Do multiheme cytochromes containing close-packed heme groups have a band structure formed from the heme π and π∗ orbitals?

Abstract

Multiheme cytochromes (MHCs) are bacterial electron-transfer proteins. We show from optical spectra and calculations that some of these cytochromes probably contain occupied and unoccupied bands formed from heme π and π∗ orbitals that span the protein. In the fully oxidised proteins, the unoccupied π∗-bands are energetically above the redox-active frontier orbitals, which according to NMR data and calculations, are formed of Fe³⁺ t_2g and porphyrin π-orbitals. These orbitals on different hemes are electronically coupled according to EPR data and calculations, but only weakly so. We suggest a role for the heme bands in the electronic conductivity of single MHCs in bioelectronic junctions that is distinct from the role of the redox-active Fe³⁺ t_2g and porphyrin π-orbitals in physiological electron transfer.