Synthetic peptide monolayers: Probing structure, attachment, and hydration conduction with STM

Although the scanning tunneling microscope (STM) has been used in many studies of proteins and other biological molecules, the interpretation of the data has been hampered by a limited understanding of image contrast and tip-sample interactions. It has been recognized that adsorbed water can carry the currents necessary for STM imaging, and it has been speculated that the mechanism of charge transport is based on the exchange of protons along hydrogen-bonded networks. While a water film facilitates the imaging of proteins, it hampers efforts to use STM to probe the electronic structure of redox proteins. The physics of charge transfer in proteins and the intricacies of imaging large molecules with STM can be studied using synthetic proteins. A protein designed with engineerable electron transfer properties, a hydrophilic exterior and mechanisms for attachment to surfaces has been studied using both STM and fixed-electrode conductivity measurements. The effects of immobilization strategy on image quality were investigated by preparing monolayers using self-assembly, Langmuir-Blodgett and monolayer insertion techniques and imaging these films in both humid air and vacuum. The molecules were easily imaged in humid air due to the adsorbed water film, but dehydration in vacuum introduced a conductance gap which prevented the proteins from being imaged within a wide voltage range. The molecules were subjected to large forces when the tip scanned over them without retracting from the surface and peptides were displaced in ways which reflected the strength of their attachment to the surface. In the course of these studies, new STM imaging phenomena have been observed including meniscus-limited lateral resolution, and intermittent imaging of covalently attached molecules. The conductivity of protein films deposited between fixed electrodes depended strongly on hydration, and the experimental arrangement provides a platform for further study of the mechanism of conduction within these films