Exploring the Electronic and Mechanical Properties of Protein Using Conducting Atomic Force Microscopy

In interfacing man-made electronic components with specifically folded biomacromolecules, the
perturbative effects of junction structure on any signal generated should be considered. We report herein
on the electron-transfer characteristics of the blue copper metalloprotein, azurin, as characterized at a
refined level by conducting atomic force microscopy (C-AFM). Specifically, the modulation of currentvoltage (I-V) behavior with compressional force has been examined. In the absence of assignable resonant
electron tunneling within the confined bias region, from -1 to 1 V, the I-V behavior was analyzed with a
modified Simmons formula. To interpret the variation of tunneling barrier height and barrier length obtained
by fitting with the modified Simmons formula, an atom packing density model associated with protein
mechanical deformation was proposed and simulated by molecular dynamics. The barrier heights determined
at the minimum forces necessary for stable electrical contact correlate reasonably well with those estimated
from bulk biophysical (electroanalytical and photochemical) experiments previously reported. At higher forces,
the tunnel barrier decreases to fall within the range observed with saturated organic systems. Molecular
dynamics simulations revealed changes in secondary structure and atomic density of the protein with respect
to compression. At low compression, where transport measurements are made, secondary structure is
retained, and atomic packing density is observed to increase linearly with force. These predictions, and
those made at higher compression, are consistent with both experimentally observed modulations of
tunneling barrier height with applied force and the applicability of the atom packing density model of electron
tunneling in proteins to molecular-level analyses.