Figure 4.

(a) Force-distance curves for the retraction of a 60 base DNA molecule from a guanidinium surface. The probe-PEG-DNA construct is illustrated in the inset. The stretched length of the PEG is 40 nm, so only the peaks beyond 40 nm reflect DNA adhesion. This is an overlay of a selection of typical curves. (b) Distribution of measured peak adhesion forces for those adhesion peaks that occurred after 40 nm (so were owing to DNA-guanidinium interactions). The solid line shows the distribution measured for single hydrogen bonds by Williams et al.²²