Elastic response of DNA to external electric fields in two dimensions

We investigate the elastic response of a single DNA molecule to an external electric field in two dimensions. DNA is tethered at one extremity to a solid support, confined to two dimensions by the electrostatic interaction with a fluid lipid membrane and directly visualized by fluorescence microscopy. The end-to-end distance of electrophoretically stretched DNA elongates with increasing field strength in good agreement with the worm-like chain model for semiflexible chains adapted to two dimensions. We obtain an effective charge of 0.6 e/nm. The time average contour of the stretched molecule shows a trumpet-like vertical broadening due to thermal fluctuations. Alignment of the membrane-bound DNA is reversible. The relaxation of the end-to-end distance from a highly stretched state follows a scaling behavior with a scaling exponent of 0.46 +/- 0.8 as expected for diffusive shortening.