Transverse Dynamic Force Spectroscopy: A Novel Approach to Determining the Complex Stiffness of a Single Molecule

Piconewton force sensitivity and angstrom position control offered by instruments such as atomic force microscopy (AFM) and optical tweezers have enabled the force−extension response of single molecules to be investigated. However, to fully investigate the dynamic and energetic properties of a single molecule, it is necessary to detect both conservative (elastic) and dissipative (viscous) components of the extension response. We present a transverse dynamic force microscope (TDFM) capable of measuring this complex quantity. This new force spectroscopy technique offers true control over the tip−surface distance revealing information not accessible by conventional dc atomic force spectroscopy. Results are presented for the force extension response of a single polysaccharide molecule. To the authors knowledge, this is the first time the complex mechanical properties of a single polymer molecule have been measured by TDFM. These observations are in agreement with previous dynamic AFM experiments.