Direct measurement of mechanical susceptibility in single-molecule proteins

A protein acquires its function through the specific structure of its polypeptide chain. A mechanical force of only a few tens of picoNewtons is sufficient to disrupt this structure and cause the protein to unfold. Conventional rheology of complex fluids seeks to understand a materials internal structures, energy landscape, and time scales of relaxation by measuring its bulk mechanical response to applied stress and strain as a function of frequency. By analogy, we propose a nano-rheological study of single-molecule proteins. To this end we report on a new AFM design, targeted at high speed force and position controlled measurements of single-molecule proteins. Using this new tool we are able to measure previously inaccessible properties of mechanically stable proteins. Using a broad spectrum force excitation technique we have measured the frequency dependent mechanical susceptibility of both folded and unfolded proteins as a function of applied force. Using these measurements we can begin to characterize the sources of dissipation or ``friction'' present in the protein.