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 lₚ∝1/κ² where κ is the inverse Debye-screening length.