Force-dependent persistence length of DNA-intercalator complexes measured in single molecule stretching experiments

By using optical tweezers with an adjustable trap stiffness, we have performed systematic single molecule stretching experiments with two types of DNA-intercalator complexes, in order to investigate the effects of the maximum applied forces on the mechanical response of such complexes. We have explicitly shown that even in the low-force entropic regime the persistence length of the DNA-intercalator complexes is strongly force-dependent, although such behavior is not exhibited by bare DNA molecules. We discuss the possible physicochemical effects that can lead to such results. In particular, we propose that the stretching force can promote partial denaturation on the highly distorted double-helix of the DNA-intercalator complexes, which interfere strongly in the measured values of the persistence length.