Cooperativity vs. Phase transition in a giant single DNA molecule

Single-chain observation of giant DNAs grafted with poly(N-isopropylacrylamide) (PNIPAAM) was performed using fluorescence microscopy while changing the solution temperature. The PNIPAAM-grafted DNA was prepared through the intercalation of psoralen-terminated PNIPAAM. Individual DNAs grafted with PNIPAAM exhibit a sharp but continuous transition at around 34 °C, from an elongated coil to a collapsed compact state. It is found that the width of the transition is almost the same between the level of individual DNAs and the level of ensemble DNAs. The unique nature of this transition is discussed in comparison with the discrete nature of the folding transition in native double-strand DNAs. With the addition of spermidine, a trivalent amine, the conformation of individual T4DNAs changes in a markedly discrete manner, on the level of individual giant DNAs, whereas in the ensemble, the size of T4 DNAs changes rather mildly; i.e., the transition appears to occur over a temperature range of 30 degrees. Thus, it is confirmed that the cooperative transition on the DNAs grafted with PNIPAAM exhibits a sharper transition than the change in the ensemble average for the discrete phase transition in native DNAs induced by the polycation.