Thermodynamics in folding transition of DNA

Recently, it has been found that individual giant DNA molecules exhibit a discrete transition, or first order phase-transition, between the compact folded state and the elongated coiled state, i.e., the folding transition. In order to clarify the thermodynamics in the folding transition of single DNA molecules, we have studied the temperature effect on the bimodal distribution of conformation for the ensemble of T4DNA chains (166 kbps) in both poly(ethylene glycol) (PEG) and spermidine. (SPD), using single-chain observation with fluorescence microscopy. From the van't Hoff relationship, the entropy change in the transition from the compact stare to the unfolded state is deduced as, DeltaS = +11, +38 k/molecule in the aqueous solution of PEG with sodium chloride and potassium chloride, respectively, where k is Boltzmann's constant, whereas, DeltaS with SPD is estimated to be -32 k/molecule. The values of DeltaS with the transition are discussed in term of the translational entropy of counterions together with the hydration effect.