Theoretical analysis of the helix-coil transition in positively superhelical DNA at high temperatures

This paper presents a statistical mechanical analysis of strand separation in a topologically constrained (i.e., superhelical) DNA homopolymer. Our calculations show that positive superhelicity can efficiently stabilize the B-form DNA duplex at temperatures substantially above the transition temperature ${\mathit{T}}_{\mathit{m}}$ for denaturation of the linear molecule. Moderate superhelix densities (σ<0.08) suffice to keep a DNA molecule virtually entirely in the B-form conformation at temperatures where, if it were relaxed, it would be substantially denatured. This behavior persists up to a predicted critical temperature ${\mathit{T}}_{\mathit{c}}$>${\mathit{T}}_{\mathit{m}}$. When T>${\mathit{T}}_{\mathit{c}}$ the molecule remains entirely denatured regardless of the amount of superhelicity imposed. The value of ${\mathit{T}}_{\mathit{c}}$ is shown to depend on the torsional stiffness C associated with interstrand twisting of the unpaired strands within denatured regions. Thus, experiments that measure ${\mathit{T}}_{\mathit{c}}$ would provide another method to evaluate C. © 1996 The American Physical Society.