Kinetics of the B-A transition of DNA: analysis of potential contributions to a reaction barrier.

Because of open problems in the relation between results obtained by relaxation experiments and molecular dynamics simulations on the B-A transition of DNA, relaxation measurements of the B-A dynamics have been extended to a wider range of conditions. Field-induced reaction effects are measured selectively by the magic angle technique using a novel cell construction preventing perturbations from cell window anisotropy. The kinetics was recorded for the case of poly[d(AT)] up to the salt concentration limit of 4.4 mM, where aggregation does not yet interfere. Now experimental data on the B-A dynamics are available for poly[d(AT)] at salt concentrations of 0.18, 0.73, 2.44 and 4.4 mM. In all cases, a spectrum of time constants is found, ranging from ~ 10 μs up to components approaching ~ 1 ms. The relatively small dependence of these data on the salt concentration indicates that electrostatic effects on the kinetics are not as strong as may be expected. The ethanol content at the transition center is a linear function of the logarithm of the salt concentration, and the slope is close to that expected from polyelectrolyte theory. The B-A transition dynamics was also measured in D2 O at a salt concentration of 2.4 mM: the center of the transition is found at 20.0 mol/l H2 O and at 20.1 mol/l D2 O with an estimated accuracy of ± 0.1 mol/l; the spectrum of time constants at the respective transition centers is very similar. The experimental results are discussed regarding the data obtained by molecular dynamics simulations.