“…The B 1 3 B t transition involves the correlated switching of a and g torsions (Table 1) and little, if any, reorientation of neighboring basepairs (Figure 2(b), (d), and (f)). The B t 3 B 2 step, by contrast, entails relatively minor changes in backbone torsions (e, z, b), but large (anti to high anti) variations in the glycosyl rotation w and accompanying sharp jumps in base-pair Roll and Slide ($40 and $4-5 A Ê , respectively), which are expected to introduce an extremely high energy barrier at the borders between the two equi-potential forms (Olson et al, 1999; see Figure 7). We therefore hypothesize that the``energetically expensive'' switch into the B 2 conformation may account for the sharp phase transition observed at 60-70 % DNA extension (Cluzel et al, 1996;Smith et al, 1996).…”