Heme Hopping Falls Short: What Explains Anti-Arrhenius Conductivity in a Multi-heme Cytochrome Nanowire?

A helical homopolymer of the outer-membrane cytochrome type S (OmcS) was proposed to electrically connect a common soil bacterium, Geobacter sulfurreducens, with minerals and other microbes for biogeochemically important processes. OmcS exhibits a surprising rise in conductivity upon cooling from 300 to 270 K that has recently been attributed to a restructuring of H-bonds, which in turn modulates heme redox potentials. This proposal is more thoroughly examine herein by (1) analyzing H-bonding at 13 temperatures encompassing the entire experimental range; (2) computing redox potentials with quantum mechanics/molecular mechanics for 10-times more (3000) configurations sampled from 3-times longer (2 s) molecular dynamics, as well as 3 s of constant redox and pH molecular dynamics; and (3) modeling redox conduction with both single-particle diffusion and multi-particle flux kinetic schemes. Upon cooling by 30 K, the connectivity of the intra-protein H-bonding network was highly (86%) similar. An increase in the density and static dielectric constant of the filaments hydration shell caused a -0.002 V/K shift in heme redox potentials, and a factor of 2 decrease in charge mobility. Revision of a too-far negative redox potential in prior work (-0.521 V; expected = -0.350 - +0.150 V; new Calc. = -0.214 V vs. SHE) caused the mobility to be greater at high versus low temperature, opposite to the original prediction. These solution-phase redox conduction models failed to reproduce the experimental conductivity of electrode-absorbed, partially dehydrated, and possibly aggregated OmcS filaments. Some improvement was seen by neglecting reorganization energy from the solvent to model dehydration. Correct modeling of the physical state is suggested to be a prerequisite for reaching a verdict on the operative charge transport mechanism and the molecular basis of its temperature response.