Protamine Promotes Direct Electron Transfer Between Shewanella oneidensis Cells and Carbon Nanomaterials in Bacterial Biocomposites

Abstract

In this work, we studied the direct electron transfer reactions occurring in a biocomposite resulting from the self-assembly of Shewanella oneidensis MR-1 cells with different carbon nanomaterials (multi-walled carbon nanotubes, graphene oxide, Ketjen black or ordered mesoporous carbon) and protamine. The system has been studied with dynamic light scattering, scanning electron microscopy, epifluorescence microscopy and electrochemistry. Although protamine is known to have an antimicrobial activity, the interaction with carbon nanomaterials largely limits this effect. Formate and lactate oxidation and fumarate reduction have been studied. In the presence of 50 mM formate or 50 mM fumarate, the anodic current was lower than the cathodic one with the biocomposite prepared with protamine, while it was found similar with the biocomposite prepared with cytochromes c 1 or c 3 DvH, suggesting different electron transfer pathways. The redox potential observed for fumarate reduction (half-wave potential) was found between À 0.1 V and À 0.2 V vs. SHE with an onset potential close to 0 V vs. SHE with MWCNT, i. e. very close to 0.030 V vs. SHE, the formal redox potential for fumarate/succinate interconversion. A maximum current density in the range of 1 A m À 2 was reached for fumarate reduction. These data highlight a new way to promote direct electron transfer reactions in electroactive artificial biofilms.[a] Dr.

Challenges and applications of nitrate-reducing microbial biocathodes

“…It helps attaching cells to the solid electrode and therefore shortens the gap between cell membrane and the electrode surface for direct transfer. Non-electroactive proteins such as protamine, can also promote electron transfer, probably by influencing the architecture of the biofilm-like material [57]. In case of indirect electron transfer, the polymeric substances are beneficial because it accumulates electron shuttles between bacteria and electron donors or acceptors [58].…”