The flexibility of A-form DNA.

We have determined the rise per base pair and persistence length of A-form DNA in trifluoroethanol solutions for fragments 350-900 base pairs in length that best describe rotational diffusion coefficients determined by transient electric birefringence. The 2.6 A spacing between base pairs found in crystal and fiber A-form structures is preserved in solution. The persistence length is about 1500 A, or about three times longer than for B-form DNA. There is no apparent electrostatic contribution to the persistence length in the salt concentration range 0.2-2.0 mM Na cacodylate. This suggests an even closer association between DNA and its neutralizing counterions than predicted by condensation theory, perhaps due to a sheath of trifluoroethanol excluded water surrounding the A-form helix.