Conducting Atomic Force Microscopy of Protein Wires

Scanning tunneling microscopy (STM) studies have shown that protein wires based on the consensus tetratricopetide repeat (CTPR) conduct with an electrical resistance proportional to length of the protein, consistent with long-range hopping transport along the long axis of the protein. However, the attempts to measure similar currents across proteins mounted in fixed-gap devices have not been successful. Here, the study reports conducting atomic force microscopy studies of CTPR8 and CTPR4, ≈8 and 4 nm in length, finding that substantial contact force (>50 nN) is required to observe conduction in CTPR8. Scrape-tests indicate that the probe penetrates the film by ≈1 nm at this force. Importantly, electrical contacts that resemble the STM data are rare-approximately 1% of all contacts. STM reports only successful contacts, so it does not predict statistically how the proteins will behave in a fixed junction.