions and their complexes with enzymes.The main results obtained are as follows: 1. The interaction mechanism of La3+ or Eu3+ with MP-8(1) The UV-vis absorption spectroscopic results indicated that after La3+ or Eu3+ interact with MP-8, the peak position of the Soret band of MP-8 shifts to the red direction and the absorbance decreases. It means that because of the large exposed extent and the negative charge of the carbonyl oxygen of the metacetonic acid group in the heme group of the MP-8 molecule, La3+ or Eu3+ coordinates firstly and strongly with the carbonyl oxygen of the metacetonic acid group in the heme group. It leads to the increase in the non-planarity of the porphyrin group in the heme group, the decrease in the energy and the probability for the transition. La3+ or Eu3+ interact weakly with the carbonyl oxygen of the amido group in the peptide chain of the MP-8 molecule because the exposed extent of the carbonyl oxygen in the peptide chain is small due to the hydrogen bond in the peptide chain. Furthermore, the carbonyl oxygen in the peptide chain is far away from the porphyrin group in the heme group. Therefore, this interaction affects the structure of the prophyrin group slightly. In addition, the relationship curve of themolar ratio of La3+ or Eu3+ and MP-8 with the absorbance of the Soret band demonstrated that two La3+ or Eu3+ ions coordinate with the carbonyl oxygen of the metacetonic acid group in the heme group of one MP-8 molecule. (2) Comparing the cyclic voltammograms of the Eu3+ solution and the Eu3+ + MP-8 solution with the molar ratio of 10, it was found that the area of cathodic peak of Eu3+ for the Eu3+ + MP-8 solution is 20% less than that for the Eu3+ solution, indicating about 20% Eu3+ has strongly coordinated with the oxygen containing group in the MP-8 molecule. It further demonstrated that two Eu3+ ions coordinate with the carbonyl oxygen of the metacetonic acid group in the heme group of one MP-8 molecule. Thus, their electrochemical reaction could not be proceeded in the original potential range.2. The effect of La3+ or Eu3+ on the electrochemical behavior and electrocatalytic activity of MP-8After La3+ or Eu3+ is added into the MP-8 solution, the difference of the anodic and cathodic peak potentials, A Ep of MP-8 decreases and the peak currents increases, indicating that La3+ or Eu3+ can promote the electrochemical reaction of MP-8. In addition, the La3+ can increase in the electrocatalytic activity for the reduction of O2, because when La3+ interacts with MP-8, La3+ would coordinate strongly with the carbonyl oxygen of the metacetonic acid group in the heme group of the MP-8 molecule. It would increase the non-planarity of the porphyrin group in the heme group and the exposed extent of Fe of the porphyrin group in the heme group, leading to the increase in the reversibility of the electrochemical reaction and the electrocatalytic activity of MP-8.3. The effect of the aggregation extent of the MP-8 molecules on the interaction of La3+ or Eu3+ with MP... |
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