Molecules were stretched in 10 mM Tris buffer (pH 7.0), 250 mM NaCl, 10 mM MgCl 2 , or alternatively in 10 mM Tris-acetate (pH 7.4), 60 mM NH 4 Cl, 6 mM b-mercapto, 20 mM MgCl 2 . Unfolding/refolding parameters were statistically indistinguishable for both conditions. The spring constant of the optical tweezers was 0.1-0.2 pN nm -1 .