Low-polar solvent strikingly stiffens double-stranded RNA and reverses its twist-stretch coupling.

Cellular environments are crowded systems with reduced solvent polarity, yet how solvent polarity shapes RNA elasticity remains unclear. In this work, our high-precision magnetic tweezers and all-atom molecular dynamics simulations showed that decreasing solvent polarity with ethanol as a model cosolvent produces a biphasic response for double-stranded (ds) RNA: moderate ethanol concentration softens dsRNA, causing a slight decrease in bending persistence length P and stretch modulus S, but high ethanol concentration markedly stiffens dsRNA, reflected by the about twofold increase in P and about fourfold increase in S. Furthermore, the twist-stretch coupling of dsRNA is strikingly reversed by ethanol of high concentration. The transition originates from the ethanol-enhanced ion neutralization giving way to major-groove clamping by monovalent ions as solvent polarity decreases. Further molecular dynamics simulations mimicking reduced water polarity by scaling atomic charges reproduce these effects, establishing solvent polarity control possibly as a general mechanism for dsRNA in cells and a guiding principle for RNA-based nanostructure design.