Gated electron transfer reactions of truncated hemoglobin from Bacillus subtilis differently orientated on SAM-modified electrodes

Abstract

Electron transfer (ET) reactions of truncated hemoglobin from Bacillus subtilis (trHb-Bs) are suggested to be implicated in biological redox signalling and actuating processes that may be used in artificial environment-sensing bioelectronic devices. Here, kinetics of ET in trHb-Bs covalently attached via its surface amino acid residues either to COOH- or NH₂-terminated (CH₂)_2–16 alkanethiol SAM assembled on gold are shown to depend on the alkanethiol length and functionalization, not being limited by electron tunnelling through the SAMs but gated by ET preceding reactions due to conformational changes in the heme active site/at the interface. ET gating was sensitive to the properties of SAMs that trHb-Bs interacted with. The ET rate constant k_s for a 1e⁻/H⁺ reaction between the SAM-modified electrode and heme of trHb-Bs was 789 and 110 s⁻¹ after extrapolation to a zero length SAM, while the formal redox potential shifted 142 and 31 mV, for NH₂- and COOH-terminated SAMs, respectively. Such domain-specific sensitivity and responsivity of redox reactions in trHb-Bs may be of immediate biological relevance and suggest the existence of bioelectronic regulative mechanisms of ET proceeding in vivo at the protein–protein charged interfaces that modulate the protein reactivity in biological redox signalling and actuating events.