Duplex-tetraplex equilibrium between a hairpin and two interacting hairpins of d(A-G)10 at neutral pH.

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d(A-G)10 forms two helical structures at neutrality, at low ionic strength a single-hairpin duplex, and at higher ionic strength a double-hairpin tetraplex. An ionic strength-dependent equilibrium between these forms is indicated by native PAGE, which also reveals additional single-stranded species below 0.3 M Na+, probably corresponding to partially denatured states. The equilibrium also depends upon oligomer concentration: at very low concentrations, d(A-G)10 migrates faster than the random coil d(C-T)10, probably because it is a more compact single hairpin; at high concentrations, it co-migrates with the linear duplex d(A-G)10 x d(C-T)10, probably because it is a two-hairpin tetraplex. Molecular weights measured by equilibrium sedimentation in 0.1 M Na+, pH 7, reveal a mixture of monomer and dimer species at 1 degree C, but only a monomer at 40 degrees C; in 0.6 M Na+, pH 7, only a dimer species is observed at 4 degrees C. That the single- and double-stranded species are hairpin helices, is indicated by preferential S1 nuclease cleavage at the center of the oligomer(s), i.e., the loop of the hairpin(s). The UV melting transition below 0.3 M Na+ or K+, exhibits a dTm/dlog[Na+/K+] of 33 or 36 degrees C, respectively, consistent with conversion of a two-hairpin tetraplex to a single-hairpin duplex with extrahelical residues. When [Na+/K+] > or = 0.3 M, dTm/dlog [Na+/K+] is 19 or 17 degrees C, respectively, consistent with conversion of a two-hairpin tetraplex directly to single strands. A two-hairpin structure stabilized by G-tetrads is indicated by differential scanning calorimetry in 0.15 M Na+/5 mM Mg2+, with deltaH of formation per mole of the two-hairpin tetraplex of -116.9 kcal or -29.2 kcal/mol of G-tetrad.

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