  We use optical tweezers to perform stretching experiments on DNA molecules
when interacting with the drugs daunomycin and ethidium bromide, which
intercalate the DNA molecule. These experiments are performed in the low-force
regime from zero up to 2 pN. Our results show that the persistence length of
the DNA-drug complexes increases strongly as the drug concentration increases
up to some critical value. Above this critical value, the persistence length
decreases abruptly and remains practically constant for larger drug
concentrations. The contour length of the molecules increases monotonically and
saturates as drugs concentration increases. Measured in- tercalants critical
concentrations for the persistence length transition coincide with reported
values for the helix-coil transition of DNA-drug complexes, obtained from
sedimentation experiments.
