Dynamic force spectroscopy of photoswitch-modified DNA.

We apply a combination of photoswitch-modified DNA and AFM-based pulling measurements to study the force-induced melting of double-stranded DNA in the unzipping geometry. We measure the differences in peak rupture force for azobenzene-modified DNA, as the incorporated azobenzenes are photoswitched reversibly between the trans and the cis form. Fitting our rupture force versus loading rate data, we obtain off rate (koff) at zero force values in the range of ∼10 s(-1). We show that the change in peak rupture force and koff induced by destabilizing the DNA duplex depends on the position of the destabilizing azobenzene photoswitch relative to the force-loading site. When the azobenzenes are proximal to the unzipping end, the decrease in peak force and koff upon azobenzene photoisomerization is significantly larger than when the azobenzene is distal to the site of force loading. We interpret these results as experimental evidence supporting the picture that the destabilization of a double-stranded DNA by a photoswitch isomerization is localized to a small bubble around the photoswitch.