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 $P_{\mathrm{WLC}}(r)$ for a wormlike chain. The $l_p$ values change dramatically over a narrow range of ${\mathrm{Mg}}^{2+}$ concentration from $\sim 21\AA$ in the unfolded state ($U$) to $\sim 10\AA$ in the compact ($I_C$) and native states. Variations in $l_p$ with increasing ${\mathrm{Na}}^{+}$ concentration are more gradual. In accord with the predictions of polyelectrolyte theory we find $l_p\propto 1/{\kappa }^2$ where $\kappa$ is the inverse Debye-screening length.