  We present imaging and force spectroscopy measurements of DNA molecules
adsorbed on functionalized mica. By means of Non-Contact mode AFM (NC-AFM) in
Ultra High Vacuum (UHV), the frequency shift (\Delta f) versus separation (z)
curves were measured providing a quantitative measurement of both force and
energy of the tip-DNA interaction. Similarly, topographic images of the
adsorbed DNA molecules in constant frequency shift mode were collected. The
high resolution force measurements confirm the imaging contrast difference
between the substrate and DNA. The force curves measured along the DNA molecule
can be divided into two classes showing marked differences in the minimum of
the interaction force and energy, indicating that NC-AFM could deliver chemical
contrast along the DNA molecule.

Biological Physics;General Physics