DUPLEX-TETRAPLEX EQUILIBRIUM BETWEEN A HAIRPIN AND 2 INTERACTING HAIRPINS OF D(A-G)(10) AT NEUTRAL PH

d(A-G)(10) forms two helical structures at neutrality, at low ionic st rength a single-hairpin duplex, and at higher ionic strength a double- hairpin tetraplex. An ionic strength-dependent equilibrium between the se forms is indicated by native PAGE, which also reveals additional si ngle-stranded species below 0.3 M Na+, probably corresponding to parti ally denatured states. The equilibrium also depends upon oligomer conc entration: 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 sin gle hairpin; at high concentrations, it co-migrates with the linear du plex d(A-G)(10). d(C-T)(10), probably because it is a two-hairpin tetr aplex. Molecular weights measured by equilibrium sedimentation in 0.1 M Na+, pH 7, reveal a mixture of monomer and dimer species at 1 degree s C, but only a monomer at 40 degrees C; in 0.6 M Na+, pH 7, only a di mer species is observed at 4 degrees C. That the single- and double-st randed species are hairpin helices, is indicated by preferential S1 nu clease cleavage at the center of the oligomer(s), i.e., the loop of th e hairpin(s). The UV melting transition below 0.3 M Na+ or K+, exhibit s a dT(m)/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+] greater than or equal to 0. 3 M, dT(m)/dlog [Na+/K+] is 19 or 17 degrees C, respectively, consiste nt with conversion of a two-hairpin tetraplex directly to single stran ds. A two-hairpin structure stabilized by G-tetrads is indicated by di fferential scanning calorimetry in 0.15 M Na+/5 mM Mg2+, with Delta H of formation per mole of the two-hairpin tetraplex of -116.9 kcal or - 29.2 kcal/mol of G-tetrad.