We have studied the interaction of the DNA molecule with the ligand 3,3'-diaminobenzidine (DAB) by performing single molecule stretching experiments with optical tweezers and dynamic light scattering (DLS) on the DNA-DAB complexes. In the stretching experiments, the persistence and contour lengths of the complexes were measured as a function of DAB concentration, allowing one to infer the main binding mechanism and also to determine the physicochemical parameters of the interaction. In the DLS experiments, the effective size of the complexes, measured as the hydrodynamic radius, was monitored as a function of DAB concentration. We found a qualitative agreement between the results obtained from the two techniques by comparing the behaviors of the hydrodynamics radius and the radius of gyration, since this last one can be expressed as a function of the persistence and contour lengths.