Effect of peripheral trifluoromethyl groups in artificial iron porphycene cofactor on ligand binding properties of myoglobin

An iron porphycene, a structural isomer of iron porphyrin, with trifluromethyl groups at the peripheral position of the framework was incorporated into sperm whale apomyoglobin. The prepared myoglobin shows the higher O₂ affinity than the native protein. However, the oxygen affinity of the reconstituted myoglobin is lower than that of the myoglobin having an iron porphycene without trifluromethyl groups, which is mainly originated from the enhancement of the O₂ dissociation. The CO affinity of the myoglobin with the trifluoromethylated iron porphycene is similar to that observed for the reference protein having the iron porphycene without trifluoromethyl groups, although their C-O stretching frequencies are significantly different. The relationship between the electronic states of the porphycene ring and the ligand bindings is discussed.