Conformational Flexibility of DNA

Pulsed Electron–Electron Double Resonance (PELDOR) on double-stranded DNA (ds-DNA) was used to investigate the conformational flexibility of helical DNA. Stretching, twisting, and bending flexibility of ds-DNA was determined by incorporation of two rigid nitroxide spin labels into a series of 20 base pair (bp) DNA duplexes. Orientation-selective PELDOR experiments performed at both X-band (9 GHz/0.3 T) and G-band (180 GHz/6.4 T) with spin label distances in the range of 2–4 nm allowed us to differentiate between different simple models of DNA dynamics existing in the literature. All of our experimental results are in full agreement with a dynamic model for ds-DNA molecules, where stretching of the molecule leads to a slightly reduced radius of the helix induced by a cooperative twist–stretch coupling.