Interfacial and long-range electron transfer at the mineral-microbe interface

The electron transfer mechanisms of multiheme cytochromes were examined with scanning tunneling microscopy (STM). To simulate bacterial metal reduction mediated by proteins in direct contact with mineral surfaces, monolayers of purified decaheme cytochromes from the metal-reducing bacterium Shewanella oneidensis were prepared on Au(111) surfaces. Recombinant tetracysteine sequences were added to two outermembrane decaheme cytochromes (OmcA and MtrC) from S. oneidensis MR-1 to ensure chemical immobilization on Au(111). STM images of the cytochrome monolayers showed good coverage and their shapes/sizes matched that predicted by their respective molecular masses. Current-voltage (I-V) tunneling spectroscopy revealed that OmcA and MtrC exhibit characteristic tunneling spectra. Theoretical modeling of the single-molecule tunneling spectra revealed a distinct tunneling mechanism for each cytochrome: OmcA mediates tunneling current coherently whereas MtrC temporarily traps electrons via orbital-mediated tunneling. These mechanisms suggest a superexchange electron transfer mechanism for OmcA and a redox-specific (i.e. heme-mediated) electron transfer mechanism for MtrC at mineral surfaces during bacterial metal reduction. Additionally, a novel electrochemical STM configuration was designed to measure tunneling current from multiheme cytochromes to hematite (001) surfaces in various electrolyte solutions. Current-distance (I-s) profiles on hematite (001) reveal predictable electric double layer structure that changes with ionic strength. The addition of the small tetraheme cytochrome c (STC) from S. oneidensis on insulated Au tips resulted in modified tunneling profiles that suggest STC significantly modulates the double layer. This observation is relevant to understanding metal reduction in cases where terminal metal-reducing enzymes are unable to come in direct contact with reducible mineral surfaces. Electronic coupling to the mineral surface might therefore be mediated by a localized ion swarm specific to the mineral surface.

Name thesis:degree_name

Ph. D.

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Geosciences

Department dc:contributor.department

Geosciences

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2008

Author dc:creator

• Wigginton, Nicholas Scott

Chair dc:contributor.committeechair

• Hochella, Michael F. Jr.

Committee members dc:contributor.committeemember

• Lower, Brian H.
• Rosso, Kevin M.
• Heflin, James R.

• geomicrobiology
• shewanella
• scanning tunneling microscopy
• hematite
• biogeochemistry

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Dc Identifier Other

etd-04292008-185236

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/27441