Cooperative lengths of DNA during melting

The mean cooperative length of domains of DNA, determined from the variance in (G + C) content in derivative melting curves of large bacterial DNAs, varies from 230 base pairs (bp) for (A ? T)-rich domains to 580 bp for (G ? C) domains. These values correspond to values for the cooperativity parameter of 2(±2) × 10^?5 and 3(±2) × 10^?6, respectively, and to +7.2 and +9.6 kcal for the free energy of helix interruption in those regions.