A set of 10 double-stranded RNAs ranging in length from 92 to 317 base pairs has been synthesized, and their rotational diffusion times were measured by transient electric birefringence. A hydrodynamic analysis of the birefringence decay data yields a helix persistence length of 720 ± 70 Å, based on a helix with an effective hydrodynamic diameter of 26 Å and a rise per base pair of 2.7 Å in the presence of Mg2+ ions. Thus, duplex RNA is somewhat stiffer than DNA, for which the persistence length is 450-500 Å. The measurements also suggest that (i) current hydrodynamic theory for the rotational decay times is applicable to RNA molecules that are longer than 100 base pairs and (ii) weak Mg2+-RNA association decreases the rise per base pair slightly, by no more than 0.1 Å.
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