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 worm-like chain (WLC). The $l_p$ values change dramatically over a narrow range of \Mg concentration from $\sim$21 Åin the unfolded state (\textbf{U}) to $\sim$10 Åin the compact ($\mathrm{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 $l_p \propto 1/ κ^2$ where $κ$ is the inverse Debye-screening length.
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