Modulation of the Thermodynamic Signatures of an RNA Thermometer by Osmolytes and Salts.

Folding of ribonucleic acids (RNAs) is driven by several factors, such as base pairing and stacking, chain entropy, and ion-mediated electrostatics, which have been studied in great detail. However, the power of background molecules in the cellular milieu is often neglected. Herein, we study the effect of common osmolytes on the folding equilibrium of a hairpin-structured RNA and, using pressure perturbation, provide novel thermodynamic and volumetric insights into the modulation mechanism. The presence of TMAO causes an increased thermal stability and a more positive volume change for the helix-to-coil transition, whereas urea destabilizes the hairpin and leads to an increased expansibility of the unfolded state. Further, we find a strong interplay between water, salt, and osmolyte in driving the thermodynamics and defining the temperature and pressure stability limit of the RNA. Our results support a universal working mechanism of TMAO and urea to (de)stabilize proteins and the RNA.