238 6.5  Scanning Probe Microscopy and Force Spectroscopy

force feedback to clamp the molecular force to a preset value to enable definitive observation

of force-​dependent kinetics of unfolding and subsequently refolding of molecular domains

(details of analytical methods to model these processes are discussed in Chapter 8).

6.5.5  AFM “CUT AND PASTE”

The spatial reproducibility and resolution of many AFM systems is high enough such that

single molecules may be pulled clear from a surface, moved laterally by anything from a few

nanometers to several microns, and then controllably repositioned by pressing the tip back

into the surface (Figure 6.7c).

This in effect is a molecular cut-​and-​paste device (Kufer et al., 2008). By combining AFM

cut and paste with the specificity of DNA base pairing, it has been possible to use a comple­

mentary DNA strand conjugated to an AFM tip to specifically capture surface-​bound DNA

constructs from a “depot” region on the surface of the sample and repositioned elsewhere on

the surface, offering future potential for smart designed synthetic biology applications at the

nanometer length scale (discussed in Chapter 9).

FIGURE 6.7  Atomic force microscopy (AFM) molecular manipulation methods. (a) AFM force

spectroscopy using a “fishing” approach to tether a peptide construct, consisting of repeat “Ig

domain” subunits, which results in (b), a characteristic sawtooth response of the force versus

extension relation. (c) An AFM tip can be used to controllably relocate single DNA “cargo”

molecules to different regions of a gold-​coated coverslip.