Ferriprotoporphyrin IX Oxygen Activation/Insertion Reactions: The Nature of the Interaction between Ferriprotoporphyrin IX and Aniline (Substrate)

Add time:08/21/2019    Source:sciencedirect.com

The interaction between aniline and ferriprotoporphyrin IX in alkaline solution has been investigated using pyridine and the N-methyl pyridinium ion as site-specific inhibitors of oxygen activation. Pyridine inhibits oxygen activation in a noncompetitive manner with respect to aniline (K1 = 1.24 mol −1 dm3 at 30°C) while the N-methyl pyridinium ion inhibited in a manner consistent with two sites for aniline binding, only one of which was competitively inhibited (K1 = 317mol-l dm3 at 30°C). A comprehensive reinvestigation of the interaction of pyridine and N-methyl pyridinium ions with alkaline ferriprotoporphyrin IX has shown that two molecules of each ligand bind per hemin dimer in a strongly cooperative manner. The association constant for the first pyridinium ion bound is K1a = 176 mol−1 dm3 at 30°C, while that for the first pyridine molecule bound is K1a = 0.580 mol−1 dm3 at 30°C; these are both close to the observed inhibition association constants (K1). Thermodynamic parameters for the interactions have been evaluated and compared to previous literature values. On the basis of these results a model is proposed for aniline interaction with the ferriprotoporphyrin dimer IX which involves the binding of two molecules of aniline to the ferriprotoporphyrin IX tetrapyrrole ring system by planar π bonding interactions with the rings having the propionate groups attached.

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