  In this work, we show that by varying the experimental conditions, the
driving amplitude, a dynamic force microscope allows DNA properties to be
selectively imaged. The substrate on which the DNA is fixed is a silica surface
grafted with silanes molecules ended with amine groups. Use of small
oscillation amplitudes favors the attractive interaction between the tip and
the sample, while use of large amplitudes renders the contribution of the
attractive interaction negligible. Particularly, at small amplitudes, the
images show that the attractive interaction is strongly enhanced along the DNA.
This enhancement is found to be amenable with a model considering a narrow
strip of randomly oriented dipoles on each side of the molecule. This work
should provide new insights on the DNA interaction and conformational changes
with localized charges.
