Helix–coil dynamics of a Z‐helix hairpin

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Abstract
The helix–coil transition of a Z‐helix hairpin formed from d(C‐G)5T4(C‐G)5 has been characterized by equilibrium melting and temperature jump experiments in 5M NaClO4 and 10 mM Na2HPO4, pH 7.0. The melting curve can be represented by a simple all‐or‐none transition with a midpoint at 81.6 ± 0.4°C and an enthalpy change of 287 ± 15 kJ/mole. The temperature jump relaxation can be described by single exponentials at a reasonable accuracy. Amplitudes measured as a function of temperature provide equilibrium parameters consistent with those derived from equilibrium melting curves. The rate constants of Z‐helix formation are found in the range from 1800 s−1 at 70°C to 800 s−1 at 90°C and are associated with an activation enthalpy of −(50 ± 10) kJ/mole, whereas the rate constants of helix dissociation are found in the range from 200 s−1 at 70°C to 4500 s−1 at 90°C with an activation enthalpy +235 kJ/mole. These parameters are consistent with a requirement of 3–4 base pairs for helix nucleation. Apparently nucleation occurs in the Z‐helix conformation, because a separate slow step corresponding to a B to Z transition has not been observed. In summary, the dynamics of the Z‐helix–coil transition is very similar to that of previously investigated right‐handed double helices.