Hairpin structures formed by seven DNA inverted repeats have been studied by PAGE, UV(CD)-spectroscopy and nuclease cleavage. The hairpins consisted of (CG)3 stems and loops of 2, 3 and 4 residues. Thermal stabilities (T(m)) have been determined in low and high ionic strength buffers, where the hairpins were structured in the B- and Z-DNA form respectively. The thermodynamic parameters of hairpin formation have been obtained by a two-state analysis of the hairpin-coil transitions. It is found that, on increasing the number of bases in the loop from 2 to 3 and 4, the T(m)s of the B-hairpins decrease, whereas the T(m)s of the same hairpins in the Z-form increase. This confirms previous evidence (1,2) that in a hairpin molecule the size and structure of the loop are modulated by the conformation of the helical stem. Moreover, B-hairpins with loops comprising 2, 3 and 4 bases have been digested with the single-strand-specific nuclease from mung bean. In our experimental conditions (0-degrees-C) the nuclease preferentially cleaves the unbonded nucleotides of the loops. However, the rates of loop hydrolysis, which roughly follow a first-order kinetics, markedly depend on the size of the loop. At a ratio of 3 enzyme units/mu-g DNA, the half-lives of hairpins which are expected to form loops of 4, 3 and 2 residues are 90, 145 and 440 minutes respectively. Thermostability and enzymatic digestion data suggest that two-membered loops can be formed in B-hairpins but not in Z-hairpins.
