Differential scanning microcalorimetry (DSC), UV absorption and circular dichroism (CD) have been used to study structure and stability of linear (lin), open circular (oc), supercoiled (cd) and relaxed circular duplex (rd) DNA and calf thymus (CT) DNA. Investigations were made in low salt buffer and in the presence of 7.2 M NaClO4. The chaotropic action of perchlorate promotes a reduction of the overall stability of DNA, which permits a direct determination of the transition enthalpies of all four DNA configurations. The stabilities against thermal denaturation have been found to increase in the series lin approximately oc less than cd less than rd. These relative stabilities can be rationalized on the basis of the linkage between supercoiling and secondary structural changes in topologically constrained duplex DNA. On the basis of these studies, a model of the melting process could be suggested that is consistent with the energetic and spectroscopic data.

The The percentage of closed circular DNA was obtained from scans at 250 nm of the negatives of polaroid photos taken of the agarose gels under UV illumination. The sample solutions used for calorimetry contained between 75% and 95% closed circular DNA.Determination of Superhelix Density. Superhelix density was determined by cesium chloride density gradient centrifugation (40) and by fluorescence titration (41).Calorimetry. An LKB batch microcalorimeter was used for the experiments. Measurements were performed at 37°C.With our conditions of thermostatting the instrument, heat pulses of 50 ,uJ can be well resolved. The sample cells were charged with approximately 4 ml of the plasmid DNA solu- 1312The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

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