Redox hopping

A charge-transport mechanism in which electrons move through a chain of discrete redox cofactors — such as the stacked hemes of a multi-heme cytochrome — by a series of thermally activated, non-adiabatic electron-transfer steps between adjacent sites, each step governed by Marcus theory. Charge is localized on one cofactor and hops to the next; it is mechanistically distinct from band-like or metallic conduction, where charge is delocalized across the material rather than localized-and-hopping.

Matters to this vault as the physical mechanism the Geobacter nanowire dispute turns on. The cytochrome-polymer model holds that the filaments conduct by heme-to-heme hopping (claim-geobacter-conductive-filaments-are-omcs-cytochrome-polymers); the 2024 electrical-consistency challenge computes an upper bound on how much current that hopping can carry and finds the measured conductivities far exceed it (claim-2024-paper-disputes-geobacter-cytochrome-nanowire-conductivity, claim-guberman-pfeffer-0-14-pa-ceiling-is-computed-not-measured). It is the foil to the non-redox, metal-like conduction reported for cable bacteria (claim-cable-bacteria-conduct-through-nickel-protein-wire) — two different answers to how a protein wire can move an electron. Both sit under extracellular electron transfer.

References

• claim-geobacter-conductive-filaments-are-omcs-cytochrome-polymers
• claim-2024-paper-disputes-geobacter-cytochrome-nanowire-conductivity
• claim-guberman-pfeffer-0-14-pa-ceiling-is-computed-not-measured
• claim-guberman-pfeffer-blames-measurement-not-cytochrome-identity
• claim-cable-bacteria-conduct-through-nickel-protein-wire (contrasting non-redox, metallic mechanism)