Persistence length changes dramatically as RNA folds

We determine the persistence length lp 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 PWLC(r) for a wormlike chain. The lp values change dramatically over a narrow range of Mg2+ concentration from ∼21 in the unfolded state (U) to ∼10 in the compact (IC) and native states. Variations in lp with increasing Na+ concentration are more gradual. In accord with the predictions of polyelectrolyte theory we find lp1/κ2 where κ is the inverse Debye-screening length.