  There is a long-standing experimental observation that the melting of
topologically constrained DNA, such as circular-closed plasmids, is less abrupt
than that of linear molecules. This finding points to an intriguing role of
topology in the physics of DNA denaturation, which is however poorly
understood. Here, we shed light on this issue by combining large-scale Brownian
Dynamics simulations with an analytically solvable phenomenological Landau mean
field theory. We find that the competition between melting and supercoiling
leads to phase coexistence of denatured and intact phases at the single
molecule level. This coexistence occurs in a wide temperature range, thereby
accounting for the broadening of the transition. Finally, our simulations show
an intriguing topology-dependent scaling law governing the growth of
denaturation bubbles in supercoiled plasmids, which can be understood within
the proposed mean field theory.
