DISCRETE COIL-GLOBULE TRANSITION OF SINGLE DUPLEX DNAS INDUCED BY POLYAMINES

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 presen t study we observed individual DNA chains in an aqueous environment by fluorescence microscopy. The long-axis lengths of individual T4DNA (1 66 kbp) were calibrated to obtain a size distribution. The main purpos e of the present study was to determine the effect of the valence of c ations on the coil-globule transition. We used the following multivale nt cations to induce the compaction of long DNA chains: 1,3-diaminopro pane (bivalent), spermidine (trivalent), and spermine (tetravalent). O ur results showed that the collapse of isolated DNA chains induced by either bivalent or multivalent cations is discrete. The critical conce ntration of cation for inducing the transition was lowest for the tetr avalent cation and highest for the bivalent cation. We also compare th e properties of the transition observed experimentally with a theoreti cal calculation including the effects on condensation of multivalent c ations and ion-exchange reaction.