A transition from a twice-folded structure to a hairpin in oligo(dG) in the presence of magnesium ions.

A long (dG)n stretch can fold twice forming an intramolecular tetrahelix stabilized by guanine tetrads and stacking interactions between them (G-structure). In this paper, we show that magnesium ions induce a transition of the (dG)n stretch from the G-structure to the hairpin stabilized by Hoogsteen-like G.G base pairs (G-hairpin). This transition between the G-structure and the G-hairpin is detected by chemical probing. The characteristic time of the transition for a (dG)28 at 4 degrees C exceeds five hours, which allowed us to separate these two forms by electrophoresis at low temperature. We believe that the slowness of the transition is due to the fact that half of the deoxyguanosines in the insert must change their conformation from syn to anti or vice versa.