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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.
