Effect of elastic energy on the folding of an RNA hairpin

► We use a zipping model to analyse the effect of elastic energy on the folding of an RNA hairpin. ► Isotonic unfolding occurs at a well-defined critical load. ► Isometric unfolding produces a series of sharp tension peaks as the hairpin unzips base by base. ► Series compliance broadens the tension peaks and produces hysteresis. ► A stiff force transducer and handles could improve the resolution of single-molecule experiments.