Thermodynamics of the collapsing phase transition in a single duplex DNA molecule

We observed changes in the conformation of individual single giant DNAs (T4DNA) using fluorescence microscopy at different temperatures, 3 similar to 53 degrees C, in an aqueous solution containing 1.2 mM spermidine. We found that individual DNAs take either elongated coil or collapsed globule conformation and that the relative population of the globule state increases with an increase in temperature. Based on this temperature dependence, the change in entropy for the coil-globule transition was deduced to be Delta S = 32 +/- 8 kg, where kg is the Boltzmann constant. In other words, at Te = 301 K the change in enthalpy is Delta H =32 +/- 8k(B)T(C) = 19 +/- 5 kcal/mol chains. The thermodynamics upon the transition in a single T4 molecule from the elongated coil to the collapsed,globule is discussed in terms of statistical physics by considering the different contributions to the change in entropy; i.e., elasticity, mixing with the solvent, the translation of counterions, hydration, and charge neutralization. We conclude that the small positive change in entropy for the transition is attributed to the competition between the large positive value originated from the translational entropy on the exchange of counterions and large negative values from the other contributions.