Diheme Cytochrome c: Structure-Function Correlation and Effect of Heme-Heme Interactions.

We explore here the structure-function relationship of the diheme cytochrome c using synthetic diheme analogs which serve as a convenient tool to investigate various aspects of Nature's sophisticated design in vitro. A large series of diiron ethane-bridged porphyrin dimers, both in the oxidized and the reduced states, are synthesized and their structural, chemical, and electrochemical properties have been scrutinized. Interestingly, the iron-to-iron nonbonding separation observed in such dihemes ranges from 9.49 to 10.06 Å which is very similar to the separation of 9.4 and 9.9 Å observed in the crystal structures of diheme cytochromes c isolated from Geobacter sulfurreducens and Haemophilus influenza, respectively. The Fe/Fe redox couple in the diheme complex is shifted toward more positive than their monomeric analog. Present study unmasks the electronic structure and properties of diheme centers and also highlights the significance of their structural arrangement and axial ligand orientation, and heme-to-heme separation. The Atoms in Molecules (AIM) analysis suggests long-range attractive dispersion forces between the heme units for the observed structure and properties in dihemes.