Table 1. Most probable folding and unfolding forces, determined from measurements at a velocity of 50 nm/s. See Supplementary Figure S2 for an example of unfolding and folding forces distribution.

Hairpin	Unfolding force	Folding force
	(pN)	(pN)
DNA10	6.4	6.2
DNA18	6.1	3.6
RNA10	13.6	8.8
RNA18	14.1	5.8

The presented force values correspond to the maxima of a Gaussian fit of the measured distributions of the forces where unfolding or folding occurs. The unfolding force distributions are build from 77 measured unfolding events for DNA10, 93 events for DNA18, 179 events for RNA10 and 146 events for RNA18. The folding force distributions are based on 79 measured folding events for DNA10, 70 events for DNA18, 148 events for RNA10 and 114 events for RNA18. The widths of these distributions are about 2.5 pN (FWHM).