d(GCGTAGC), an Equilibrium Between a Hairpin Structure and an Unusual Duplex

The DNA heptamer d(GCGTAGC), as determined by NMR spectroscopy, occurs in aqueous solution in a temperature-and salt-dependent equilibrium between a monoloop hairpin (where the unpaired thymidine, forming the loop, is stacked on top of the guanine (G3) base) and a duplex with the unpaired thymidine residues from opposite strands intercalating and costacking between shared G:A pairs.