Effect of on on the overstretching transition of double-stranded DNA: Evidence of force-induced DNA melting

When a single molecule of double-stranded DNA is stretched beyond its B-for m contour length, the measured force shows a highly cooperative overstretch ing transition. We have investigated the source of this transition by alter ing helix stability with solution pH. As solution pH was increased from pH 6.0 to pH 10.6 in 250 mM NaCl, the overstretching transition force decrease d from 67.0 +/- 0.8 pN to 56.2 +/- 0.8 pN, whereas the transition width rem ained nearly constant. As the pH was lowered from pH 6.0 to pH 3.1, the ove rstretching force decreased from 67.0 +/- 0.8 pN to 47.0 +/- 1.0 pN, but th e transition width increased from 3.0 +/- 0.6 pN to 16.0 +/- 3 pN, These re sults quantitatively agree with a model that asserts that DNA strand dissoc iation, or melting, occurs during the overstretching transition.