Thresholds in field-induced reactions of linear biopolymers. Strong chain-length dependence of field effects in DNA.

We have analysed the field-induced conformation change of DNA by absorbance measurements at the magic angle. Conformation changes are observed when the electric field strength exceeds a clearly defined threshold value. The threshold values increase with increasing salt concentration and show a linear dependence upon the logarithm of the ionic strength. Measurements with homogeneous DNA samples of different chain lengths N show that the threshold increases with decreasing N; at a given ionic strength the threshold is a linear function of the logarithm of N. The threshold value observed for a circular DNA molecule with a chain length Nc fits to these data with an effective length Nc/2. This result indicates that the length of maximal extension is important for the field-induced reaction and suggests, together with the other results, that the field-induced reaction is mainly driven by a polarization of the ion atmosphere along the axis of DNA. Some data are also given for the dynamics of the reaction: at high electric field pulses the first step is a fast destacking and tilting of the bases followed by a slow unwinding process. For short pulses the reaction is almost completely reversible with a characteristic time constant of about 3 microseconds for the back reaction.