We separate double stranded lambda phage DNA by applying a fixed force at a constant temperature ranging from 15 to $50°\mathrm{C}$, and measure the minimum force required to separate the two strands. The measurements also offer information on the free energy of double stranded DNA (dsDNA) at temperatures where dsDNA does not thermally denature in the absence of force. While parts of the phase diagram can be explained using existing models and free energy parameters, others deviate significantly. Possible reasons for the deviations between theory and experiment are considered.

• Received 24 October 2003

DOI:https://doi.org/10.1103/PhysRevLett.93.078101