The porphyrin ring rather than the metal ion dictates long-range electron transport across proteins suggesting coherence-assisted mechanism

Abstract

The fundamental biological process of electron transfer (ET) takes place across proteins with common ET pathways of several nanometers. Recent discoveries push this limit and show long-range extracellular ET over several micrometers. Here, we aim in deciphering how protein-bound intramolecular cofactors can facilitate such long-range ET. In contrast to natural systems, our protein-based platform enables us to modulate important factors associated with ET in a facile manner, such as the type of the cofactor and its quantity within the protein. We choose here the biologically relevant protoporphyrin molecule as the electron mediator. Unlike natural systems having only Fe-containing protoporphyrins, i.e., heme, as electron mediators, we use here porphyrins with different metal centers, or lacking a metal center. We show that the metal redox center has no role in ET and that ET is mediated solely by the conjugated backbone of the molecule. We further discuss several ET mechanisms, accounting to our observations with possible contribution of coherent processes. Our findings contribute to our understanding of the participation of heme molecules in long-range biological ET.

The Role of Metal Ions in the Electron Transport through Azurin-Based Junctions

“…In the case of the Zn-azurin and for the Apo, part of the contribution to the transmission at the Fermi level seems indeed to be provided by these residues as well. As an aside, it is worth mentioning that recent experiments on protoporphyrins have revealed that the metal redox center has no role in the electron transfer, which was instead found to be mediated solely by the conjugated backbone of the molecule [68]. Our results are based on DFT, which is known to be affected by uncertainties concerning the size of the HOMO-LUMO gap because of self-interaction errors in the standard exchange-correlation functionals and image charge effects [69][70][71][72][73][74].…”

Long-Range Charge Transport in Homogeneous and Alternating-Rigidity Chains

“…Long-range charge transport has been reported in the G. Sulfurreducens bacteria up to a micrometer scale 54 . Studies have shown that the geobacteria's long-range transport cannot be replicated by solely using hopping or coherent transport mechanisms 55 .…”

“…Studies have shown that the geobacteria's long-range transport cannot be replicated by solely using hopping or coherent transport mechanisms 55 . Newly-proposed mechanisms have been suggested based on the alternating rigidity of the protein conducting the charge in biological nanowires 54,56 . Inspired by biological nanowires, in this section, we explore the use of probe "blocks" to mimic modular nanowires of alternating rigidity and study the resulting effect on the resistance of the system.…”

“…Refs. [54][55][56] , and references therein. It was argued that charge transport in protein nanowires cannot be satisfactory described by either a metallic-like conduction, or a completely classical-diffusive mechanism.…”

“…Rather, it was argued that a mixed coherent-incoherent mechanism takes place, with charge delocalization over "islands" 55 . This conduction process could be facilitated by the varying rigidity of the molecule 54,56 .…”