Conducting Atomic Force Microscopy of Protein Wires.

Area of Science:

• Biophysics
• Molecular Electronics
• Materials Science

Background:

• Scanning tunneling microscopy (STM) suggests protein wires exhibit length-dependent electrical resistance.
• Previous attempts to measure protein conductivity in fixed-gap devices have been unsuccessful.
• Consensus tetratricopetide repeat (CTPR) proteins are investigated for their conductive properties.

Purpose of the Study:

• To investigate the electrical conductivity of CTPR8 and CTPR4 proteins using conducting atomic force microscopy (CAFM).
• To determine the contact force requirements for observing electrical conduction in protein films.
• To reconcile discrepancies between STM and fixed-gap junction measurements of protein conductivity.

Main Methods:

• Utilized conducting atomic force microscopy (CAFM) to study CTPR8 (≈8 nm) and CTPR4 (≈4 nm) proteins.
• Applied substantial contact forces (>50 nN) to establish electrical contact.
• Performed scrape-tests to assess probe penetration depth at high forces.

Main Results:

• Significant contact force (>50 nN) was necessary to observe electrical conduction in CTPR8 proteins.
• Scrape-tests revealed approximately 1 nm of film penetration at the required forces.
• Successful, STM-like electrical contacts were rare, occurring in only ~1% of all CAFM measurements.

Conclusions:

• High contact forces are critical for measuring protein conductivity in fixed-junction setups, a factor not apparent in STM studies.
• The low success rate of reliable contacts highlights challenges in protein-based electronic devices.
• STM studies may overestimate the ease of achieving conductive protein contacts due to reporting bias.