Polypurine DNAs and RNAs form secondary structures which may be tetra-stranded

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Abstract
Polypurine DNAs and RNAs containing at least 33% guanine form a stable secondary structure at neutral pH and moderate ionic strengths. The t m 's of the polymers increase with increasing guanine content. To eliminate possible structures three novel polymers, d(Gn 2 A) n , d(Gm 6 A) n and d(IA) n* , as well as the random copolymer r(G.A) n were studied. Both d(Gn 2 A) n and d(IA) n can form a secondary structure whereas d(Gm 6 A) n and r(G,A) n cannot. Model building suggested two possible structures, one a duplex and the other a tetra-stranded polymer. The latter is considered to be the more likely, since previous X-ray diffraction studies have shown that rG n and rI n are tetra-stranded. Circular dichroism spectra are also consistent with such an interpretation.