Publications

Recently, it has become clear that single, long duplex DNAs exhibit a large discrete transition between elongated coil and compacted globule states. To obtain further insight into this phenomenon, in the present study we observed individual DNA chains in an aqueous environment by fluorescence microscopy. The long-axis lengths of individual T4DNA (166 kbp) were calibrated to obtain a size distribution. The main purpose of the present study was to determine the effect of the valence of cations on the coil−globule transition. We used the following multivalent cations to induce the compaction of long DNA chains: 1,3-diaminopropane (bivalent), spermidine (trivalent), and spermine (tetravalent). Our results showed that the collapse of isolated DNA chains induced by either bivalent or multivalent cations is discrete. The critical concentration of cation for inducing the transition was lowest for the tetravalent cation and highest for the bivalent cation. We also compare the properties of the transition observed experimentally with a theoretical calculation including the effects on condensation of multivalent cations and ion-exchange reaction.