Fig. 3.

Force curves for antibody-tethered tips. Histograms of the measured adhesion forces for an antibody-tethered tip probing a field of MMTV nucleosomal array molecules (a) and the same tip scanning in the presence of 30 μg/ml of a peptide from the N-terminal tail of histone H3, showing how the blocked antibody does not adhere to the sample surface (b). Two hundred force curves were analyzed for each histogram (bars at the origins are the numbers of force curves in which no adhesion force was measured). (Insets) Typical examples of force-versus-distance curves obtained. The curve in a shows the characteristic single-molecule binding curve characteristic of PEG stretching, whereas the curve in b shows negligible adhesion. The average adhesion forces are 57 ± 20 pN (absence of H3 tail peptide), 6 pN (presence of tail peptide), 116 ± 63 pN (PEG tether alone), and 118 ± 134 pN for the bare (clean) tip. Thus, the tethered antibody plays an important role in reducing nonspecific adhesion.