Non-specific binding of Na$^+$ and Mg$^{2+}$ to RNA determined by force spectroscopy methods

TL;DR

This study uses optical tweezers to quantify how monovalent and divalent ions influence RNA hairpin stability, validating models of ion binding and providing thermodynamic parameters across various ionic conditions.

Contribution

It introduces a force spectroscopy approach to measure RNA thermodynamics under different ionic conditions, validating the Tightly Bound Ion model and quantifying ion effects.

Findings

Validated the Tightly Bound Ion model for RNA-ion interactions

Measured the persistence length of ssRNA

Quantified non-specific ion binding effects on RNA stability

Abstract

RNA duplex stability depends strongly on ionic conditions, and inside cells RNAs are exposed to both monovalent and multivalent ions. Despite recent advances, we do not have general methods to quantitatively account for the effects of monovalent and multivalent ions on RNA stability, and the thermodynamic parameters for secondary structure prediction have only been derived at 1M [Na$^+$]. Here, by mechanically unfolding and folding a 20 bp RNA hairpin using optical tweezers, we study the RNA thermodynamics and kinetics at different monovalent and mixed monovalent/Mg$^{2+}$ salt conditions. We measure the unfolding and folding rupture forces and apply Kramers theory to extract accurate information about the hairpin free energy landscape under tension at a wide range of ionic conditions. We obtain non-specific corrections for the free energy of formation of the RNA hairpin and measure how the distance of the transition state to the folded state changes with force and ionic strength. We experimentally validate the Tightly Bound Ion model and obtain values for the persistence length of ssRNA. Finally, we test the approximate rule by which the non-specific binding affinity of divalent cations at a given concentration is equivalent to that of monovalent cations taken at 100 fold that concentration for small molecular constructs.