Solvent effects on the dynamics of (dG‐dC) 3

The kinetics of the coil‐to‐helix transition of (dG‐dC) 3 in M NaCl, 45 m M sodium cacodylate, pH 7, were measured in H 2 O, D 2 O, 10 mol % ethanol, 10 mol % urea, and 10 mol % glycerol. At 43°C in H 2 O the recombination rate is 1.3 ± 0.2 × 10 7 M −1 s −1 ; the dissociation rate is 68 ± 10 s −1 . The destabilization of the helix in 10 mol % ethanol and 10 mol % urea relative to water is primarily due to a large increase in the helix‐dissociation rate. In 10 mol % glycerol, the destabilization of the helix is due to a decrease in the recombination rate and an increase in the dissociation rate. Above 20°C, two exponential decays longer than 1 μs are observed after a temperature jump. The slower relaxation time is 4–10 times faster than the bimolecular component and is independent of oligomer concentration. We attribute this relaxation to a rapid equilibrium between two helical states. At low temperatures and oligomer concentrations of 1 m M or greater, the helices aggregate in 1 M NaCl. Experimental data are presented under conditions where aggregation is unimportant and evidence is given that the Δ H ‐determined spectroscopically is unaffected by aggregation.