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Nucleic Acids Research, 1990, Vol. 18, No. 20 6057-6060
© 1990

CHEMISTRY

The stability of polypurine tetraplexes in the presence of mono- and divalent cations

Jeremy S. Lee

Department of Biochemistry, University of Saskatchewan Saskatoon, Saskatchewan, S7N 0W0, Canada

Received June 18, 1990. Revised September 12, 1990. Accepted September 12, 1990.

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.


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