Persistence length changes dramatically as RNA folds

We determine the persistence length l(p) for a bacterial group I ribozyme as a function of concentration of monovalent and divalent cations by fitting the distance distribution functions P(r) obtained from small angle x-ray scattering intensity data to the asymptotic form of the calculated P-WLC(r) for a wormlike chain. The l(p) values change dramatically over a narrow range of Mg2+ concentration from similar to 21 angstrom in the unfolded state (U) to similar to 10 angstrom in the compact (I-C) and native states. Variations in l(p) with increasing Na+ concentration are more gradual. In accord with the predictions of polyelectrolyte theory we find l(p)proportional to 1/kappa(2) where kappa is the inverse Debye-screening length.