Thermodynamics in the folding phase-transition of single T4DNA molecules in poly(ethylene glycol) solution

To clarify the thermodynamics in the folding phase-transition of single DNA molecules, we studied the effect of temperature on the bimodal distribution of conformation for the ensemble of T4DNA chains (166 kbps) at a fixed concentration of poly(ethylene glycol) using single-chain observation with fluorescence microscopy. From the vanʹt Hoff-type relationship, the entropy change in the unfolding transition is ΔS=+11.3 k/molecule, and the change in the heat content is ΔH=+4.77×10−20 J/molecule at Tc=306 K, where k is the Boltzmannʹs constant. The positive value of ΔS with the unfolding transition is discussed in relation to the increase in the translational entropy of counterions.