Abstract
It has been suggested that the structure that results when double-stranded DNA (dsDNA) is pulled from the 3'3' ends differs from that which results when it is pulled from the 5'5' ends. In this work, we demonstrate, using lambda phage dsDNA, that the overstretched states do indeed show different properties, suggesting that they correspond to different structures. For 3'3' pulling versus 5'5' pulling, the following differences are observed: (i) the forces at which half of the molecules in the ensemble have made a complete force-induced transition to single stranded DNA are 141 +/- 3 pN and 122 +/- 4 pN, respectively; (ii) the extension vs. force curve for overstretched DNA has a marked change in slope at 127 +/- 3 pN for 3'3' and 110 +/- 3 pN for 5'5'; (iii) the hysteresis (H) in the extension vs. force curves at 150 mM NaCl is 0.3 +/- 0.8 pN microm for 3'3' versus 13 +/- 8 pN for 5'5'; and (iv) 3'3' and 5'5' molecules show different changes in hysteresis due to interactions with beta-cyclodextrin, a molecule that is known to form stable host-guest complexes with rotated base pairs, and glyoxal that is known to bind stably to unpaired bases. These differences and additional findings are well-accommodated by the corresponding structures predicted on theoretical grounds.