Figure 6.

Theoretical modeling of the conformational energy of (dG)₁₈⋅(dC)₁₈. (A) Energy as a function of the twist per base pair. Base pair breaking and expulsion occurred at ≈70° (solid line) coupled to a sudden relaxation of the strain energy built up in the backbones. The discontinuity at 130° corresponds to a rearrangement of the backbones so that the phosphate anionic oxygens point outwards. This introduced C3′-endo puckering in the cytidine strand. When Watson–Crick hydrogen bonding was maintained with distance constraints the energy rose rapidly beyond a twist of 70° (dotted line). (B) Energy as a function of the rise per bp for overtwisted DNA (twist constrained at 138°). The rise could be varied between ≈4 and 6 Å (80% of the maximum extension of a single strand of DNA) with little energy change. The discontinuity at 5.5 Å corresponds to the reintroduction of a C2′-endo sugar for the cytidines, allowing an increase in the intrastrand P-P distance.