Guberman-Pfeffer's 0.14 pA/filament redox-current ceiling is a Marcus-theory computation, not a measured quantity

electromicrobiologygeobactermicrobial-nanowirescytochromemarcus-theoryredox-conductionscientific-controversy

Guberman-Pfeffer's 2024 challenge to the reported electrical conductivities of Geobacter cytochrome nanowires (claim-2024-paper-disputes-geobacter-cytochrome-nanowire-conductivity) is built on a comparison against a specific figure — a "biologically reasonable maximum redox current of 0.14 pA/filament." That ceiling is not an independent experimental measurement. It is computed by the author from generic, protein-independent heme-to-heme electron-transfer rates.

The paper states: "Theory and experiment both suggest that 1 × 10⁸ and 1 × 10⁹ s⁻¹ are protein-independent, order-of-magnitude estimates for ground-state heme-to-heme electron transfer in T- and slip-stacked geometries, respectively... Using these generic rates, the experimentally characterized 300 nm-long filaments are predicted to support protein-limited currents of ~0.14 pA." The rates rest on non-adiabatic Marcus theory applied to the heme packing geometries the cryo-EM structures resolve (claim-geobacter-conductive-filaments-are-omcs-cytochrome-polymers), and are cross-checked against independent kinetic analyses of ultrafast transient-absorption measurements on photosensitized variants of two unrelated multi-heme proteins from Shewanella oneidensis — the small tetraheme cytochrome (STC; van Wonderen et al. 2019) and the metal-reducing cytochrome type C (MtrC; van Wonderen et al. 2021) — which the paper reports are in "excellent agreement." The ceiling is therefore claimed to be corroborated across independent structural and kinetic sources rather than being a single lab's number.

This is what gives the argument its distinctive force. Because the ceiling is a computed upper bound on redox hopping, it does not depend on what the filament is made of — only on whether heme-to-heme hopping physics can carry the current someone measured. It is a different kind of weapon from a better micrograph, and it is what keeps the magnitude and mechanism of Geobacter conduction contested even though the filaments' structural identity is well resolved.