THE STABILITY OF POLYPURINE TETRAPLEXES IN THE PRESENCE OF MONOVALENT AND DIVALENT-CATIONS

As with other guanine-rich sequences, poly[d(GGA)], poly[d(GA)] and poly[d(GAA)] probably form four-stranded or tetraplex structures. Thermal denaturation profiles were measured for these polymers at pH8 in the presence of Na+ , NH4+, K+, Cs+, Mg2+, Ca2;+ and Ba2+. For poly[d(GA)], Na+ , NH4+, K+ stabilize the tetraplex to similar extents and the Tm increases with increasing ionic strength. In contrast the Tms with Mg2;+, Ca2;+ and Ba2+ are significantly different and reach maxima at about 5mM of cation. The tetraplex from poly [d(GAA)] behaves In a similar manner. Thermal denaturation profiles for poly[d(GGA)] yield transitions whose hyperchromicity depends both on the concentration and nature of the ion. A reversible cooperative transition is not observed at concentrations greater than 0.15M K+ ,1mM Ca2+ or 0.3mM Ba2;+ and hysteresis is evident at some concentrations. These results are consistent with the idea that K+ and ions of a similar size can form a coordination complex with the 6-Keto group of eight guanines (G8;-DNA). Unlike the tetraplex polymer this G8;-DNA does not melt cooperatively. © 1990 Oxford University Press.