Direct Mechanical Measurements of the Elasticity of Single DNA Molecules by Using Magnetic Beads

TLDR

DNA molecules were tethered between a glass surface and a magnetic bead, and their equilibrium positions were monitored under controlled magnetic and hydrodynamic forces to generate force–extension data. Force–extension curves at 10⁻¹⁴–10⁻¹¹ N and varying salt concentrations revealed deviations from the freely jointed chain model, indicating intrinsic local curvature, while intercalators had minimal impact and a bend‑inducing platinum complex markedly altered elasticity, supporting the natural curvature hypothesis.

Abstract

Single DNA molecules were chemically attached by one end to a glass surface and by their other end to a magnetic bead. Equilibrium positions of the beads were observed in an optical microscope while the beads were acted on by known magnetic and hydrodynamic forces. Extension versus force curves were obtained for individual DNA molecules at three different salt concentrations with forces between 10 -14 and 10 -11 newtons. Deviations from the force curves predicted by the freely jointed chain model suggest that DNA has significant local curvature in solution. Ethidium bromide and 4′,6-diamidino-2-phenylindole had little effect on the elastic response of the molecules, but their extent of intercalation was directly measured. Conversely, the effect of bend-inducing cis -diamminedichloroplatinum (II) was large and supports the hypothesis of natural curvature in DNA.